internals/SPLAY.md: internal API documentation

Closes #14563

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@@ -665,6 +665,7 @@ ReactOS
 README
 realloc
 Realtime
+rebalances
 rebase
 RECV
 recv

diff --git a/docs/Makefile.am b/docs/Makefile.am
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  internals/MQTT.md                              \
  internals/NEW-PROTOCOL.md                      \
  internals/README.md                            \
+ internals/SPLAY.md                             \
  internals/WEBSOCKET.md
 
 EXTRA_DIST =                                    \

diff --git a/docs/internals/SPLAY.md b/docs/internals/SPLAY.md
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+<!--
+Copyright (C) Daniel Stenberg, <daniel@haxx.se>, 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.