Lesson 38 · Coalescing typed runs

Coalescing typed runs

Undoing one character at a time is tedious - you want undo to remove a whole typed word at once. Today you coalesce consecutive typing into a single undo step.

The goal

Group a run of consecutive character inserts into one undo entry, broken by any other kind of edit.

Start here - the target

TO DO

Scenario: Consecutive typing undoes as one step

Givenan editor over an empty buffer

Whenthe characters of "cat" are typed, then one Undo

Thenthe undo depth after typing is 1, and the single Undo removes the whole word, leaving Text() "" with the cursor at Row 0, Col 0

Andtyping "ab", then Enter, then "cd" gives undo depth 3, and the first Undo removes only "cd" (leaving Text() "ab\n" with the cursor at Row 1, Col 0)

Background

Character-by-character undo is technically correct and practically miserable: nobody wants to press undo eight times to remove "elephant". Editors coalesce a run of consecutive keystrokes into one undo step. The mechanism is a small flag: the first character of a run records a snapshot and marks that a run is in progress, and while that flag is set, further characters skip recording - so the whole word shares the single snapshot taken before its first letter. One undo, one word.

The run has to end somewhere, and the rule is that any other kind of edit breaks it: pressing Enter, backspacing, or deleting records its own snapshot and clears the flag, so the next typed character starts a fresh group. That is why "ab", Enter, "cd" produces three undo entries, and undoing once peels off just the "cd". Undo and redo reset the flag too, so a run never bridges across them. This is the last piece of behavior; the coalesced, cursor-restoring, branch-aware undo you now have is exactly what a real editor ships. Next, the editor drives itself from a script.

Make it work

// InsertChar records a snapshot only when NOT already mid-run:
func (e *Editor) InsertChar(c byte) {
  if !e.coalescing { e.pushUndo(); e.coalescing = true }
  /* insert + advance */ ; e.Dirty = true
}
// Enter/Backspace/DeleteForward pushUndo() and set e.coalescing = false.
// Undo/Redo also reset e.coalescing = false so the next run is fresh.

CheckpointDONE

Consecutive typing undoes as a single word - chapter's undo model is complete. Commit and stop here.