Merge branch 'prop140_21643_diff_only_squashed'

14 files changed, 2701 insertions, 4 deletions

diff --git a/src/common/log.c b/src/common/log.c
index 2878ab5349..6a5819064a 100644
--- a/src/common/log.c
+++ b/src/common/log.c
@@ -1177,7 +1177,7 @@ static const char *domain_list[] = {
   "GENERAL", "CRYPTO", "NET", "CONFIG", "FS", "PROTOCOL", "MM",
   "HTTP", "APP", "CONTROL", "CIRC", "REND", "BUG", "DIR", "DIRSERV",
   "OR", "EDGE", "ACCT", "HIST", "HANDSHAKE", "HEARTBEAT", "CHANNEL",
-  "SCHED", "GUARD", NULL
+  "SCHED", "GUARD", "CONSDIFF", NULL
 };
 
 /** Return a bitmask for the log domain for which <b>domain</b> is the name,
diff --git a/src/common/torlog.h b/src/common/torlog.h
index ce7aff5301..6e374b1c11 100644
--- a/src/common/torlog.h
+++ b/src/common/torlog.h
@@ -101,8 +101,10 @@
 #define LD_SCHED     (1u<<22)
 /** Guard nodes */
 #define LD_GUARD     (1u<<23)
+/** Generation and application of consensus diffs. */
+#define LD_CONSDIFF  (1u<<24)
 /** Number of logging domains in the code. */
-#define N_LOGGING_DOMAINS 24
+#define N_LOGGING_DOMAINS 25
 
 /** This log message is not safe to send to a callback-based logger
  * immediately.  Used as a flag, not a log domain. */
diff --git a/src/or/consdiff.c b/src/or/consdiff.c
new file mode 100644
index 0000000000..13b267f4a0
--- /dev/null
+++ b/src/or/consdiff.c
@@ -0,0 +1,1301 @@
+/* Copyright (c) 2014, Daniel Martí
+ * Copyright (c) 2014, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file consdiff.c
+ * \brief Consensus diff implementation, including both the generation and the
+ * application of diffs in a minimal ed format.
+ *
+ * The consensus diff application is done in consdiff_apply_diff, which relies
+ * on apply_ed_diff for the main ed diff part and on some digest helper
+ * functions to check the digest hashes found in the consensus diff header.
+ *
+ * The consensus diff generation is more complex. consdiff_gen_diff generates
+ * it, relying on gen_ed_diff to generate the ed diff and some digest helper
+ * functions to generate the digest hashes.
+ *
+ * gen_ed_diff is the tricky bit. In it simplest form, it will take quadratic
+ * time and linear space to generate an ed diff given two smartlists. As shown
+ * in its comment section, calling calc_changes on the entire two consensuses
+ * will calculate what is to be added and what is to be deleted in the diff.
+ * Its comment section briefly explains how it works.
+ *
+ * In our case specific to consensuses, we take advantage of the fact that
+ * consensuses list routers sorted by their identities. We use that
+ * information to avoid running calc_changes on the whole smartlists.
+ * gen_ed_diff will navigate through the two consensuses identity by identity
+ * and will send small couples of slices to calc_changes, keeping the running
+ * time near-linear. This is explained in more detail in the gen_ed_diff
+ * comments.
+ *
+ * The allocation strategy tries to save time and memory by avoiding needless
+ * copies.  Instead of actually splitting the inputs into separate strings, we
+ * allocate cdline_t objects, each of which represents a line in the original
+ * object or in the output.  We use memarea_t allocators to manage the
+ * temporary memory we use when generating or applying diffs.
+ **/
+
+#define CONSDIFF_PRIVATE
+
+#include "or.h"
+#include "consdiff.h"
+#include "memarea.h"
+#include "routerparse.h"
+
+static const char* ns_diff_version = "network-status-diff-version 1";
+static const char* hash_token = "hash";
+
+static char *consensus_join_lines(const smartlist_t *inp);
+
+/** Return true iff a and b have the same contents. */
+STATIC int
+lines_eq(const cdline_t *a, const cdline_t *b)
+{
+  return a->len == b->len && fast_memeq(a->s, b->s, a->len);
+}
+
+/** Return true iff a has the same contents as the nul-terminated string b. */
+STATIC int
+line_str_eq(const cdline_t *a, const char *b)
+{
+  const size_t len = strlen(b);
+  tor_assert(len <= UINT32_MAX);
+  cdline_t bline = { b, (uint32_t)len };
+  return lines_eq(a, &bline);
+}
+
+/** Add a cdline_t to <b>lst</b> holding as its contents the nul-terminated
+ * string s.  Use the provided memory area for storage. */
+STATIC void
+smartlist_add_linecpy(smartlist_t *lst, memarea_t *area, const char *s)
+{
+  size_t len = strlen(s);
+  const char *ss = memarea_memdup(area, s, len);
+  cdline_t *line = memarea_alloc(area, sizeof(cdline_t));
+  line->s = ss;
+  line->len = (uint32_t)len;
+  smartlist_add(lst, line);
+}
+
+/** Compute the digest of <b>cons</b>, and store the result in
+ * <b>digest_out</b>. Return 0 on success, -1 on failure. */
+/* This is a separate, mockable function so that we can override it when
+ * fuzzing. */
+MOCK_IMPL(STATIC int,
+consensus_compute_digest,(const char *cons,
+                          consensus_digest_t *digest_out))
+{
+  int r = crypto_digest256((char*)digest_out->sha3_256,
+                           cons, strlen(cons), DIGEST_SHA3_256);
+  return r;
+}
+
+/** Return true iff <b>d1</b> and <b>d2</b> contain the same digest */
+/* This is a separate, mockable function so that we can override it when
+ * fuzzing. */
+MOCK_IMPL(STATIC int,
+consensus_digest_eq,(const uint8_t *d1,
+                     const uint8_t *d2))
+{
+  return fast_memeq(d1, d2, DIGEST256_LEN);
+}
+
+/** Create (allocate) a new slice from a smartlist. Assumes that the start
+ * and the end indexes are within the bounds of the initial smartlist. The end
+ * element is not part of the resulting slice. If end is -1, the slice is to
+ * reach the end of the smartlist.
+ */
+STATIC smartlist_slice_t *
+smartlist_slice(const smartlist_t *list, int start, int end)
+{
+  int list_len = smartlist_len(list);
+  tor_assert(start >= 0);
+  tor_assert(start <= list_len);
+  if (end == -1) {
+    end = list_len;
+  }
+  tor_assert(start <= end);
+
+  smartlist_slice_t *slice = tor_malloc(sizeof(smartlist_slice_t));
+  slice->list = list;
+  slice->offset = start;
+  slice->len = end - start;
+  return slice;
+}
+
+/** Helper: Compute the longest common subsequence lengths for the two slices.
+ * Used as part of the diff generation to find the column at which to split
+ * slice2 while still having the optimal solution.
+ * If direction is -1, the navigation is reversed. Otherwise it must be 1.
+ * The length of the resulting integer array is that of the second slice plus
+ * one.
+ */
+STATIC int *
+lcs_lengths(const smartlist_slice_t *slice1, const smartlist_slice_t *slice2,
+            int direction)
+{
+  size_t a_size = sizeof(int) * (slice2->len+1);
+
+  /* Resulting lcs lengths. */
+  int *result = tor_malloc_zero(a_size);
+  /* Copy of the lcs lengths from the last iteration. */
+  int *prev = tor_malloc(a_size);
+
+  tor_assert(direction == 1 || direction == -1);
+
+  int si = slice1->offset;
+  if (direction == -1) {
+    si += (slice1->len-1);
+  }
+
+  for (int i = 0; i < slice1->len; ++i, si+=direction) {
+
+    const cdline_t *line1 = smartlist_get(slice1->list, si);
+    /* Store the last results. */
+    memcpy(prev, result, a_size);
+
+    int sj = slice2->offset;
+    if (direction == -1) {
+      sj += (slice2->len-1);
+    }
+
+    for (int j = 0; j < slice2->len; ++j, sj+=direction) {
+
+      const cdline_t *line2 = smartlist_get(slice2->list, sj);
+      if (lines_eq(line1, line2)) {
+        /* If the lines are equal, the lcs is one line longer. */
+        result[j + 1] = prev[j] + 1;
+      } else {
+        /* If not, see what lcs parent path is longer. */
+        result[j + 1] = MAX(result[j], prev[j + 1]);
+      }
+    }
+  }
+  tor_free(prev);
+  return result;
+}
+
+/** Helper: Trim any number of lines that are equally at the start or the end
+ * of both slices.
+ */
+STATIC void
+trim_slices(smartlist_slice_t *slice1, smartlist_slice_t *slice2)
+{
+  while (slice1->len>0 && slice2->len>0) {
+    const cdline_t *line1 = smartlist_get(slice1->list, slice1->offset);
+    const cdline_t *line2 = smartlist_get(slice2->list, slice2->offset);
+    if (!lines_eq(line1, line2)) {
+      break;
+    }
+    slice1->offset++; slice1->len--;
+    slice2->offset++; slice2->len--;
+  }
+
+  int i1 = (slice1->offset+slice1->len)-1;
+  int i2 = (slice2->offset+slice2->len)-1;
+
+  while (slice1->len>0 && slice2->len>0) {
+    const cdline_t *line1 = smartlist_get(slice1->list, i1);
+    const cdline_t *line2 = smartlist_get(slice2->list, i2);
+    if (!lines_eq(line1, line2)) {
+      break;
+    }
+    i1--;
+    slice1->len--;
+    i2--;
+    slice2->len--;
+  }
+}
+
+/** Like smartlist_string_pos, but uses a cdline_t, and is restricted to the
+ * bounds of the slice.
+ */
+STATIC int
+smartlist_slice_string_pos(const smartlist_slice_t *slice,
+                           const cdline_t *string)
+{
+  int end = slice->offset + slice->len;
+  for (int i = slice->offset; i < end; ++i) {
+    const cdline_t *el = smartlist_get(slice->list, i);
+    if (lines_eq(el, string)) {
+      return i;
+    }
+  }
+  return -1;
+}
+
+/** Helper: Set all the appropriate changed booleans to true. The first slice
+ * must be of length 0 or 1. All the lines of slice1 and slice2 which are not
+ * present in the other slice will be set to changed in their bool array.
+ * The two changed bool arrays are passed in the same order as the slices.
+ */
+STATIC void
+set_changed(bitarray_t *changed1, bitarray_t *changed2,
+            const smartlist_slice_t *slice1, const smartlist_slice_t *slice2)
+{
+  int toskip = -1;
+  tor_assert(slice1->len == 0 || slice1->len == 1);
+
+  if (slice1->len == 1) {
+    const cdline_t *line_common = smartlist_get(slice1->list, slice1->offset);
+    toskip = smartlist_slice_string_pos(slice2, line_common);
+    if (toskip == -1) {
+      bitarray_set(changed1, slice1->offset);
+    }
+  }
+  int end = slice2->offset + slice2->len;
+  for (int i = slice2->offset; i < end; ++i) {
+    if (i != toskip) {
+      bitarray_set(changed2, i);
+    }
+  }
+}
+
+/*
+ * Helper: Given that slice1 has been split by half into top and bot, we want
+ * to fetch the column at which to split slice2 so that we are still on track
+ * to the optimal diff solution, i.e. the shortest one. We use lcs_lengths
+ * since the shortest diff is just another way to say the longest common
+ * subsequence.
+ */
+static int
+optimal_column_to_split(const smartlist_slice_t *top,
+                        const smartlist_slice_t *bot,
+                        const smartlist_slice_t *slice2)
+{
+  int *lens_top = lcs_lengths(top, slice2, 1);
+  int *lens_bot = lcs_lengths(bot, slice2, -1);
+  int column=0, max_sum=-1;
+
+  for (int i = 0; i < slice2->len+1; ++i) {
+    int sum = lens_top[i] + lens_bot[slice2->len-i];
+    if (sum > max_sum) {
+      column = i;
+      max_sum = sum;
+    }
+  }
+  tor_free(lens_top);
+  tor_free(lens_bot);
+
+  return column;
+}
+
+/**
+ * Helper: Figure out what elements are new or gone on the second smartlist
+ * relative to the first smartlist, and store the booleans in the bitarrays.
+ * True on the first bitarray means the element is gone, true on the second
+ * bitarray means it's new.
+ *
+ * In its base case, either of the smartlists is of length <= 1 and we can
+ * quickly see what elements are new or are gone. In the other case, we will
+ * split one smartlist by half and we'll use optimal_column_to_split to find
+ * the optimal column at which to split the second smartlist so that we are
+ * finding the smallest diff possible.
+ */
+STATIC void
+calc_changes(smartlist_slice_t *slice1,
+             smartlist_slice_t *slice2,
+             bitarray_t *changed1, bitarray_t *changed2)
+{
+  trim_slices(slice1, slice2);
+
+  if (slice1->len <= 1) {
+    set_changed(changed1, changed2, slice1, slice2);
+
+  } else if (slice2->len <= 1) {
+    set_changed(changed2, changed1, slice2, slice1);
+
+  /* Keep on splitting the slices in two. */
+  } else {
+    smartlist_slice_t *top, *bot, *left, *right;
+
+    /* Split the first slice in half. */
+    int mid = slice1->len/2;
+    top = smartlist_slice(slice1->list, slice1->offset, slice1->offset+mid);
+    bot = smartlist_slice(slice1->list, slice1->offset+mid,
+        slice1->offset+slice1->len);
+
+    /* Split the second slice by the optimal column. */
+    int mid2 = optimal_column_to_split(top, bot, slice2);
+    left = smartlist_slice(slice2->list, slice2->offset, slice2->offset+mid2);
+    right = smartlist_slice(slice2->list, slice2->offset+mid2,
+        slice2->offset+slice2->len);
+
+    calc_changes(top, left, changed1, changed2);
+    calc_changes(bot, right, changed1, changed2);
+    tor_free(top);
+    tor_free(bot);
+    tor_free(left);
+    tor_free(right);
+  }
+}
+
+/* This table is from crypto.c. The SP and PAD defines are different. */
+#define NOT_VALID_BASE64 255
+#define X NOT_VALID_BASE64
+#define SP NOT_VALID_BASE64
+#define PAD NOT_VALID_BASE64
+static const uint8_t base64_compare_table[256] = {
+  X, X, X, X, X, X, X, X, X, SP, SP, SP, X, SP, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  SP, X, X, X, X, X, X, X, X, X, X, 62, X, X, X, 63,
+  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, X, X, X, PAD, X, X,
+  X, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
+  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, X, X, X, X, X,
+  X, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+  41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
+};
+
+/** Helper: Get the identity hash from a router line, assuming that the line
+ * at least appears to be a router line and thus starts with "r ".
+ *
+ * If an identity hash is found, store it (without decoding it) in
+ * <b>hash_out</b>, and return 0.  On failure, return -1.
+ */
+STATIC int
+get_id_hash(const cdline_t *line, cdline_t *hash_out)
+{
+  if (line->len < 2)
+    return -1;
+
+  /* Skip the router name. */
+  const char *hash = memchr(line->s + 2, ' ', line->len - 2);
+  if (!hash) {
+    return -1;
+  }
+
+  hash++;
+  const char *hash_end = hash;
+  /* Stop when the first non-base64 character is found. Use unsigned chars to
+   * avoid negative indexes causing crashes.
+   */
+  while (base64_compare_table[*((unsigned char*)hash_end)]
+           != NOT_VALID_BASE64 &&
+         hash_end < line->s + line->len) {
+    hash_end++;
+  }
+
+  /* Empty hash. */
+  if (hash_end == hash) {
+    return -1;
+  }
+
+  hash_out->s = hash;
+  /* Always true because lines are limited to this length */
+  tor_assert(hash_end - hash <= UINT32_MAX);
+  hash_out->len = (uint32_t)(hash_end - hash);
+
+  return 0;
+}
+
+/** Helper: Check that a line is a valid router entry. We must at least be
+ * able to fetch a proper identity hash from it for it to be valid.
+ */
+STATIC int
+is_valid_router_entry(const cdline_t *line)
+{
+  if (line->len < 2 || fast_memneq(line->s, "r ", 2))
+    return 0;
+  cdline_t tmp;
+  return (get_id_hash(line, &tmp) == 0);
+}
+
+/** Helper: Find the next router line starting at the current position.
+ * Assumes that cur is lower than the length of the smartlist, i.e. it is a
+ * line within the bounds of the consensus. The only exception is when we
+ * don't want to skip the first line, in which case cur will be -1.
+ */
+STATIC int
+next_router(const smartlist_t *cons, int cur)
+{
+  int len = smartlist_len(cons);
+  tor_assert(cur >= -1 && cur < len);
+
+  if (++cur >= len) {
+    return len;
+  }
+
+  const cdline_t *line = smartlist_get(cons, cur);
+  while (!is_valid_router_entry(line)) {
+    if (++cur >= len) {
+      return len;
+    }
+    line = smartlist_get(cons, cur);
+  }
+  return cur;
+}
+
+/** Helper: compare two base64-encoded identity hashes, which may be of
+ * different lengths. Comparison ends when the first non-base64 char is found.
+ */
+STATIC int
+base64cmp(const cdline_t *hash1, const cdline_t *hash2)
+{
+  /* NULL is always lower, useful for last_hash which starts at NULL. */
+  if (!hash1->s && !hash2->s) {
+    return 0;
+  }
+  if (!hash1->s) {
+    return -1;
+  }
+  if (!hash2->s) {
+    return 1;
+  }
+
+  /* Don't index with a char; char may be signed. */
+  const unsigned char *a = (unsigned char*)hash1->s;
+  const unsigned char *b = (unsigned char*)hash2->s;
+  const unsigned char *a_end = a + hash1->len;
+  const unsigned char *b_end = b + hash2->len;
+  while (1) {
+    uint8_t av = base64_compare_table[*a];
+    uint8_t bv = base64_compare_table[*b];
+    if (av == NOT_VALID_BASE64) {
+      if (bv == NOT_VALID_BASE64) {
+        /* Both ended with exactly the same characters. */
+        return 0;
+      } else {
+        /* hash2 goes on longer than hash1 and thus hash1 is lower. */
+        return -1;
+      }
+    } else if (bv == NOT_VALID_BASE64) {
+      /* hash1 goes on longer than hash2 and thus hash1 is greater. */
+      return 1;
+    } else if (av < bv) {
+      /* The first difference shows that hash1 is lower. */
+      return -1;
+    } else if (av > bv) {
+      /* The first difference shows that hash1 is greater. */
+      return 1;
+    } else {
+      a++;
+      b++;
+      if (a == a_end) {
+        if (b == b_end) {
+          return 0;
+        } else {
+          return -1;
+        }
+      } else if (b == b_end) {
+        return 1;
+      }
+    }
+  }
+}
+
+/** Structure used to remember the previous and current identity hash of
+ * the "r " lines in a consensus, to enforce well-ordering. */
+typedef struct router_id_iterator_t {
+  cdline_t last_hash;
+  cdline_t hash;
+} router_id_iterator_t;
+
+/**
+ * Initializer for a router_id_iterator_t.
+ */
+#define ROUTER_ID_ITERATOR_INIT { { NULL, 0 }, { NULL, 0 } }
+
+/** Given an index *<b>idxp</b> into the consensus at <b>cons</b>, advance
+ * the index to the next router line ("r ...") in the consensus, or to
+ * an index one after the end of the list if there is no such line.
+ *
+ * Use <b>iter</b> to record the hash of the found router line, if any,
+ * and to enforce ordering on the hashes.  If the hashes are mis-ordered,
+ * return -1.  Else, return 0.
+ **/
+static int
+find_next_router_line(const smartlist_t *cons,
+                      const char *consname,
+                      int *idxp,
+                      router_id_iterator_t *iter)
+{
+  *idxp = next_router(cons, *idxp);
+  if (*idxp < smartlist_len(cons)) {
+    memcpy(&iter->last_hash, &iter->hash, sizeof(cdline_t));
+    if (get_id_hash(smartlist_get(cons, *idxp), &iter->hash) < 0 ||
+        base64cmp(&iter->hash, &iter->last_hash) <= 0) {
+      log_warn(LD_CONSDIFF, "Refusing to generate consensus diff because "
+               "the %s consensus doesn't have its router entries sorted "
+               "properly.", consname);
+      return -1;
+    }
+  }
+  return 0;
+}
+
+/** Generate an ed diff as a smartlist from two consensuses, also given as
+ * smartlists. Will return NULL if the diff could not be generated, which can
+ * happen if any lines the script had to add matched "." or if the routers
+ * were not properly ordered.
+ *
+ * All cdline_t objects in the resulting object are either references to lines
+ * in one of the inputs, or are newly allocated lines in the provided memarea.
+ *
+ * This implementation is consensus-specific. To generate an ed diff for any
+ * given input in quadratic time, you can replace all the code until the
+ * navigation in reverse order with the following:
+ *
+ *   int len1 = smartlist_len(cons1);
+ *   int len2 = smartlist_len(cons2);
+ *   bitarray_t *changed1 = bitarray_init_zero(len1);
+ *   bitarray_t *changed2 = bitarray_init_zero(len2);
+ *   cons1_sl = smartlist_slice(cons1, 0, -1);
+ *   cons2_sl = smartlist_slice(cons2, 0, -1);
+ *   calc_changes(cons1_sl, cons2_sl, changed1, changed2);
+ */
+STATIC smartlist_t *
+gen_ed_diff(const smartlist_t *cons1, const smartlist_t *cons2,
+            memarea_t *area)
+{
+  int len1 = smartlist_len(cons1);
+  int len2 = smartlist_len(cons2);
+  smartlist_t *result = smartlist_new();
+
+  /* Initialize the changed bitarrays to zero, so that calc_changes only needs
+   * to set the ones that matter and leave the rest untouched.
+   */
+  bitarray_t *changed1 = bitarray_init_zero(len1);
+  bitarray_t *changed2 = bitarray_init_zero(len2);
+  int i1=-1, i2=-1;
+  int start1=0, start2=0;
+
+  /* To check that hashes are ordered properly */
+  router_id_iterator_t iter1 = ROUTER_ID_ITERATOR_INIT;
+  router_id_iterator_t iter2 = ROUTER_ID_ITERATOR_INIT;
+
+  /* i1 and i2 are initialized at the first line of each consensus. They never
+   * reach past len1 and len2 respectively, since next_router doesn't let that
+   * happen. i1 and i2 are advanced by at least one line at each iteration as
+   * long as they have not yet reached len1 and len2, so the loop is
+   * guaranteed to end, and each pair of (i1,i2) will be inspected at most
+   * once.
+   */
+  while (i1 < len1 || i2 < len2) {
+
+    /* Advance each of the two navigation positions by one router entry if not
+     * yet at the end.
+     */
+    if (i1 < len1) {
+      if (find_next_router_line(cons1, "base", &i1, &iter1) < 0) {
+        goto error_cleanup;
+      }
+    }
+
+    if (i2 < len2) {
+      if (find_next_router_line(cons2, "target", &i2, &iter2) < 0) {
+        goto error_cleanup;
+      }
+    }
+
+    /* If we have reached the end of both consensuses, there is no need to
+     * compare hashes anymore, since this is the last iteration.
+     */
+    if (i1 < len1 || i2 < len2) {
+
+      /* Keep on advancing the lower (by identity hash sorting) position until
+       * we have two matching positions. The only other possible outcome is
+       * that a lower position reaches the end of the consensus before it can
+       * reach a hash that is no longer the lower one. Since there will always
+       * be a lower hash for as long as the loop runs, one of the two indexes
+       * will always be incremented, thus assuring that the loop must end
+       * after a finite number of iterations. If that cannot be because said
+       * consensus has already reached the end, both are extended to their
+       * respecting ends since we are done.
+       */
+      int cmp = base64cmp(&iter1.hash, &iter2.hash);
+      while (cmp != 0) {
+        if (i1 < len1 && cmp < 0) {
+          if (find_next_router_line(cons1, "base", &i1, &iter1) < 0) {
+            goto error_cleanup;
+          }
+          if (i1 == len1) {
+            /* We finished the first consensus, so grab all the remaining
+             * lines of the second consensus and finish up.
+             */
+            i2 = len2;
+            break;
+          }
+        } else if (i2 < len2 && cmp > 0) {
+          if (find_next_router_line(cons2, "target", &i2, &iter2) < 0) {
+            goto error_cleanup;
+          }
+          if (i2 == len2) {
+            /* We finished the second consensus, so grab all the remaining
+             * lines of the first consensus and finish up.
+             */
+            i1 = len1;
+            break;
+          }
+        } else {
+          i1 = len1;
+          i2 = len2;
+          break;
+        }
+        cmp = base64cmp(&iter1.hash, &iter2.hash);
+      }
+    }
+
+    /* Make slices out of these chunks (up to the common router entry) and
+     * calculate the changes for them.
+     * Error if any of the two slices are longer than 10K lines. That should
+     * never happen with any pair of real consensuses. Feeding more than 10K
+     * lines to calc_changes would be very slow anyway.
+     */
+#define MAX_LINE_COUNT (10000)
+    if (i1-start1 > MAX_LINE_COUNT || i2-start2 > MAX_LINE_COUNT) {
+      log_warn(LD_CONSDIFF, "Refusing to generate consensus diff because "
+          "we found too few common router ids.");
+      goto error_cleanup;
+    }
+
+    smartlist_slice_t *cons1_sl = smartlist_slice(cons1, start1, i1);
+    smartlist_slice_t *cons2_sl = smartlist_slice(cons2, start2, i2);
+    calc_changes(cons1_sl, cons2_sl, changed1, changed2);
+    tor_free(cons1_sl);
+    tor_free(cons2_sl);
+    start1 = i1, start2 = i2;
+  }
+
+  /* Navigate the changes in reverse order and generate one ed command for
+   * each chunk of changes.
+   */
+  i1=len1-1, i2=len2-1;
+  char buf[128];
+  while (i1 > 0 || i2 > 0) {
+
+    int start1x, start2x, end1, end2, added, deleted;
+
+    /* We are at a point were no changed bools are true, so just keep going. */
+    if (!(i1 >= 0 && bitarray_is_set(changed1, i1)) &&
+        !(i2 >= 0 && bitarray_is_set(changed2, i2))) {
+      if (i1 >= 0) {
+        i1--;
+      }
+      if (i2 >= 0) {
+        i2--;
+      }
+      continue;
+    }
+
+    end1 = i1, end2 = i2;
+
+    /* Grab all contiguous changed lines */
+    while (i1 >= 0 && bitarray_is_set(changed1, i1)) {
+      i1--;
+    }
+    while (i2 >= 0 && bitarray_is_set(changed2, i2)) {
+      i2--;
+    }
+
+    start1x = i1+1, start2x = i2+1;
+    added = end2-i2, deleted = end1-i1;
+
+    if (added == 0) {
+      if (deleted == 1) {
+        tor_snprintf(buf, sizeof(buf), "%id", start1x+1);
+        smartlist_add_linecpy(result, area, buf);
+      } else {
+        tor_snprintf(buf, sizeof(buf), "%i,%id", start1x+1, start1x+deleted);
+        smartlist_add_linecpy(result, area, buf);
+      }
+    } else {
+      int i;
+      if (deleted == 0) {
+        tor_snprintf(buf, sizeof(buf), "%ia", start1x);
+        smartlist_add_linecpy(result, area, buf);
+      } else if (deleted == 1) {
+        tor_snprintf(buf, sizeof(buf), "%ic", start1x+1);
+        smartlist_add_linecpy(result, area, buf);
+      } else {
+        tor_snprintf(buf, sizeof(buf), "%i,%ic", start1x+1, start1x+deleted);
+        smartlist_add_linecpy(result, area, buf);
+      }
+
+      for (i = start2x; i <= end2; ++i) {
+        cdline_t *line = smartlist_get(cons2, i);
+        if (line_str_eq(line, ".")) {
+          log_warn(LD_CONSDIFF, "Cannot generate consensus diff because "
+              "one of the lines to be added is \".\".");
+          goto error_cleanup;
+        }
+        smartlist_add(result, line);
+      }
+      smartlist_add_linecpy(result, area, ".");
+    }
+  }
+
+  bitarray_free(changed1);
+  bitarray_free(changed2);
+
+  return result;
+
+ error_cleanup:
+
+  bitarray_free(changed1);
+  bitarray_free(changed2);
+
+  smartlist_free(result);
+
+  return NULL;
+}
+
+/* Helper: Read a base-10 number between 0 and INT32_MAX from <b>s</b> and
+ * store it in <b>num_out</b>.  Advance <b>s</b> to the characer immediately
+ * after the number.  Return 0 on success, -1 on failure. */
+static int
+get_linenum(const char **s, int *num_out)
+{
+  int ok;
+  char *next;
+  *num_out = (int) tor_parse_long(*s, 10, 0, INT32_MAX, &ok, &next);
+  if (ok && next) {
+    *s = next;
+    return 0;
+  } else {
+    return -1;
+  }
+}
+
+/** Apply the ed diff, starting at <b>diff_starting_line</b>, to the consensus
+ * and return a new consensus, also as a line-based smartlist. Will return
+ * NULL if the ed diff is not properly formatted.
+ *
+ * All cdline_t objects in the resulting object are references to lines
+ * in one of the inputs; nothing is copied.
+ */
+STATIC smartlist_t *
+apply_ed_diff(const smartlist_t *cons1, const smartlist_t *diff,
+              int diff_starting_line)
+{
+  int diff_len = smartlist_len(diff);
+  int j = smartlist_len(cons1);
+  smartlist_t *cons2 = smartlist_new();
+
+  for (int i=diff_starting_line; i<diff_len; ++i) {
+    const cdline_t *diff_cdline = smartlist_get(diff, i);
+    char diff_line[128];
+
+    if (diff_cdline->len > sizeof(diff_line) - 1) {
+      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+               "an ed command was far too long");
+      goto error_cleanup;
+    }
+    /* Copy the line to make it nul-terminated. */
+    memcpy(diff_line, diff_cdline->s, diff_cdline->len);
+    diff_line[diff_cdline->len] = 0;
+    const char *ptr = diff_line;
+    int start = 0, end = 0;
+    if (get_linenum(&ptr, &start) < 0) {
+      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+               "an ed command was missing a line number.");
+      goto error_cleanup;
+    }
+    if (*ptr == ',') {
+      /* Two-item range */
+      ++ptr;
+      if (get_linenum(&ptr, &end) < 0) {
+        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+                 "an ed command was missing a range end line number.");
+        goto error_cleanup;
+      }
+      /* Incoherent range. */
+      if (end <= start) {
+        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+                 "an invalid range was found in an ed command.");
+        goto error_cleanup;
+      }
+    } else {
+      /* We'll take <n1> as <n1>,<n1> for simplicity. */
+      end = start;
+    }
+
+    if (end > j) {
+      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+          "its commands are not properly sorted in reverse order.");
+      goto error_cleanup;
+    }
+
+    if (*ptr == '\0') {
+      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+               "a line with no ed command was found");
+      goto error_cleanup;
+    }
+
+    if (*(ptr+1) != '\0') {
+      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+          "an ed command longer than one char was found.");
+      goto error_cleanup;
+    }
+
+    char action = *ptr;
+
+    switch (action) {
+      case 'a':
+      case 'c':
+      case 'd':
+        break;
+      default:
+        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+            "an unrecognised ed command was found.");
+        goto error_cleanup;
+    }
+
+    /* Add unchanged lines. */
+    for (; j && j > end; --j) {
+      cdline_t *cons_line = smartlist_get(cons1, j-1);
+      smartlist_add(cons2, cons_line);
+    }
+
+    /* Ignore removed lines. */
+    if (action == 'c' || action == 'd') {
+      while (--j >= start) {
+        /* Skip line */
+      }
+    }
+
+    /* Add new lines in reverse order, since it will all be reversed at the
+     * end.
+     */
+    if (action == 'a' || action == 'c') {
+      int added_end = i;
+
+      i++; /* Skip the line with the range and command. */
+      while (i < diff_len) {
+        if (line_str_eq(smartlist_get(diff, i), ".")) {
+          break;
+        }
+        if (++i == diff_len) {
+          log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+              "it has lines to be inserted that don't end with a \".\".");
+          goto error_cleanup;
+        }
+      }
+
+      int added_i = i-1;
+
+      /* It would make no sense to add zero new lines. */
+      if (added_i == added_end) {
+        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
+            "it has an ed command that tries to insert zero lines.");
+        goto error_cleanup;
+      }
+
+      while (added_i > added_end) {
+        cdline_t *added_line = smartlist_get(diff, added_i--);
+        smartlist_add(cons2, added_line);
+      }
+    }
+  }
+
+  /* Add remaining unchanged lines. */
+  for (; j > 0; --j) {
+    cdline_t *cons_line = smartlist_get(cons1, j-1);
+    smartlist_add(cons2, cons_line);
+  }
+
+  /* Reverse the whole thing since we did it from the end. */
+  smartlist_reverse(cons2);
+  return cons2;
+
+ error_cleanup:
+
+  smartlist_free(cons2);
+
+  return NULL;
+}
+
+/** Generate a consensus diff as a smartlist from two given consensuses, also
+ * as smartlists. Will return NULL if the consensus diff could not be
+ * generated. Neither of the two consensuses are modified in any way, so it's
+ * up to the caller to free their resources.
+ */
+smartlist_t *
+consdiff_gen_diff(const smartlist_t *cons1,
+                  const smartlist_t *cons2,
+                  const consensus_digest_t *digests1,
+                  const consensus_digest_t *digests2,
+                  memarea_t *area)
+{
+  smartlist_t *ed_diff = gen_ed_diff(cons1, cons2, area);
+  /* ed diff could not be generated - reason already logged by gen_ed_diff. */
+  if (!ed_diff) {
+    goto error_cleanup;
+  }
+
+  /* See that the script actually produces what we want. */
+  smartlist_t *ed_cons2 = apply_ed_diff(cons1, ed_diff, 0);
+  if (!ed_cons2) {
+    /* LCOV_EXCL_START -- impossible if diff generation is correct */
+    log_warn(LD_BUG|LD_CONSDIFF, "Refusing to generate consensus diff because "
+        "the generated ed diff could not be tested to successfully generate "
+        "the target consensus.");
+    goto error_cleanup;
+    /* LCOV_EXCL_STOP */
+  }
+
+  int cons2_eq = 1;
+  if (smartlist_len(cons2) == smartlist_len(ed_cons2)) {
+    SMARTLIST_FOREACH_BEGIN(cons2, const cdline_t *, line1) {
+      const cdline_t *line2 = smartlist_get(ed_cons2, line1_sl_idx);
+      if (! lines_eq(line1, line2) ) {
+        cons2_eq = 0;
+        break;
+      }
+    } SMARTLIST_FOREACH_END(line1);
+  } else {
+    cons2_eq = 0;
+  }
+  smartlist_free(ed_cons2);
+  if (!cons2_eq) {
+    /* LCOV_EXCL_START -- impossible if diff generation is correct. */
+    log_warn(LD_BUG|LD_CONSDIFF, "Refusing to generate consensus diff because "
+        "the generated ed diff did not generate the target consensus "
+        "successfully when tested.");
+    goto error_cleanup;
+    /* LCOV_EXCL_STOP */
+  }
+
+  char cons1_hash_hex[HEX_DIGEST256_LEN+1];
+  char cons2_hash_hex[HEX_DIGEST256_LEN+1];
+  base16_encode(cons1_hash_hex, HEX_DIGEST256_LEN+1,
+                (const char*)digests1->sha3_256, DIGEST256_LEN);
+  base16_encode(cons2_hash_hex, HEX_DIGEST256_LEN+1,
+                (const char*)digests2->sha3_256, DIGEST256_LEN);
+
+  /* Create the resulting consensus diff. */
+  char buf[160];
+  smartlist_t *result = smartlist_new();
+  tor_snprintf(buf, sizeof(buf), "%s", ns_diff_version);
+  smartlist_add_linecpy(result, area, buf);
+  tor_snprintf(buf, sizeof(buf), "%s %s %s", hash_token,
+      cons1_hash_hex, cons2_hash_hex);
+  smartlist_add_linecpy(result, area, buf);
+  smartlist_add_all(result, ed_diff);
+  smartlist_free(ed_diff);
+  return result;
+
+ error_cleanup:
+
+  if (ed_diff) {
+    /* LCOV_EXCL_START -- ed_diff is NULL except in unreachable cases above */
+    smartlist_free(ed_diff);
+    /* LCOV_EXCL_STOP */
+  }
+
+  return NULL;
+}
+
+/** Fetch the digest of the base consensus in the consensus diff, encoded in
+ * base16 as found in the diff itself. digest1_out and digest2_out must be of
+ * length DIGEST256_LEN or larger if not NULL.
+ */
+int
+consdiff_get_digests(const smartlist_t *diff,
+                     char *digest1_out,
+                     char *digest2_out)
+{
+  smartlist_t *hash_words = NULL;
+  const cdline_t *format;
+  char cons1_hash[DIGEST256_LEN], cons2_hash[DIGEST256_LEN];
+  char *cons1_hash_hex, *cons2_hash_hex;
+  if (smartlist_len(diff) < 2) {
+    log_info(LD_CONSDIFF, "The provided consensus diff is too short.");
+    goto error_cleanup;
+  }
+
+  /* Check that it's the format and version we know. */
+  format = smartlist_get(diff, 0);
+  if (!line_str_eq(format, ns_diff_version)) {
+    log_warn(LD_CONSDIFF, "The provided consensus diff format is not known.");
+    goto error_cleanup;
+  }
+
+  /* Grab the base16 digests. */
+  hash_words = smartlist_new();
+  {
+    const cdline_t *line2 = smartlist_get(diff, 1);
+    char *h = tor_memdup_nulterm(line2->s, line2->len);
+    smartlist_split_string(hash_words, h, " ", 0, 0);
+    tor_free(h);
+  }
+
+  /* There have to be three words, the first of which must be hash_token. */
+  if (smartlist_len(hash_words) != 3 ||
+      strcmp(smartlist_get(hash_words, 0), hash_token)) {
+    log_info(LD_CONSDIFF, "The provided consensus diff does not include "
+        "the necessary digests.");
+    goto error_cleanup;
+  }
+
+  /* Expected hashes as found in the consensus diff header. They must be of
+   * length HEX_DIGEST256_LEN, normally 64 hexadecimal characters.
+   * If any of the decodings fail, error to make sure that the hashes are
+   * proper base16-encoded digests.
+   */
+  cons1_hash_hex = smartlist_get(hash_words, 1);
+  cons2_hash_hex = smartlist_get(hash_words, 2);
+  if (strlen(cons1_hash_hex) != HEX_DIGEST256_LEN ||
+      strlen(cons2_hash_hex) != HEX_DIGEST256_LEN) {
+    log_info(LD_CONSDIFF, "The provided consensus diff includes "
+        "base16-encoded digests of incorrect size.");
+    goto error_cleanup;
+  }
+
+  if (base16_decode(cons1_hash, DIGEST256_LEN,
+          cons1_hash_hex, HEX_DIGEST256_LEN) != DIGEST256_LEN ||
+      base16_decode(cons2_hash, DIGEST256_LEN,
+          cons2_hash_hex, HEX_DIGEST256_LEN) != DIGEST256_LEN) {
+    log_info(LD_CONSDIFF, "The provided consensus diff includes "
+        "malformed digests.");
+    goto error_cleanup;
+  }
+
+  if (digest1_out) {
+    memcpy(digest1_out, cons1_hash, DIGEST256_LEN);
+  }
+  if (digest2_out) {
+    memcpy(digest2_out, cons2_hash, DIGEST256_LEN);
+  }
+
+  SMARTLIST_FOREACH(hash_words, char *, cp, tor_free(cp));
+  smartlist_free(hash_words);
+  return 0;
+
+ error_cleanup:
+
+  if (hash_words) {
+    SMARTLIST_FOREACH(hash_words, char *, cp, tor_free(cp));
+    smartlist_free(hash_words);
+  }
+  return 1;
+}
+
+/** Apply the consensus diff to the given consensus and return a new
+ * consensus, also as a line-based smartlist. Will return NULL if the diff
+ * could not be applied. Neither the consensus nor the diff are modified in
+ * any way, so it's up to the caller to free their resources.
+ */
+char *
+consdiff_apply_diff(const smartlist_t *cons1,
+                    const smartlist_t *diff,
+                    const consensus_digest_t *digests1)
+{
+  smartlist_t *cons2 = NULL;
+  char *cons2_str = NULL;
+  char e_cons1_hash[DIGEST256_LEN];
+  char e_cons2_hash[DIGEST256_LEN];
+
+  if (consdiff_get_digests(diff, e_cons1_hash, e_cons2_hash) != 0) {
+    goto error_cleanup;
+  }
+
+  /* See that the consensus that was given to us matches its hash. */
+  if (!consensus_digest_eq(digests1->sha3_256,
+                           (const uint8_t*)e_cons1_hash)) {
+    char hex_digest1[HEX_DIGEST256_LEN+1];
+    char e_hex_digest1[HEX_DIGEST256_LEN+1];
+    log_warn(LD_CONSDIFF, "Refusing to apply consensus diff because "
+        "the base consensus doesn't match the digest as found in "
+        "the consensus diff header.");
+    base16_encode(hex_digest1, HEX_DIGEST256_LEN+1,
+                  (const char *)digests1->sha3_256, DIGEST256_LEN);
+    base16_encode(e_hex_digest1, HEX_DIGEST256_LEN+1,
+                  e_cons1_hash, DIGEST256_LEN);
+    log_warn(LD_CONSDIFF, "Expected: %s; found: %s",
+             hex_digest1, e_hex_digest1);
+    goto error_cleanup;
+  }
+
+  /* Grab the ed diff and calculate the resulting consensus. */
+  /* Skip the first two lines. */
+  cons2 = apply_ed_diff(cons1, diff, 2);
+
+  /* ed diff could not be applied - reason already logged by apply_ed_diff. */
+  if (!cons2) {
+    goto error_cleanup;
+  }
+
+  cons2_str = consensus_join_lines(cons2);
+
+  consensus_digest_t cons2_digests;
+  if (consensus_compute_digest(cons2_str, &cons2_digests) < 0) {
+    /* LCOV_EXCL_START -- digest can't fail */
+    log_warn(LD_CONSDIFF, "Could not compute digests of the consensus "
+        "resulting from applying a consensus diff.");
+    goto error_cleanup;
+    /* LCOV_EXCL_STOP */
+  }
+
+  /* See that the resulting consensus matches its hash. */
+  if (!consensus_digest_eq(cons2_digests.sha3_256,
+                           (const uint8_t*)e_cons2_hash)) {
+    log_warn(LD_CONSDIFF, "Refusing to apply consensus diff because "
+        "the resulting consensus doesn't match the digest as found in "
+        "the consensus diff header.");
+    char hex_digest2[HEX_DIGEST256_LEN+1];
+    char e_hex_digest2[HEX_DIGEST256_LEN+1];
+    base16_encode(hex_digest2, HEX_DIGEST256_LEN+1,
+        (const char *)cons2_digests.sha3_256, DIGEST256_LEN);
+    base16_encode(e_hex_digest2, HEX_DIGEST256_LEN+1,
+        e_cons2_hash, DIGEST256_LEN);
+    log_warn(LD_CONSDIFF, "Expected: %s; found: %s",
+             hex_digest2, e_hex_digest2);
+    goto error_cleanup;
+  }
+
+  goto done;
+
+ error_cleanup:
+  tor_free(cons2_str); /* Sets it to NULL */
+
+ done:
+  if (cons2) {
+    smartlist_free(cons2);
+  }
+
+  return cons2_str;
+}
+
+/** Any consensus line longer than this means that the input is invalid. */
+#define CONSENSUS_LINE_MAX_LEN (1<<20)
+
+/**
+ * Helper: For every NL-terminated line in <b>s</b>, add a cdline referring to
+ * that line (without trailing newline) to <b>out</b>.  Return -1 if there are
+ * any non-NL terminated lines; 0 otherwise.
+ *
+ * Unlike tor_split_lines, this function avoids ambiguity on its
+ * handling of a final line that isn't NL-terminated.
+ *
+ * All cdline_t objects are allocated in the provided memarea.  Strings
+ * are not copied: if <b>s</b> changes or becomes invalid, then all
+ * generated cdlines will become invalid.
+ */
+STATIC int
+consensus_split_lines(smartlist_t *out, const char *s, memarea_t *area)
+{
+  while (*s) {
+    const char *eol = strchr(s, '\n');
+    if (!eol) {
+      /* File doesn't end with newline. */
+      return -1;
+    }
+    if (eol - s > CONSENSUS_LINE_MAX_LEN) {
+      /* Line is far too long. */
+      return -1;
+    }
+    cdline_t *line = memarea_alloc(area, sizeof(cdline_t));
+    line->s = s;
+    line->len = (uint32_t)(eol - s);
+    smartlist_add(out, line);
+    s = eol+1;
+  }
+  return 0;
+}
+
+/** Given a list of cdline_t, return a newly allocated string containing
+ * all of the lines, terminated with NL, concatenated.
+ *
+ * Unlike smartlist_join_strings(), avoids lossy operations on empty
+ * lists.  */
+static char *
+consensus_join_lines(const smartlist_t *inp)
+{
+  size_t n = 0;
+  SMARTLIST_FOREACH(inp, const cdline_t *, cdline, n += cdline->len + 1);
+  n += 1;
+  char *result = tor_malloc(n);
+  char *out = result;
+  SMARTLIST_FOREACH_BEGIN(inp, const cdline_t *, cdline) {
+    memcpy(out, cdline->s, cdline->len);
+    out += cdline->len;
+    *out++ = '\n';
+  } SMARTLIST_FOREACH_END(cdline);
+  *out++ = '\0';
+  tor_assert(out == result+n);
+  return result;
+}
+
+/** Given two consensus documents, try to compute a diff between them.  On
+ * success, retun a newly allocated string containing that diff.  On failure,
+ * return NULL. */
+char *
+consensus_diff_generate(const char *cons1,
+                        const char *cons2)
+{
+  consensus_digest_t d1, d2;
+  smartlist_t *lines1 = NULL, *lines2 = NULL, *result_lines = NULL;
+  int r1, r2;
+  char *result = NULL;
+
+  r1 = consensus_compute_digest(cons1, &d1);
+  r2 = consensus_compute_digest(cons2, &d2);
+  if (BUG(r1 < 0 || r2 < 0))
+    return NULL; // LCOV_EXCL_LINE
+
+  memarea_t *area = memarea_new();
+  lines1 = smartlist_new();
+  lines2 = smartlist_new();
+  if (consensus_split_lines(lines1, cons1, area) < 0)
+    goto done;
+  if (consensus_split_lines(lines2, cons2, area) < 0)
+    goto done;
+
+  result_lines = consdiff_gen_diff(lines1, lines2, &d1, &d2, area);
+
+ done:
+  if (result_lines) {
+    result = consensus_join_lines(result_lines);
+    smartlist_free(result_lines);
+  }
+
+  memarea_drop_all(area);
+  smartlist_free(lines1);
+  smartlist_free(lines2);
+
+  return result;
+}
+
+/** Given a consensus document and a diff, try to apply the diff to the
+ * consensus.  On success return a newly allocated string containing the new
+ * consensus.  On failure, return NULL. */
+char *
+consensus_diff_apply(const char *consensus,
+                     const char *diff)
+{
+  consensus_digest_t d1;
+  smartlist_t *lines1 = NULL, *lines2 = NULL;
+  int r1;
+  char *result = NULL;
+  memarea_t *area = memarea_new();
+
+  r1 = consensus_compute_digest(consensus, &d1);
+  if (BUG(r1 < 0))
+    return NULL; // LCOV_EXCL_LINE
+
+  lines1 = smartlist_new();
+  lines2 = smartlist_new();
+  if (consensus_split_lines(lines1, consensus, area) < 0)
+    goto done;
+  if (consensus_split_lines(lines2, diff, area) < 0)
+    goto done;
+
+  result = consdiff_apply_diff(lines1, lines2, &d1);
+
+ done:
+  smartlist_free(lines1);
+  smartlist_free(lines2);
+  memarea_drop_all(area);
+
+  return result;
+}
+
diff --git a/src/or/consdiff.h b/src/or/consdiff.h
new file mode 100644
index 0000000000..e9d175136e
--- /dev/null
+++ b/src/or/consdiff.h
@@ -0,0 +1,93 @@
+/* Copyright (c) 2014, Daniel Martí
+ * Copyright (c) 2014, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_CONSDIFF_H
+#define TOR_CONSDIFF_H
+
+#include "or.h"
+
+char *consensus_diff_generate(const char *cons1,
+                              const char *cons2);
+char *consensus_diff_apply(const char *consensus,
+                           const char *diff);
+
+#ifdef CONSDIFF_PRIVATE
+struct memarea_t;
+
+/** Line type used for constructing consensus diffs.  Each of these lines
+ * refers to a chunk of memory allocated elsewhere, and is not necessarily
+ * NUL-terminated: this helps us avoid copies and save memory. */
+typedef struct cdline_t {
+  const char *s;
+  uint32_t len;
+} cdline_t;
+
+typedef struct consensus_digest_t {
+  uint8_t sha3_256[DIGEST256_LEN];
+} consensus_digest_t;
+
+STATIC smartlist_t *consdiff_gen_diff(const smartlist_t *cons1,
+                                      const smartlist_t *cons2,
+                                      const consensus_digest_t *digests1,
+                                      const consensus_digest_t *digests2,
+                                      struct memarea_t *area);
+STATIC char *consdiff_apply_diff(const smartlist_t *cons1,
+                                 const smartlist_t *diff,
+                                 const consensus_digest_t *digests1);
+STATIC int consdiff_get_digests(const smartlist_t *diff,
+                                char *digest1_out,
+                                char *digest2_out);
+
+/** Data structure to define a slice of a smarltist. */
+typedef struct smartlist_slice_t {
+  /**
+   * Smartlist that this slice is made from.
+   * References the whole original smartlist that the slice was made out of.
+   * */
+  const smartlist_t *list;
+  /** Starting position of the slice in the smartlist. */
+  int offset;
+  /** Length of the slice, i.e. the number of elements it holds. */
+  int len;
+} smartlist_slice_t;
+STATIC smartlist_t *gen_ed_diff(const smartlist_t *cons1,
+                                const smartlist_t *cons2,
+                                struct memarea_t *area);
+STATIC smartlist_t *apply_ed_diff(const smartlist_t *cons1,
+                                  const smartlist_t *diff,
+                                  int start_line);
+STATIC void calc_changes(smartlist_slice_t *slice1, smartlist_slice_t *slice2,
+                         bitarray_t *changed1, bitarray_t *changed2);
+STATIC smartlist_slice_t *smartlist_slice(const smartlist_t *list,
+                                          int start, int end);
+STATIC int next_router(const smartlist_t *cons, int cur);
+STATIC int *lcs_lengths(const smartlist_slice_t *slice1,
+                        const smartlist_slice_t *slice2,
+                        int direction);
+STATIC void trim_slices(smartlist_slice_t *slice1, smartlist_slice_t *slice2);
+STATIC int base64cmp(const cdline_t *hash1, const cdline_t *hash2);
+STATIC int get_id_hash(const cdline_t *line, cdline_t *hash_out);
+STATIC int is_valid_router_entry(const cdline_t *line);
+STATIC int smartlist_slice_string_pos(const smartlist_slice_t *slice,
+                                      const cdline_t *string);
+STATIC void set_changed(bitarray_t *changed1, bitarray_t *changed2,
+                        const smartlist_slice_t *slice1,
+                        const smartlist_slice_t *slice2);
+STATIC int consensus_split_lines(smartlist_t *out, const char *s,
+                                 struct memarea_t *area);
+STATIC void smartlist_add_linecpy(smartlist_t *lst, struct memarea_t *area,
+                                  const char *s);
+STATIC int lines_eq(const cdline_t *a, const cdline_t *b);
+STATIC int line_str_eq(const cdline_t *a, const char *b);
+
+MOCK_DECL(STATIC int,
+          consensus_compute_digest,(const char *cons,
+                                    consensus_digest_t *digest_out));
+MOCK_DECL(STATIC int,
+          consensus_digest_eq,(const uint8_t *d1,
+                               const uint8_t *d2));
+#endif
+
+#endif
+
diff --git a/src/or/include.am b/src/or/include.am
index 4e54deca55..d65d0d3657 100644
--- a/src/or/include.am
+++ b/src/or/include.am
@@ -36,6 +36,7 @@ LIBTOR_A_SOURCES = \
 	src/or/connection.c				\
 	src/or/connection_edge.c			\
 	src/or/connection_or.c				\
+	src/or/consdiff.c				\
 	src/or/control.c				\
 	src/or/cpuworker.c				\
 	src/or/dircollate.c				\
@@ -151,6 +152,7 @@ ORHEADERS = \
 	src/or/connection.h				\
 	src/or/connection_edge.h			\
 	src/or/connection_or.h				\
+	src/or/consdiff.h				\
 	src/or/control.h				\
 	src/or/cpuworker.h				\
 	src/or/dircollate.h				\
diff --git a/src/test/Makefile.nmake b/src/test/Makefile.nmake
index 0ba56d7036..575198388b 100644
--- a/src/test/Makefile.nmake
+++ b/src/test/Makefile.nmake
@@ -12,7 +12,7 @@ LIBS = ..\..\..\build-alpha\lib\libevent.lib \
  crypt32.lib gdi32.lib user32.lib
 
 TEST_OBJECTS = test.obj test_addr.obj test_channel.obj test_channeltls.obj \
-        test_containers.obj \
+        test_consdiff.obj test_containers.obj \
 	test_controller_events.obj test_crypto.obj test_data.obj test_dir.obj \
 	test_checkdir.obj test_microdesc.obj test_pt.obj test_util.obj \
         test_config.obj test_connection.obj \
diff --git a/src/test/bench.c b/src/test/bench.c
index c3a9d1c4ba..5da9168cdc 100644
--- a/src/test/bench.c
+++ b/src/test/bench.c
@@ -28,6 +28,7 @@ const char tor_git_revision[] = "";
 #include "crypto_curve25519.h"
 #include "onion_ntor.h"
 #include "crypto_ed25519.h"
+#include "consdiff.h"
 
 #if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_PROCESS_CPUTIME_ID)
 static uint64_t nanostart;
@@ -674,6 +675,27 @@ main(int argc, const char **argv)
 
   tor_threads_init();
 
+  if (argc == 4 && !strcmp(argv[1], "diff")) {
+    init_logging(1);
+    const int N = 200;
+    char *f1 = read_file_to_str(argv[2], RFTS_BIN, NULL);
+    char *f2 = read_file_to_str(argv[3], RFTS_BIN, NULL);
+    if (! f1 || ! f2) {
+      perror("X");
+      return 1;
+    }
+    for (i = 0; i < N; ++i) {
+      char *diff = consensus_diff_generate(f1, f2);
+      tor_free(diff);
+    }
+    char *diff = consensus_diff_generate(f1, f2);
+    printf("%s", diff);
+    tor_free(f1);
+    tor_free(f2);
+    tor_free(diff);
+    return 0;
+  }
+
   for (i = 1; i < argc; ++i) {
     if (!strcmp(argv[i], "--list")) {
       list = 1;
diff --git a/src/test/fuzz/fuzz_diff.c b/src/test/fuzz/fuzz_diff.c
new file mode 100644
index 0000000000..c241f63dc4
--- /dev/null
+++ b/src/test/fuzz/fuzz_diff.c
@@ -0,0 +1,67 @@
+/* Copyright (c) 2016, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#define CONSDIFF_PRIVATE
+
+#include "orconfig.h"
+#include "or.h"
+#include "consdiff.h"
+
+#include "fuzzing.h"
+
+static int
+mock_consensus_compute_digest_(const char *c, consensus_digest_t *d)
+{
+  (void)c;
+  memset(d->sha3_256, 3, sizeof(d->sha3_256));
+  return 0;
+}
+
+int
+fuzz_init(void)
+{
+  MOCK(consensus_compute_digest, mock_consensus_compute_digest_);
+  return 0;
+}
+
+int
+fuzz_cleanup(void)
+{
+  UNMOCK(consensus_compute_digest);
+  return 0;
+}
+
+int
+fuzz_main(const uint8_t *stdin_buf, size_t data_size)
+{
+#define SEP "=====\n"
+#define SEPLEN strlen(SEP)
+  const uint8_t *separator = tor_memmem(stdin_buf, data_size, SEP, SEPLEN);
+  if (! separator)
+    return 0;
+  size_t c1_len = separator - stdin_buf;
+  char *c1 = tor_memdup_nulterm(stdin_buf, c1_len);
+  size_t c2_len = data_size - c1_len - SEPLEN;
+  char *c2 = tor_memdup_nulterm(separator + SEPLEN, c2_len);
+
+  char *c3 = consensus_diff_generate(c1, c2);
+
+  if (c3) {
+    char *c4 = consensus_diff_apply(c1, c3);
+    tor_assert(c4);
+    if (strcmp(c2, c4)) {
+      printf("%s\n", escaped(c1));
+      printf("%s\n", escaped(c2));
+      printf("%s\n", escaped(c3));
+      printf("%s\n", escaped(c4));
+    }
+    tor_assert(! strcmp(c2, c4));
+    tor_free(c3);
+    tor_free(c4);
+  }
+  tor_free(c1);
+  tor_free(c2);
+
+  return 0;
+}
+
diff --git a/src/test/fuzz/fuzz_diff_apply.c b/src/test/fuzz/fuzz_diff_apply.c
new file mode 100644
index 0000000000..8d7bf751bf
--- /dev/null
+++ b/src/test/fuzz/fuzz_diff_apply.c
@@ -0,0 +1,65 @@
+/* Copyright (c) 2016, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#define CONSDIFF_PRIVATE
+
+#include "orconfig.h"
+#include "or.h"
+#include "consdiff.h"
+
+#include "fuzzing.h"
+
+static int
+mock_consensus_compute_digest_(const char *c, consensus_digest_t *d)
+{
+  (void)c;
+  memset(d->sha3_256, 3, sizeof(d->sha3_256));
+  return 0;
+}
+
+static int
+mock_consensus_digest_eq_(const uint8_t *a, const uint8_t *b)
+{
+  (void)a;
+  (void)b;
+  return 1;
+}
+
+int
+fuzz_init(void)
+{
+  MOCK(consensus_compute_digest, mock_consensus_compute_digest_);
+  MOCK(consensus_digest_eq, mock_consensus_digest_eq_);
+  return 0;
+}
+
+int
+fuzz_cleanup(void)
+{
+  UNMOCK(consensus_compute_digest);
+  UNMOCK(consensus_digest_eq);
+  return 0;
+}
+
+int
+fuzz_main(const uint8_t *stdin_buf, size_t data_size)
+{
+#define SEP "=====\n"
+#define SEPLEN strlen(SEP)
+  const uint8_t *separator = tor_memmem(stdin_buf, data_size, SEP, SEPLEN);
+  if (! separator)
+    return 0;
+  size_t c1_len = separator - stdin_buf;
+  char *c1 = tor_memdup_nulterm(stdin_buf, c1_len);
+  size_t c2_len = data_size - c1_len - SEPLEN;
+  char *c2 = tor_memdup_nulterm(separator + SEPLEN, c2_len);
+
+  char *c3 = consensus_diff_apply(c1, c2);
+
+  tor_free(c1);
+  tor_free(c2);
+  tor_free(c3);
+
+  return 0;
+}
+
diff --git a/src/test/fuzz/include.am b/src/test/fuzz/include.am
index 806710879b..97afd18215 100644
--- a/src/test/fuzz/include.am
+++ b/src/test/fuzz/include.am
@@ -48,6 +48,22 @@ src_test_fuzz_fuzz_descriptor_CFLAGS = $(FUZZING_CFLAGS)
 src_test_fuzz_fuzz_descriptor_LDFLAGS = $(FUZZING_LDFLAG)
 src_test_fuzz_fuzz_descriptor_LDADD = $(FUZZING_LIBS)
 
+src_test_fuzz_fuzz_diff_SOURCES = \
+	src/test/fuzz/fuzzing_common.c \
+	src/test/fuzz/fuzz_diff.c
+src_test_fuzz_fuzz_diff_CPPFLAGS = $(FUZZING_CPPFLAGS)
+src_test_fuzz_fuzz_diff_CFLAGS = $(FUZZING_CFLAGS)
+src_test_fuzz_fuzz_diff_LDFLAGS = $(FUZZING_LDFLAG)
+src_test_fuzz_fuzz_diff_LDADD = $(FUZZING_LIBS)
+
+src_test_fuzz_fuzz_diff_apply_SOURCES = \
+	src/test/fuzz/fuzzing_common.c \
+	src/test/fuzz/fuzz_diff_apply.c
+src_test_fuzz_fuzz_diff_apply_CPPFLAGS = $(FUZZING_CPPFLAGS)
+src_test_fuzz_fuzz_diff_apply_CFLAGS = $(FUZZING_CFLAGS)
+src_test_fuzz_fuzz_diff_apply_LDFLAGS = $(FUZZING_LDFLAG)
+src_test_fuzz_fuzz_diff_apply_LDADD = $(FUZZING_LIBS)
+
 src_test_fuzz_fuzz_http_SOURCES = \
 	src/test/fuzz/fuzzing_common.c \
 	src/test/fuzz/fuzz_http.c
@@ -99,6 +115,8 @@ src_test_fuzz_fuzz_vrs_LDADD = $(FUZZING_LIBS)
 FUZZERS = \
 	src/test/fuzz/fuzz-consensus \
 	src/test/fuzz/fuzz-descriptor \
+	src/test/fuzz/fuzz-diff \
+	src/test/fuzz/fuzz-diff-apply \
 	src/test/fuzz/fuzz-extrainfo \
 	src/test/fuzz/fuzz-http \
 	src/test/fuzz/fuzz-hsdescv2 \
@@ -106,7 +124,6 @@ FUZZERS = \
 	src/test/fuzz/fuzz-microdesc \
 	src/test/fuzz/fuzz-vrs
 
-
 LIBFUZZER = /home/nickm/build/libfuzz/libFuzzer.a
 LIBFUZZER_CPPFLAGS = $(FUZZING_CPPFLAGS) -DLLVM_FUZZ
 LIBFUZZER_CFLAGS = $(FUZZING_CFLAGS)
@@ -128,6 +145,20 @@ src_test_fuzz_lf_fuzz_descriptor_CFLAGS = $(LIBFUZZER_CFLAGS)
 src_test_fuzz_lf_fuzz_descriptor_LDFLAGS = $(LIBFUZZER_LDFLAG)
 src_test_fuzz_lf_fuzz_descriptor_LDADD = $(LIBFUZZER_LIBS)
 
+src_test_fuzz_lf_fuzz_diff_SOURCES = \
+	$(src_test_fuzz_fuzz_diff_SOURCES)
+src_test_fuzz_lf_fuzz_diff_CPPFLAGS = $(LIBFUZZER_CPPFLAGS)
+src_test_fuzz_lf_fuzz_diff_CFLAGS = $(LIBFUZZER_CFLAGS)
+src_test_fuzz_lf_fuzz_diff_LDFLAGS = $(LIBFUZZER_LDFLAG)
+src_test_fuzz_lf_fuzz_diff_LDADD = $(LIBFUZZER_LIBS)
+
+src_test_fuzz_lf_fuzz_diff_apply_SOURCES = \
+	$(src_test_fuzz_fuzz_diff_apply_SOURCES)
+src_test_fuzz_lf_fuzz_diff_apply_CPPFLAGS = $(LIBFUZZER_CPPFLAGS)
+src_test_fuzz_lf_fuzz_diff_apply_CFLAGS = $(LIBFUZZER_CFLAGS)
+src_test_fuzz_lf_fuzz_diff_apply_LDFLAGS = $(LIBFUZZER_LDFLAG)
+src_test_fuzz_lf_fuzz_diff_apply_LDADD = $(LIBFUZZER_LIBS)
+
 src_test_fuzz_lf_fuzz_extrainfo_SOURCES = \
 	$(src_test_fuzz_fuzz_extrainfo_SOURCES)
 src_test_fuzz_lf_fuzz_extrainfo_CPPFLAGS = $(LIBFUZZER_CPPFLAGS)
@@ -172,6 +203,8 @@ src_test_fuzz_lf_fuzz_vrs_LDADD = $(LIBFUZZER_LIBS)
 
 LIBFUZZER_FUZZERS = \
 	src/test/fuzz/lf-fuzz-consensus \
+	src/test/fuzz/lf-fuzz-diff \
+	src/test/fuzz/lf-fuzz-diff-apply \
 	src/test/fuzz/lf-fuzz-descriptor \
 	src/test/fuzz/lf-fuzz-extrainfo \
 	src/test/fuzz/lf-fuzz-http \
@@ -198,6 +231,16 @@ src_test_fuzz_liboss_fuzz_descriptor_a_SOURCES = \
 src_test_fuzz_liboss_fuzz_descriptor_a_CPPFLAGS = $(LIBOSS_FUZZ_CPPFLAGS)
 src_test_fuzz_liboss_fuzz_descriptor_a_CFLAGS = $(LIBOSS_FUZZ_CFLAGS)
 
+src_test_fuzz_liboss_fuzz_diff_a_SOURCES = \
+	$(src_test_fuzz_fuzz_diff_SOURCES)
+src_test_fuzz_liboss_fuzz_diff_a_CPPFLAGS = $(LIBOSS_FUZZ_CPPFLAGS)
+src_test_fuzz_liboss_fuzz_diff_a_CFLAGS = $(LIBOSS_FUZZ_CFLAGS)
+
+src_test_fuzz_liboss_fuzz_diff_apply_a_SOURCES = \
+	$(src_test_fuzz_fuzz_diff_apply_SOURCES)
+src_test_fuzz_liboss_fuzz_diff_apply_a_CPPFLAGS = $(LIBOSS_FUZZ_CPPFLAGS)
+src_test_fuzz_liboss_fuzz_diff_apply_a_CFLAGS = $(LIBOSS_FUZZ_CFLAGS)
+
 src_test_fuzz_liboss_fuzz_extrainfo_a_SOURCES = \
 	$(src_test_fuzz_fuzz_extrainfo_SOURCES)
 src_test_fuzz_liboss_fuzz_extrainfo_a_CPPFLAGS = $(LIBOSS_FUZZ_CPPFLAGS)
@@ -231,6 +274,8 @@ src_test_fuzz_liboss_fuzz_vrs_a_CFLAGS = $(LIBOSS_FUZZ_CFLAGS)
 OSS_FUZZ_FUZZERS = \
 	src/test/fuzz/liboss-fuzz-consensus.a \
 	src/test/fuzz/liboss-fuzz-descriptor.a \
+	src/test/fuzz/liboss-fuzz-diff.a \
+	src/test/fuzz/liboss-fuzz-diff-apply.a \
 	src/test/fuzz/liboss-fuzz-extrainfo.a \
 	src/test/fuzz/liboss-fuzz-http.a \
 	src/test/fuzz/liboss-fuzz-hsdescv2.a \
diff --git a/src/test/include.am b/src/test/include.am
index 1c0726fd3a..23be3e8334 100644
--- a/src/test/include.am
+++ b/src/test/include.am
@@ -86,6 +86,7 @@ src_test_test_SOURCES = \
 	src/test/test_compat_libevent.c \
 	src/test/test_config.c \
 	src/test/test_connection.c \
+	src/test/test_consdiff.c \
 	src/test/test_containers.c \
 	src/test/test_controller.c \
 	src/test/test_controller_events.c \
diff --git a/src/test/test.c b/src/test/test.c
index 1f91405a61..0039818cb8 100644
--- a/src/test/test.c
+++ b/src/test/test.c
@@ -1194,6 +1194,7 @@ struct testgroup_t testgroups[] = {
   { "compat/libevent/", compat_libevent_tests },
   { "config/", config_tests },
   { "connection/", connection_tests },
+  { "consdiff/", consdiff_tests },
   { "container/", container_tests },
   { "control/", controller_tests },
   { "control/event/", controller_event_tests },
diff --git a/src/test/test.h b/src/test/test.h
index c78e272178..0cf56ab74a 100644
--- a/src/test/test.h
+++ b/src/test/test.h
@@ -189,6 +189,7 @@ extern struct testcase_t circuituse_tests[];
 extern struct testcase_t compat_libevent_tests[];
 extern struct testcase_t config_tests[];
 extern struct testcase_t connection_tests[];
+extern struct testcase_t consdiff_tests[];
 extern struct testcase_t container_tests[];
 extern struct testcase_t controller_tests[];
 extern struct testcase_t controller_event_tests[];
diff --git a/src/test/test_consdiff.c b/src/test/test_consdiff.c
new file mode 100644
index 0000000000..1b4e2ad3c1
--- /dev/null
+++ b/src/test/test_consdiff.c
@@ -0,0 +1,1097 @@
+/* Copyright (c) 2014, Daniel Martí
+ * Copyright (c) 2014, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#define CONSDIFF_PRIVATE
+
+#include "or.h"
+#include "test.h"
+
+#include "consdiff.h"
+#include "memarea.h"
+#include "log_test_helpers.h"
+
+#define tt_str_eq_line(a,b) \
+  tt_assert(line_str_eq((b),(a)))
+
+static void
+test_consdiff_smartlist_slice(void *arg)
+{
+  smartlist_t *sl = smartlist_new();
+  smartlist_slice_t *sls;
+
+  /* Create a regular smartlist. */
+  (void)arg;
+  smartlist_add(sl, (void*)1);
+  smartlist_add(sl, (void*)2);
+  smartlist_add(sl, (void*)3);
+  smartlist_add(sl, (void*)4);
+  smartlist_add(sl, (void*)5);
+
+  /* See if the slice was done correctly. */
+  sls = smartlist_slice(sl, 2, 5);
+  tt_ptr_op(sl, OP_EQ, sls->list);
+  tt_ptr_op((void*)3, OP_EQ, smartlist_get(sls->list, sls->offset));
+  tt_ptr_op((void*)5, OP_EQ,
+      smartlist_get(sls->list, sls->offset + (sls->len-1)));
+  tor_free(sls);
+
+  /* See that using -1 as the end does get to the last element. */
+  sls = smartlist_slice(sl, 2, -1);
+  tt_ptr_op(sl, OP_EQ, sls->list);
+  tt_ptr_op((void*)3, OP_EQ, smartlist_get(sls->list, sls->offset));
+  tt_ptr_op((void*)5, OP_EQ,
+      smartlist_get(sls->list, sls->offset + (sls->len-1)));
+
+ done:
+  tor_free(sls);
+  smartlist_free(sl);
+}
+
+static void
+test_consdiff_smartlist_slice_string_pos(void *arg)
+{
+  smartlist_t *sl = smartlist_new();
+  smartlist_slice_t *sls;
+  memarea_t *area = memarea_new();
+
+  /* Create a regular smartlist. */
+  (void)arg;
+  consensus_split_lines(sl, "a\nd\nc\na\nb\n", area);
+
+  /* See that smartlist_slice_string_pos respects the bounds of the slice. */
+  sls = smartlist_slice(sl, 2, 5);
+  cdline_t a_line = { "a", 1 };
+  tt_int_op(3, OP_EQ, smartlist_slice_string_pos(sls, &a_line));
+  cdline_t d_line = { "d", 1 };
+  tt_int_op(-1, OP_EQ, smartlist_slice_string_pos(sls, &d_line));
+
+ done:
+  tor_free(sls);
+  smartlist_free(sl);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_lcs_lengths(void *arg)
+{
+  smartlist_t *sl1 = smartlist_new();
+  smartlist_t *sl2 = smartlist_new();
+  smartlist_slice_t *sls1, *sls2;
+  int *lengths1, *lengths2;
+  memarea_t *area = memarea_new();
+
+  /* Expected lcs lengths in regular and reverse order. */
+  int e_lengths1[] = { 0, 1, 2, 3, 3, 4 };
+  int e_lengths2[] = { 0, 1, 1, 2, 3, 4 };
+
+  (void)arg;
+  consensus_split_lines(sl1, "a\nb\nc\nd\ne\n", area);
+  consensus_split_lines(sl2, "a\nc\nd\ni\ne\n", area);
+
+  sls1 = smartlist_slice(sl1, 0, -1);
+  sls2 = smartlist_slice(sl2, 0, -1);
+
+  lengths1 = lcs_lengths(sls1, sls2, 1);
+  lengths2 = lcs_lengths(sls1, sls2, -1);
+  tt_mem_op(e_lengths1, OP_EQ, lengths1, sizeof(int) * 6);
+  tt_mem_op(e_lengths2, OP_EQ, lengths2, sizeof(int) * 6);
+
+ done:
+  tor_free(lengths1);
+  tor_free(lengths2);
+  tor_free(sls1);
+  tor_free(sls2);
+  smartlist_free(sl1);
+  smartlist_free(sl2);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_trim_slices(void *arg)
+{
+  smartlist_t *sl1 = smartlist_new();
+  smartlist_t *sl2 = smartlist_new();
+  smartlist_t *sl3 = smartlist_new();
+  smartlist_t *sl4 = smartlist_new();
+  smartlist_slice_t *sls1, *sls2, *sls3, *sls4;
+  memarea_t *area = memarea_new();
+
+  (void)arg;
+  consensus_split_lines(sl1, "a\nb\nb\nb\nd\n", area);
+  consensus_split_lines(sl2, "a\nc\nc\nc\nd\n", area);
+  consensus_split_lines(sl3, "a\nb\nb\nb\na\n", area);
+  consensus_split_lines(sl4, "c\nb\nb\nb\nc\n", area);
+  sls1 = smartlist_slice(sl1, 0, -1);
+  sls2 = smartlist_slice(sl2, 0, -1);
+  sls3 = smartlist_slice(sl3, 0, -1);
+  sls4 = smartlist_slice(sl4, 0, -1);
+
+  /* They should be trimmed by one line at each end. */
+  tt_int_op(5, OP_EQ, sls1->len);
+  tt_int_op(5, OP_EQ, sls2->len);
+  trim_slices(sls1, sls2);
+  tt_int_op(3, OP_EQ, sls1->len);
+  tt_int_op(3, OP_EQ, sls2->len);
+
+  /* They should not be trimmed at all. */
+  tt_int_op(5, OP_EQ, sls3->len);
+  tt_int_op(5, OP_EQ, sls4->len);
+  trim_slices(sls3, sls4);
+  tt_int_op(5, OP_EQ, sls3->len);
+  tt_int_op(5, OP_EQ, sls4->len);
+
+ done:
+  tor_free(sls1);
+  tor_free(sls2);
+  tor_free(sls3);
+  tor_free(sls4);
+  smartlist_free(sl1);
+  smartlist_free(sl2);
+  smartlist_free(sl3);
+  smartlist_free(sl4);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_set_changed(void *arg)
+{
+  smartlist_t *sl1 = smartlist_new();
+  smartlist_t *sl2 = smartlist_new();
+  bitarray_t *changed1 = bitarray_init_zero(4);
+  bitarray_t *changed2 = bitarray_init_zero(4);
+  smartlist_slice_t *sls1, *sls2;
+  memarea_t *area = memarea_new();
+
+  (void)arg;
+  consensus_split_lines(sl1, "a\nb\na\na\n", area);
+  consensus_split_lines(sl2, "a\na\na\na\n", area);
+
+  /* Length of sls1 is 0. */
+  sls1 = smartlist_slice(sl1, 0, 0);
+  sls2 = smartlist_slice(sl2, 1, 3);
+  set_changed(changed1, changed2, sls1, sls2);
+
+  /* The former is not changed, the latter changes all of its elements. */
+  tt_assert(!bitarray_is_set(changed1, 0));
+  tt_assert(!bitarray_is_set(changed1, 1));
+  tt_assert(!bitarray_is_set(changed1, 2));
+  tt_assert(!bitarray_is_set(changed1, 3));
+
+  tt_assert(!bitarray_is_set(changed2, 0));
+  tt_assert(bitarray_is_set(changed2, 1));
+  tt_assert(bitarray_is_set(changed2, 2));
+  tt_assert(!bitarray_is_set(changed2, 3));
+  bitarray_clear(changed2, 1);
+  bitarray_clear(changed2, 2);
+
+  /* Length of sls1 is 1 and its element is in sls2. */
+  tor_free(sls1);
+  sls1 = smartlist_slice(sl1, 0, 1);
+  set_changed(changed1, changed2, sls1, sls2);
+
+  /* The latter changes all elements but the (first) common one. */
+  tt_assert(!bitarray_is_set(changed1, 0));
+  tt_assert(!bitarray_is_set(changed1, 1));
+  tt_assert(!bitarray_is_set(changed1, 2));
+  tt_assert(!bitarray_is_set(changed1, 3));
+
+  tt_assert(!bitarray_is_set(changed2, 0));
+  tt_assert(!bitarray_is_set(changed2, 1));
+  tt_assert(bitarray_is_set(changed2, 2));
+  tt_assert(!bitarray_is_set(changed2, 3));
+  bitarray_clear(changed2, 2);
+
+  /* Length of sls1 is 1 and its element is not in sls2. */
+  tor_free(sls1);
+  sls1 = smartlist_slice(sl1, 1, 2);
+  set_changed(changed1, changed2, sls1, sls2);
+
+  /* The former changes its element, the latter changes all elements. */
+  tt_assert(!bitarray_is_set(changed1, 0));
+  tt_assert(bitarray_is_set(changed1, 1));
+  tt_assert(!bitarray_is_set(changed1, 2));
+  tt_assert(!bitarray_is_set(changed1, 3));
+
+  tt_assert(!bitarray_is_set(changed2, 0));
+  tt_assert(bitarray_is_set(changed2, 1));
+  tt_assert(bitarray_is_set(changed2, 2));
+  tt_assert(!bitarray_is_set(changed2, 3));
+
+ done:
+  bitarray_free(changed1);
+  bitarray_free(changed2);
+  smartlist_free(sl1);
+  smartlist_free(sl2);
+  tor_free(sls1);
+  tor_free(sls2);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_calc_changes(void *arg)
+{
+  smartlist_t *sl1 = smartlist_new();
+  smartlist_t *sl2 = smartlist_new();
+  smartlist_slice_t *sls1, *sls2;
+  bitarray_t *changed1 = bitarray_init_zero(4);
+  bitarray_t *changed2 = bitarray_init_zero(4);
+  memarea_t *area = memarea_new();
+
+  (void)arg;
+  consensus_split_lines(sl1, "a\na\na\na\n", area);
+  consensus_split_lines(sl2, "a\na\na\na\n", area);
+
+  sls1 = smartlist_slice(sl1, 0, -1);
+  sls2 = smartlist_slice(sl2, 0, -1);
+  calc_changes(sls1, sls2, changed1, changed2);
+
+  /* Nothing should be set to changed. */
+  tt_assert(!bitarray_is_set(changed1, 0));
+  tt_assert(!bitarray_is_set(changed1, 1));
+  tt_assert(!bitarray_is_set(changed1, 2));
+  tt_assert(!bitarray_is_set(changed1, 3));
+
+  tt_assert(!bitarray_is_set(changed2, 0));
+  tt_assert(!bitarray_is_set(changed2, 1));
+  tt_assert(!bitarray_is_set(changed2, 2));
+  tt_assert(!bitarray_is_set(changed2, 3));
+
+  smartlist_clear(sl2);
+  consensus_split_lines(sl2, "a\nb\na\nb\n", area);
+  tor_free(sls1);
+  tor_free(sls2);
+  sls1 = smartlist_slice(sl1, 0, -1);
+  sls2 = smartlist_slice(sl2, 0, -1);
+  calc_changes(sls1, sls2, changed1, changed2);
+
+  /* Two elements are changed. */
+  tt_assert(!bitarray_is_set(changed1, 0));
+  tt_assert(bitarray_is_set(changed1, 1));
+  tt_assert(bitarray_is_set(changed1, 2));
+  tt_assert(!bitarray_is_set(changed1, 3));
+  bitarray_clear(changed1, 1);
+  bitarray_clear(changed1, 2);
+
+  tt_assert(!bitarray_is_set(changed2, 0));
+  tt_assert(bitarray_is_set(changed2, 1));
+  tt_assert(!bitarray_is_set(changed2, 2));
+  tt_assert(bitarray_is_set(changed2, 3));
+  bitarray_clear(changed1, 1);
+  bitarray_clear(changed1, 3);
+
+  smartlist_clear(sl2);
+  consensus_split_lines(sl2, "b\nb\nb\nb\n", area);
+  tor_free(sls1);
+  tor_free(sls2);
+  sls1 = smartlist_slice(sl1, 0, -1);
+  sls2 = smartlist_slice(sl2, 0, -1);
+  calc_changes(sls1, sls2, changed1, changed2);
+
+  /* All elements are changed. */
+  tt_assert(bitarray_is_set(changed1, 0));
+  tt_assert(bitarray_is_set(changed1, 1));
+  tt_assert(bitarray_is_set(changed1, 2));
+  tt_assert(bitarray_is_set(changed1, 3));
+
+  tt_assert(bitarray_is_set(changed2, 0));
+  tt_assert(bitarray_is_set(changed2, 1));
+  tt_assert(bitarray_is_set(changed2, 2));
+  tt_assert(bitarray_is_set(changed2, 3));
+
+ done:
+  bitarray_free(changed1);
+  bitarray_free(changed2);
+  smartlist_free(sl1);
+  smartlist_free(sl2);
+  tor_free(sls1);
+  tor_free(sls2);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_get_id_hash(void *arg)
+{
+  (void)arg;
+
+  cdline_t line1 = { "r name", 6 };
+  cdline_t line2 = { "r name _hash_isnt_base64 etc", 28 };
+  cdline_t line3 = { "r name hash+valid+base64 etc", 28 };
+  cdline_t tmp;
+
+  /* No hash. */
+  tt_int_op(-1, OP_EQ, get_id_hash(&line1, &tmp));
+  /* The hash contains characters that are not base64. */
+  tt_int_op(-1, OP_EQ, get_id_hash(&line2, &tmp));
+
+  /* valid hash. */
+  tt_int_op(0, OP_EQ, get_id_hash(&line3, &tmp));
+  tt_ptr_op(tmp.s, OP_EQ, line3.s + 7);
+  tt_uint_op(tmp.len, OP_EQ, line3.len - 11);
+
+ done:
+  ;
+}
+
+static void
+test_consdiff_is_valid_router_entry(void *arg)
+{
+  /* Doesn't start with "r ". */
+  (void)arg;
+  cdline_t line0 = { "foo", 3 };
+  tt_int_op(0, OP_EQ, is_valid_router_entry(&line0));
+
+  /* These are already tested with get_id_hash, but make sure it's run
+   * properly. */
+
+  cdline_t line1 = { "r name", 6 };
+  cdline_t line2 = { "r name _hash_isnt_base64 etc", 28 };
+  cdline_t line3 = { "r name hash+valid+base64 etc", 28 };
+  tt_int_op(0, OP_EQ, is_valid_router_entry(&line1));
+  tt_int_op(0, OP_EQ, is_valid_router_entry(&line2));
+  tt_int_op(1, OP_EQ, is_valid_router_entry(&line3));
+
+ done:
+  ;
+}
+
+static void
+test_consdiff_next_router(void *arg)
+{
+  smartlist_t *sl = smartlist_new();
+  memarea_t *area = memarea_new();
+  (void)arg;
+  smartlist_add_linecpy(sl, area, "foo");
+  smartlist_add_linecpy(sl, area,
+      "r name hash+longer+than+27+chars+and+valid+base64 etc");
+  smartlist_add_linecpy(sl, area, "foo");
+  smartlist_add_linecpy(sl, area, "foo");
+  smartlist_add_linecpy(sl, area,
+      "r name hash+longer+than+27+chars+and+valid+base64 etc");
+  smartlist_add_linecpy(sl, area, "foo");
+
+  /* Not currently on a router entry line, finding the next one. */
+  tt_int_op(1, OP_EQ, next_router(sl, 0));
+  tt_int_op(4, OP_EQ, next_router(sl, 2));
+
+  /* Already at the beginning of a router entry line, ignore it. */
+  tt_int_op(4, OP_EQ, next_router(sl, 1));
+
+  /* There are no more router entries, so return the line after the last. */
+  tt_int_op(6, OP_EQ, next_router(sl, 4));
+  tt_int_op(6, OP_EQ, next_router(sl, 5));
+
+ done:
+  smartlist_free(sl);
+  memarea_drop_all(area);
+}
+
+static int
+base64cmp_wrapper(const char *a, const char *b)
+{
+  cdline_t aa = { a, a ? (uint32_t) strlen(a) : 0 };
+  cdline_t bb = { b, b ? (uint32_t) strlen(b) : 0 };
+  return base64cmp(&aa, &bb);
+}
+
+static void
+test_consdiff_base64cmp(void *arg)
+{
+  /* NULL arguments. */
+  (void)arg;
+  tt_int_op(0, OP_EQ, base64cmp_wrapper(NULL, NULL));
+  tt_int_op(-1, OP_EQ, base64cmp_wrapper(NULL, "foo"));
+  tt_int_op(1, OP_EQ, base64cmp_wrapper("bar", NULL));
+
+  /* Nil base64 values. */
+  tt_int_op(0, OP_EQ, base64cmp_wrapper("", ""));
+  tt_int_op(0, OP_EQ, base64cmp_wrapper("_", "&"));
+
+  /* Exact same valid strings. */
+  tt_int_op(0, OP_EQ, base64cmp_wrapper("abcABC/+", "abcABC/+"));
+  /* Both end with an invalid base64 char other than '\0'. */
+  tt_int_op(0, OP_EQ, base64cmp_wrapper("abcABC/+ ", "abcABC/+ "));
+  /* Only one ends with an invalid base64 char other than '\0'. */
+  tt_int_op(-1, OP_EQ, base64cmp_wrapper("abcABC/+ ", "abcABC/+a"));
+
+  /* Comparisons that would return differently with strcmp(). */
+  tt_int_op(-1, OP_EQ, strcmp("/foo", "Afoo"));
+  tt_int_op(1, OP_EQ, base64cmp_wrapper("/foo", "Afoo"));
+  tt_int_op(1, OP_EQ, strcmp("Afoo", "0foo"));
+  tt_int_op(-1, OP_EQ, base64cmp_wrapper("Afoo", "0foo"));
+
+  /* Comparisons that would return the same as with strcmp(). */
+  tt_int_op(1, OP_EQ, strcmp("afoo", "Afoo"));
+  tt_int_op(1, OP_EQ, base64cmp_wrapper("afoo", "Afoo"));
+
+  /* Different lengths */
+  tt_int_op(-1, OP_EQ, base64cmp_wrapper("afoo", "afooo"));
+  tt_int_op(1, OP_EQ, base64cmp_wrapper("afooo", "afoo"));
+
+ done:
+  ;
+}
+
+static void
+test_consdiff_gen_ed_diff(void *arg)
+{
+  smartlist_t *cons1=NULL, *cons2=NULL, *diff=NULL;
+  int i;
+  memarea_t *area = memarea_new();
+  setup_capture_of_logs(LOG_WARN);
+
+  (void)arg;
+  cons1 = smartlist_new();
+  cons2 = smartlist_new();
+
+  /* Identity hashes are not sorted properly, return NULL. */
+  smartlist_add_linecpy(cons1, area, "r name bbbbbbbbbbbbbbbbbbbbbbbbbbb etc");
+  smartlist_add_linecpy(cons1, area, "foo");
+  smartlist_add_linecpy(cons1, area, "r name aaaaaaaaaaaaaaaaaaaaaaaaaaa etc");
+  smartlist_add_linecpy(cons1, area, "bar");
+
+  smartlist_add_linecpy(cons2, area, "r name aaaaaaaaaaaaaaaaaaaaaaaaaaa etc");
+  smartlist_add_linecpy(cons2, area, "foo");
+  smartlist_add_linecpy(cons2, area, "r name ccccccccccccccccccccccccccc etc");
+  smartlist_add_linecpy(cons2, area, "bar");
+
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because the base consensus doesn't have its router entries sorted "
+         "properly.");
+
+  /* Same, but now with the second consensus. */
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons2, cons1, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because the target consensus doesn't have its router entries sorted "
+         "properly.");
+
+  /* Same as the two above, but with the reversed thing immediately after a
+     match. (The code handles this differently) */
+  smartlist_del(cons1, 0);
+  smartlist_add_linecpy(cons1, area, "r name aaaaaaaaaaaaaaaaaaaaaaaaaaa etc");
+
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because the base consensus doesn't have its router entries sorted "
+         "properly.");
+
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons2, cons1, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because the target consensus doesn't have its router entries sorted "
+         "properly.");
+
+  /* Identity hashes are repeated, return NULL. */
+  smartlist_clear(cons1);
+
+  smartlist_add_linecpy(cons1, area, "r name bbbbbbbbbbbbbbbbbbbbbbbbbbb etc");
+  smartlist_add_linecpy(cons1, area, "foo");
+  smartlist_add_linecpy(cons1, area, "r name bbbbbbbbbbbbbbbbbbbbbbbbbbb etc");
+  smartlist_add_linecpy(cons1, area, "bar");
+
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because the base consensus doesn't have its router entries sorted "
+         "properly.");
+
+  /* We have to add a line that is just a dot, return NULL. */
+  smartlist_clear(cons1);
+  smartlist_clear(cons2);
+
+  smartlist_add_linecpy(cons1, area, "foo1");
+  smartlist_add_linecpy(cons1, area, "foo2");
+
+  smartlist_add_linecpy(cons2, area, "foo1");
+  smartlist_add_linecpy(cons2, area, ".");
+  smartlist_add_linecpy(cons2, area, "foo2");
+
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Cannot generate consensus diff "
+         "because one of the lines to be added is \".\".");
+
+#define MAX_LINE_COUNT (10000)
+  /* Too many lines to be fed to the quadratic-time function. */
+  smartlist_clear(cons1);
+  smartlist_clear(cons2);
+
+  for (i=0; i < MAX_LINE_COUNT; ++i) smartlist_add_linecpy(cons1, area, "a");
+  for (i=0; i < MAX_LINE_COUNT; ++i) smartlist_add_linecpy(cons1, area, "b");
+
+  mock_clean_saved_logs();
+  diff = gen_ed_diff(cons1, cons2, area);
+
+  tt_ptr_op(NULL, OP_EQ, diff);
+  expect_single_log_msg_containing("Refusing to generate consensus diff "
+         "because we found too few common router ids.");
+
+  /* We have dot lines, but they don't interfere with the script format. */
+  smartlist_clear(cons1);
+  smartlist_clear(cons2);
+
+  smartlist_add_linecpy(cons1, area, "foo1");
+  smartlist_add_linecpy(cons1, area, ".");
+  smartlist_add_linecpy(cons1, area, ".");
+  smartlist_add_linecpy(cons1, area, "foo2");
+
+  smartlist_add_linecpy(cons2, area, "foo1");
+  smartlist_add_linecpy(cons2, area, ".");
+  smartlist_add_linecpy(cons2, area, "foo2");
+
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  smartlist_free(diff);
+
+  /* Empty diff tests. */
+  smartlist_clear(cons1);
+  smartlist_clear(cons2);
+
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(0, OP_EQ, smartlist_len(diff));
+  smartlist_free(diff);
+
+  smartlist_add_linecpy(cons1, area, "foo");
+  smartlist_add_linecpy(cons1, area, "bar");
+
+  smartlist_add_linecpy(cons2, area, "foo");
+  smartlist_add_linecpy(cons2, area, "bar");
+
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(0, OP_EQ, smartlist_len(diff));
+  smartlist_free(diff);
+
+  /* Everything is deleted. */
+  smartlist_clear(cons2);
+
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(1, OP_EQ, smartlist_len(diff));
+  tt_str_eq_line("1,2d", smartlist_get(diff, 0));
+
+  smartlist_free(diff);
+
+  /* Everything is added. */
+  diff = gen_ed_diff(cons2, cons1, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(4, OP_EQ, smartlist_len(diff));
+  tt_str_eq_line("0a", smartlist_get(diff, 0));
+  tt_str_eq_line("foo", smartlist_get(diff, 1));
+  tt_str_eq_line("bar", smartlist_get(diff, 2));
+  tt_str_eq_line(".", smartlist_get(diff, 3));
+
+  smartlist_free(diff);
+
+  /* Everything is changed. */
+  smartlist_add_linecpy(cons2, area, "foo2");
+  smartlist_add_linecpy(cons2, area, "bar2");
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(4, OP_EQ, smartlist_len(diff));
+  tt_str_eq_line("1,2c", smartlist_get(diff, 0));
+  tt_str_eq_line("foo2", smartlist_get(diff, 1));
+  tt_str_eq_line("bar2", smartlist_get(diff, 2));
+  tt_str_eq_line(".", smartlist_get(diff, 3));
+
+  smartlist_free(diff);
+
+  /* Test 'a', 'c' and 'd' together. See that it is done in reverse order. */
+  smartlist_clear(cons1);
+  smartlist_clear(cons2);
+  consensus_split_lines(cons1, "A\nB\nC\nD\nE\n", area);
+  consensus_split_lines(cons2, "A\nC\nO\nE\nU\n", area);
+  diff = gen_ed_diff(cons1, cons2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(7, OP_EQ, smartlist_len(diff));
+  tt_str_eq_line("5a", smartlist_get(diff, 0));
+  tt_str_eq_line("U", smartlist_get(diff, 1));
+  tt_str_eq_line(".", smartlist_get(diff, 2));
+  tt_str_eq_line("4c", smartlist_get(diff, 3));
+  tt_str_eq_line("O", smartlist_get(diff, 4));
+  tt_str_eq_line(".", smartlist_get(diff, 5));
+  tt_str_eq_line("2d", smartlist_get(diff, 6));
+
+  /* TODO: small real use-cases, i.e. consensuses. */
+
+ done:
+  teardown_capture_of_logs();
+  smartlist_free(cons1);
+  smartlist_free(cons2);
+  smartlist_free(diff);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_apply_ed_diff(void *arg)
+{
+  smartlist_t *cons1=NULL, *cons2=NULL, *diff=NULL;
+  memarea_t *area = memarea_new();
+  (void)arg;
+  cons1 = smartlist_new();
+  diff = smartlist_new();
+  setup_capture_of_logs(LOG_WARN);
+
+  consensus_split_lines(cons1, "A\nB\nC\nD\nE\n", area);
+
+  /* Command without range. */
+  smartlist_add_linecpy(diff, area, "a");
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  smartlist_clear(diff);
+  expect_single_log_msg_containing("an ed command was missing a line number");
+
+  /* Range without command. */
+  smartlist_add_linecpy(diff, area, "1");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("a line with no ed command was found");
+
+  smartlist_clear(diff);
+
+  /* Range without end. */
+  smartlist_add_linecpy(diff, area, "1,");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("an ed command was missing a range "
+                                   "end line number.");
+
+  smartlist_clear(diff);
+
+  /* Incoherent ranges. */
+  smartlist_add_linecpy(diff, area, "1,1");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("an invalid range was found");
+
+  smartlist_clear(diff);
+
+  smartlist_add_linecpy(diff, area, "3,2");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("an invalid range was found");
+
+  smartlist_clear(diff);
+
+  /* Script is not in reverse order. */
+  smartlist_add_linecpy(diff, area, "1d");
+  smartlist_add_linecpy(diff, area, "3d");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("its commands are not properly sorted");
+
+  smartlist_clear(diff);
+
+  /* Script contains unrecognised commands longer than one char. */
+  smartlist_add_linecpy(diff, area, "1foo");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("an ed command longer than one char was "
+                                   "found");
+
+  smartlist_clear(diff);
+
+  /* Script contains unrecognised commands. */
+  smartlist_add_linecpy(diff, area, "1e");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("an unrecognised ed command was found");
+
+  smartlist_clear(diff);
+
+  /* Command that should be followed by at least one line and a ".", but
+   * isn't. */
+  smartlist_add_linecpy(diff, area, "0a");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("it has an ed command that tries to "
+                                   "insert zero lines.");
+
+  /* Now it is followed by a ".", but it inserts zero lines. */
+  smartlist_add_linecpy(diff, area, ".");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("it has an ed command that tries to "
+                                   "insert zero lines.");
+
+  smartlist_clear(diff);
+
+  /* Now it it inserts something, but has no terminator. */
+  smartlist_add_linecpy(diff, area, "0a");
+  smartlist_add_linecpy(diff, area, "hello");
+  mock_clean_saved_logs();
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("lines to be inserted that don't end with "
+                                   "a \".\".");
+
+  smartlist_clear(diff);
+
+  /* Test appending text, 'a'. */
+  consensus_split_lines(diff, "3a\nU\nO\n.\n0a\nV\n.\n", area);
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_int_op(8, OP_EQ, smartlist_len(cons2));
+  tt_str_eq_line("V", smartlist_get(cons2, 0));
+  tt_str_eq_line("A", smartlist_get(cons2, 1));
+  tt_str_eq_line("B", smartlist_get(cons2, 2));
+  tt_str_eq_line("C", smartlist_get(cons2, 3));
+  tt_str_eq_line("U", smartlist_get(cons2, 4));
+  tt_str_eq_line("O", smartlist_get(cons2, 5));
+  tt_str_eq_line("D", smartlist_get(cons2, 6));
+  tt_str_eq_line("E", smartlist_get(cons2, 7));
+
+  smartlist_clear(diff);
+  smartlist_free(cons2);
+
+  /* Test deleting text, 'd'. */
+  consensus_split_lines(diff, "4d\n1,2d\n", area);
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_int_op(2, OP_EQ, smartlist_len(cons2));
+  tt_str_eq_line("C", smartlist_get(cons2, 0));
+  tt_str_eq_line("E", smartlist_get(cons2, 1));
+
+  smartlist_clear(diff);
+  smartlist_free(cons2);
+
+  /* Test changing text, 'c'. */
+  consensus_split_lines(diff, "4c\nT\nX\n.\n1, 2c\nM\n.\n", area);
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_int_op(5, OP_EQ, smartlist_len(cons2));
+  tt_str_eq_line("M", smartlist_get(cons2, 0));
+  tt_str_eq_line("C", smartlist_get(cons2, 1));
+  tt_str_eq_line("T", smartlist_get(cons2, 2));
+  tt_str_eq_line("X", smartlist_get(cons2, 3));
+  tt_str_eq_line("E", smartlist_get(cons2, 4));
+
+  smartlist_clear(diff);
+  smartlist_free(cons2);
+
+  /* Test 'a', 'd' and 'c' together. */
+  consensus_split_lines(diff, "4c\nT\nX\n.\n2d\n0a\nM\n.\n", area);
+  cons2 = apply_ed_diff(cons1, diff, 0);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_int_op(6, OP_EQ, smartlist_len(cons2));
+  tt_str_eq_line("M", smartlist_get(cons2, 0));
+  tt_str_eq_line("A", smartlist_get(cons2, 1));
+  tt_str_eq_line("C", smartlist_get(cons2, 2));
+  tt_str_eq_line("T", smartlist_get(cons2, 3));
+  tt_str_eq_line("X", smartlist_get(cons2, 4));
+  tt_str_eq_line("E", smartlist_get(cons2, 5));
+
+ done:
+  teardown_capture_of_logs();
+  smartlist_free(cons1);
+  smartlist_free(cons2);
+  smartlist_free(diff);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_gen_diff(void *arg)
+{
+  char *cons1_str=NULL, *cons2_str=NULL;
+  smartlist_t *cons1=NULL, *cons2=NULL, *diff=NULL;
+  consensus_digest_t digests1, digests2;
+  memarea_t *area = memarea_new();
+  (void)arg;
+  cons1 = smartlist_new();
+  cons2 = smartlist_new();
+
+  /* Identity hashes are not sorted properly, return NULL.
+   * Already tested in gen_ed_diff, but see that a NULL ed diff also makes
+   * gen_diff return NULL. */
+  cons1_str = tor_strdup(
+      "network-status-version foo\n"
+      "r name bbbbbbbbbbbbbbbbb etc\nfoo\n"
+      "r name aaaaaaaaaaaaaaaaa etc\nbar\n"
+      "directory-signature foo bar\nbar\n"
+      );
+  cons2_str = tor_strdup(
+      "network-status-version foo\n"
+      "r name aaaaaaaaaaaaaaaaa etc\nfoo\n"
+      "r name ccccccccccccccccc etc\nbar\n"
+      "directory-signature foo bar\nbar\n"
+      );
+
+  tt_int_op(0, OP_EQ,
+      consensus_compute_digest(cons1_str, &digests1));
+  tt_int_op(0, OP_EQ,
+      consensus_compute_digest(cons2_str, &digests2));
+
+  consensus_split_lines(cons1, cons1_str, area);
+  consensus_split_lines(cons2, cons2_str, area);
+
+  diff = consdiff_gen_diff(cons1, cons2, &digests1, &digests2, area);
+  tt_ptr_op(NULL, OP_EQ, diff);
+
+  /* Check that the headers are done properly. */
+  tor_free(cons1_str);
+  cons1_str = tor_strdup(
+      "network-status-version foo\n"
+      "r name ccccccccccccccccc etc\nfoo\n"
+      "r name eeeeeeeeeeeeeeeee etc\nbar\n"
+      "directory-signature foo bar\nbar\n"
+      );
+  tt_int_op(0, OP_EQ,
+      consensus_compute_digest(cons1_str, &digests1));
+  smartlist_clear(cons1);
+  consensus_split_lines(cons1, cons1_str, area);
+  diff = consdiff_gen_diff(cons1, cons2, &digests1, &digests2, area);
+  tt_ptr_op(NULL, OP_NE, diff);
+  tt_int_op(7, OP_EQ, smartlist_len(diff));
+  tt_assert(line_str_eq(smartlist_get(diff, 0),
+                        "network-status-diff-version 1"));
+  tt_assert(line_str_eq(smartlist_get(diff, 1), "hash "
+      "06646D6CF563A41869D3B02E73254372AE3140046C5E7D83C9F71E54976AF9B4 "
+      "7AFECEFA4599BA33D603653E3D2368F648DF4AC4723929B0F7CF39281596B0C1"));
+  tt_assert(line_str_eq(smartlist_get(diff, 2), "3,4d"));
+  tt_assert(line_str_eq(smartlist_get(diff, 3), "1a"));
+  tt_assert(line_str_eq(smartlist_get(diff, 4),
+                        "r name aaaaaaaaaaaaaaaaa etc"));
+  tt_assert(line_str_eq(smartlist_get(diff, 5), "foo"));
+  tt_assert(line_str_eq(smartlist_get(diff, 6), "."));
+
+  /* TODO: small real use-cases, i.e. consensuses. */
+
+ done:
+  tor_free(cons1_str);
+  tor_free(cons2_str);
+  smartlist_free(cons1);
+  smartlist_free(cons2);
+  smartlist_free(diff);
+  memarea_drop_all(area);
+}
+
+static void
+test_consdiff_apply_diff(void *arg)
+{
+  smartlist_t *cons1=NULL, *diff=NULL;
+  char *cons1_str=NULL, *cons2 = NULL;
+  consensus_digest_t digests1;
+  (void)arg;
+  memarea_t *area = memarea_new();
+  cons1 = smartlist_new();
+  diff = smartlist_new();
+  setup_capture_of_logs(LOG_INFO);
+
+  cons1_str = tor_strdup(
+      "network-status-version foo\n"
+      "r name ccccccccccccccccc etc\nfoo\n"
+      "r name eeeeeeeeeeeeeeeee etc\nbar\n"
+      "directory-signature foo bar\nbar\n"
+      );
+  tt_int_op(0, OP_EQ,
+      consensus_compute_digest(cons1_str, &digests1));
+  consensus_split_lines(cons1, cons1_str, area);
+
+  /* diff doesn't have enough lines. */
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("too short")
+
+  /* first line doesn't match format-version string. */
+  smartlist_add_linecpy(diff, area, "foo-bar");
+  smartlist_add_linecpy(diff, area, "header-line");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("format is not known")
+
+  /* The first word of the second header line is not "hash". */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "word a b");
+  smartlist_add_linecpy(diff, area, "x");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("does not include the necessary digests")
+
+  /* Wrong number of words after "hash". */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash a b c");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("does not include the necessary digests")
+
+  /* base16 digests do not have the expected length. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash aaa bbb");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("includes base16-encoded digests of "
+                                   "incorrect size")
+
+  /* base16 digests contain non-base16 characters. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      " ????????????????????????????????????????????????????????????????"
+      " ----------------------------------------------------------------");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("includes malformed digests")
+
+  /* Invalid ed diff.
+   * As tested in apply_ed_diff, but check that apply_diff does return NULL if
+   * the ed diff can't be applied. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* sha3 of cons1. */
+      " 06646D6CF563A41869D3B02E73254372AE3140046C5E7D83C9F71E54976AF9B4"
+      /* sha256 of cons2. */
+      " 635D34593020C08E5ECD865F9986E29D50028EFA62843766A8197AD228A7F6AA");
+  smartlist_add_linecpy(diff, area, "foobar");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_single_log_msg_containing("because an ed command was missing a line "
+                                   "number")
+
+  /* Base consensus doesn't match its digest as found in the diff. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* bogus sha256. */
+      " 3333333333333333333333333333333333333333333333333333333333333333"
+      /* sha256 of cons2. */
+      " 635D34593020C08E5ECD865F9986E29D50028EFA62843766A8197AD228A7F6AA");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_log_msg_containing("base consensus doesn't match the digest "
+                            "as found");
+
+  /* Resulting consensus doesn't match its digest as found in the diff. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* sha3 of cons1. */
+      " 06646D6CF563A41869D3B02E73254372AE3140046C5E7D83C9F71E54976AF9B4"
+      /* bogus sha3. */
+      " 3333333333333333333333333333333333333333333333333333333333333333");
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_log_msg_containing("resulting consensus doesn't match the "
+                            "digest as found");
+
+#if 0
+  /* XXXX No longer possible, since we aren't using the other algorithm. */
+  /* Resulting consensus digest cannot be computed */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* sha3 of cons1. */
+      " 06646D6CF563A41869D3B02E73254372AE3140046C5E7D83C9F71E54976AF9B4"
+      /* bogus sha3. */
+      " 3333333333333333333333333333333333333333333333333333333333333333");
+  smartlist_add_linecpy(diff, area, "1,2d"); // remove starting line
+  mock_clean_saved_logs();
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_EQ, cons2);
+  expect_log_msg_containing("Could not compute digests of the consensus "
+                            "resulting from applying a consensus diff.");
+#endif
+
+  /* Very simple test, only to see that nothing errors. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* sha3 of cons1. */
+      " 06646D6CF563A41869D3B02E73254372AE3140046C5E7D83C9F71E54976AF9B4"
+      /* sha3 of cons2. */
+      " 90A418881B2FCAB3D9E60EE02E4D666D56CFA38F8A3B7AA3E0ADBA530DDA9353");
+  smartlist_add_linecpy(diff, area, "3c");
+  smartlist_add_linecpy(diff, area, "sample");
+  smartlist_add_linecpy(diff, area, ".");
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_str_op(
+      "network-status-version foo\n"
+      "r name ccccccccccccccccc etc\nsample\n"
+      "r name eeeeeeeeeeeeeeeee etc\nbar\n"
+      "directory-signature foo bar\nbar\n", OP_EQ,
+      cons2);
+  tor_free(cons2);
+
+  /* Check that lowercase letters in base16-encoded digests work too. */
+  smartlist_clear(diff);
+  smartlist_add_linecpy(diff, area, "network-status-diff-version 1");
+  smartlist_add_linecpy(diff, area, "hash"
+      /* sha3 of cons1. */
+      " 06646d6cf563a41869d3b02e73254372ae3140046c5e7d83c9f71e54976af9b4"
+      /* sha3 of cons2. */
+      " 90a418881b2fcab3d9e60ee02e4d666d56cfa38f8a3b7aa3e0adba530dda9353");
+  smartlist_add_linecpy(diff, area, "3c");
+  smartlist_add_linecpy(diff, area, "sample");
+  smartlist_add_linecpy(diff, area, ".");
+  cons2 = consdiff_apply_diff(cons1, diff, &digests1);
+  tt_ptr_op(NULL, OP_NE, cons2);
+  tt_str_op(
+      "network-status-version foo\n"
+      "r name ccccccccccccccccc etc\nsample\n"
+      "r name eeeeeeeeeeeeeeeee etc\nbar\n"
+      "directory-signature foo bar\nbar\n", OP_EQ,
+      cons2);
+  tor_free(cons2);
+
+  smartlist_clear(diff);
+
+ done:
+  teardown_capture_of_logs();
+  tor_free(cons1_str);
+  smartlist_free(cons1);
+  smartlist_free(diff);
+  memarea_drop_all(area);
+}
+
+#define CONSDIFF_LEGACY(name)                                          \
+  { #name, test_consdiff_ ## name , 0, NULL, NULL }
+
+struct testcase_t consdiff_tests[] = {
+  CONSDIFF_LEGACY(smartlist_slice),
+  CONSDIFF_LEGACY(smartlist_slice_string_pos),
+  CONSDIFF_LEGACY(lcs_lengths),
+  CONSDIFF_LEGACY(trim_slices),
+  CONSDIFF_LEGACY(set_changed),
+  CONSDIFF_LEGACY(calc_changes),
+  CONSDIFF_LEGACY(get_id_hash),
+  CONSDIFF_LEGACY(is_valid_router_entry),
+  CONSDIFF_LEGACY(next_router),
+  CONSDIFF_LEGACY(base64cmp),
+  CONSDIFF_LEGACY(gen_ed_diff),
+  CONSDIFF_LEGACY(apply_ed_diff),
+  CONSDIFF_LEGACY(gen_diff),
+  CONSDIFF_LEGACY(apply_diff),
+  END_OF_TESTCASES
+};
+