sendme: Clarify how sendme_circuit_cell_is_next() works

Commit 4ef8470fa5480d3b was actually reverted before because in the end we
needed to do this minus 1 check on the window.

This commit clarifies that in the code, takes the useful comment changes from
4ef8470fa5480d3b and makes sendme_circuit_cell_is_next() private since it
behaves in a very specific way that one external caller might expect.

Part of #30428.

Signed-off-by: David Goulet <dgoulet@torproject.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/src/core/or/circuit_st.h b/src/core/or/circuit_st.h
index a68547ecb1..499bf93d6b 100644
--- a/src/core/or/circuit_st.h
+++ b/src/core/or/circuit_st.h
@@ -115,12 +115,11 @@ struct circuit_t {
    * list can not contain more than 10 digests of DIGEST_LEN bytes (20).
    *
    * At position i in the list, the digest corresponds to the
-   * ((CIRCWINDOW_INCREMENT * i) - 1)-nth cell received since we expect the
-   * (CIRCWINDOW_INCREMENT * i)-nth cell to be the SENDME and thus containing
-   * the previous cell digest.
+   * (CIRCWINDOW_INCREMENT * i)-nth cell received since we expect a SENDME to
+   * be received containing that cell digest.
    *
-   * For example, position 2 (starting at 0) means that we've received 299
-   * cells and the 299th cell digest is kept at index 2.
+   * For example, position 2 (starting at 0) means that we've received 300
+   * cells so the 300th cell digest is kept at index 2.
    *
    * At maximum, this list contains 200 bytes plus the smartlist overhead. */
   smartlist_t *sendme_last_digests;