tor/doc/HACKING/design/01b-collections.md

## Collections in tor

### 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 module documentation for more
information; there are many convenience functions for commonly needed
operations.


### 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
src/ext/tor_queue.h .

We also provide an intrusive hashtable implementation in src/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.