Move doc/HACKING/design/01b-collections.md into doxygen.

1 files changed, 46 insertions, 0 deletions

diff --git a/src/lib/container/lib_container.dox b/src/lib/container/lib_container.dox
index fb360368da..675aaeef3f 100644
--- a/src/lib/container/lib_container.dox
+++ b/src/lib/container/lib_container.dox
@@ -2,4 +2,50 @@
 @dir /lib/container
 @brief lib/container: Hash tables, dynamic arrays, bit arrays, etc.
 
+### Smartlists: Neither lists, nor especially smart.
+
+For historical reasons, we call our dynamic-allocated array type
+`smartlist_t`.  It can grow or shrink as elements are added and removed.
+
+All smartlists hold an array of `void *`.  Whenever you expose a smartlist
+in an API you *must* document which types its pointers actually hold.
+
+<!-- It would be neat to fix that, wouldn't it? -NM  -->
+
+Smartlists are created empty with `smartlist_new()` and freed with
+`smartlist_free()`.  See the `containers.h` header documentation for more
+information; there are many convenience functions for commonly needed
+operations.
+
+For low-level operations on smartlists, see also
+\refdir{lib/smartlist_core}.
+
+<!-- TODO: WRITE more about what you can do with smartlists. -->
+
+### Digest maps, string maps, and more.
+
+Tor makes frequent use of maps from 160-bit digests, 256-bit digests,
+or nul-terminated strings to `void *`. These types are `digestmap_t`,
+`digest256map_t`, and `strmap_t` respectively.  See the containers.h
+module documentation for more information.
+
+### Intrusive lists and hashtables
+
+For performance-sensitive cases, we sometimes want to use "intrusive"
+collections: ones where the bookkeeping pointers are stuck inside the
+structures that belong to the collection.  If you've used the
+BSD-style sys/queue.h macros, you'll be familiar with these.
+
+Unfortunately, the `sys/queue.h` macros vary significantly between the
+platforms that have them, so we provide our own variants in
+`ext/tor_queue.h`.
+
+We also provide an intrusive hashtable implementation in `ext/ht.h`.
+When you're using it, you'll need to define your own hash
+functions. If attacker-induced collisions are a worry here, use the
+cryptographic siphash24g function to extract hashes.
+
+<!-- TODO: WRITE about bloom filters, namemaps, bit-arrays, order functions.
+-->
+
 **/