Lesson 32 · Replacing the match - Build a Text Editor

Replacing the match

Find is most useful paired with replace. Today you swap the current match for new text, turning search into find-and-replace and closing the chapter on a real editing power tool.

The goal

Replace the current search match with new text, moving the cursor past the replacement, and do nothing when there is no match.

Start here - the target

TO DO

Scenario: Replacing the found match

Givenan editor over "foo bar" after FindNext("bar") has set the current match at Row 0, Col 4

WhenReplaceMatch("baz") is called

ThenText() is "foo baz", the cursor is at Row 0, Col 7 (just past the replacement), and the buffer is dirty

Andwith no current match (a fresh editor that has not found anything), ReplaceMatch("x") returns false and changes nothing

Background

Replace is what turns search from navigation into editing. The match you just found already carries everything a replacement needs - its position and its length - so replacing it is: delete that span, insert the new text where it stood, and move the cursor to the end of what you inserted. Because the replacement can be a different length than the match, letting the cursor land past the inserted text keeps it sensibly placed whether the word grew or shrank.

Two details make it robust. With no current match - nothing has been found yet, or the match was cleared by an edit - there is nothing to replace, so it returns false and leaves the buffer alone. And replacing consumes the match: it is an edit, so it sets the dirty flag and clears the highlight, exactly the invalidation rule from last lesson (you cannot re-replace a span you just changed). With find, find-again in both directions, wrap-around, highlighting, and replace, the editor has the search toolkit real ones ship. All that is left is the memory that lets you take any of it back: undo.

Make it work

func (e *Editor) ReplaceMatch(repl string) bool {
  if e.matchLen == 0 { return false }        // nothing found to replace
  off := e.Buf.LineStart(e.matchRow) + e.matchCol
  e.Buf.Delete(off, e.matchLen)              // remove the old match
  e.Buf.Insert(off, repl)                    // drop the new text in
  e.Row, e.Col = e.Buf.PositionAt(off + len(repl))
  e.matchLen = 0; e.Dirty = true             // consume the match
  return true
}

CheckpointDONE

The editor can find and replace a match - chapter five is complete. Commit and stop here.