| <!-- | |
| Copyright (C) Daniel Stenberg, <[email protected]>, et al. | |
| SPDX-License-Identifier: curl | |
| --> | |
| # `splay` | |
| #include "splay.h" | |
| This is an internal module for splay tree management. A splay tree is a binary | |
| search tree with the additional property that recently accessed elements are | |
| quick to access again. A self-balancing tree. | |
| Nodes are added to the tree, they are accessed and removed from the tree and | |
| it automatically rebalances itself in each operation. | |
| ## libcurl use | |
| libcurl adds fixed timeout expiry timestamps to the splay tree, and is meant | |
| to scale up to holding a huge amount of pending timeouts with decent | |
| performance. | |
| The splay tree is used to: | |
| 1. figure out the next timeout expiry value closest in time | |
| 2. iterate over timeouts that already have expired | |
| This splay tree rebalances itself based on the time value. | |
| Each node in the splay tree points to a `struct Curl_easy`. Each `Curl_easy` | |
| struct is represented only once in the tree. To still allow each easy handle | |
| to have a large number of timeouts per handle, each handle has a sorted linked | |
| list of pending timeouts. Only the handle's timeout that is closest to expire | |
| is the timestamp used for the splay tree node. | |
| When a specific easy handle's timeout expires, the node gets removed from the | |
| splay tree and from the handle's linked list of timeouts. The next timeout for | |
| that handle is then first in line and becomes the new timeout value as the | |
| node is re-added to the splay. | |
| ## `Curl_splay` | |
| ~~~c | |
| struct Curl_tree *Curl_splay(struct curltime i, struct Curl_tree *t); | |
| ~~~ | |
| Rearranges the tree `t` after the provide time `i`. | |
| ## `Curl_splayinsert` | |
| ~~~c | |
| struct Curl_tree *Curl_splayinsert(struct curltime key, | |
| struct Curl_tree *t, | |
| struct Curl_tree *node); | |
| ~~~ | |
| This function inserts a new `node` in the tree, using the given `key` | |
| timestamp. The `node` struct has a field called `->payload` that can be set to | |
| point to anything. libcurl sets this to the `struct Curl_easy` handle that is | |
| associated with the timeout value set in `key`. | |
| The splay insert function does not allocate any memory, it assumes the caller | |
| has that arranged. | |
| It returns a pointer to the new tree root. | |
| ## `Curl_splaygetbest` | |
| ~~~c | |
| struct Curl_tree *Curl_splaygetbest(struct curltime key, | |
| struct Curl_tree *tree, | |
| struct Curl_tree **removed); | |
| ~~~ | |
| If there is a node in the `tree` that has a time value that is less than the | |
| provided `key`, this function removes that node from the tree and provides it | |
| in the `*removed` pointer (or NULL if there was no match). | |
| It returns a pointer to the new tree root. | |
| ## `Curl_splayremove` | |
| ~~~c | |
| int Curl_splayremove(struct Curl_tree *tree, | |
| struct Curl_tree *node, | |
| struct Curl_tree **newroot); | |
| ~~~ | |
| Removes a given `node` from a splay `tree`, and returns the `newroot` | |
| identifying the new tree root. | |
| Note that a clean tree without any nodes present implies a NULL pointer. | |
| ## `Curl_splayset` | |
| ~~~c | |
| void Curl_splayset(struct Curl_tree *node, void *payload); | |
| ~~~ | |
| Set a custom pointer to be stored in the splay node. This pointer is not used | |
| by the splay code itself and can be retrieved again with `Curl_splayget`. | |
| ## `Curl_splayget` | |
| ~~~c | |
| void *Curl_splayget(struct Curl_tree *node); | |
| ~~~ | |
| Get the custom pointer from the splay node that was previously set with | |
| `Curl_splayset`. If no pointer was set before, it returns NULL. | |