add undo docs

undo.md

@@ -0,0 +1,64 @@
+# Towards an Undo System
+
+date  
+2025-11-27 20:14
+
+tags  
+ked, hacks, text-editors
+
+I've been thinking about building an undo system for
+[ke](https://git.wntrmute.dev/kyle/ke). The first pass is going to be a
+linear system. Let's start with the basic definitions for an undo
+system:
+
+``` c
+typedef enum undo_kind {
+    UNDO_INSERT  = 1 << 0,
+    UNDO_UNKNOWN = 1 << 1,
+    /* more types to follow */
+} undo_kind;
+
+
+typedef struct undo_node {
+    undo_kind        op;
+    size_t           row, col;
+    abuf             text;
+    struct undo_node    *next;
+    struct undo_node    *parent;
+} undo_node;
+
+
+typedef struct undo_tree {
+    /* the start of the undo sequence */
+    undo_node   *root;
+    /* where we are currently at */
+    undo_node   *current;
+    /* the current undo operations being built */
+    undo_node   *pending;
+} undo_tree;
+```
+
+The root is anchored at the last time the file was saved; when saving,
+the tree is freed. Current points to the end of the history, and pending
+is the sequence being built.
+
+The lifecycle looks something like:
+
+- `undo_tree_new` and `undo_tree_free` --- called in `init_editor` and
+  `deathknell`, respectively. The tree is initialized with all nodes set
+  to `NULL`.
+- Once the user starts doing undoable things, `undo_begin(undo_kind)`
+  gets called, calling `undo_node_new(undo_kind)` if needed to set up
+  `tree->pending`. It may need to call `undo_commit` (below) if needed.
+- Until an `undo_commit` is called, some form of `undo_append` or
+  `undo_prepend` is called.
+- Finally, at some point `undo_commit` is called. This needs to do a few
+  things:
+  1.  If `tree->current->next` is not `NULL`, it must be freed.
+  2.  Set `tree->current->next` as pending, set `tree->current` to
+      `tree->current->next`.
+  3.  Set `tree->pending` to `NULL`.
+
+Once the e
+
+- `undo_node_new(undo_kind)` and `undo_node_free`