Lesson 29 · Finding the previous match

Finding the previous match

Search runs both ways. Today you add a backward find that moves the cursor to the nearest match before it, the mirror of finding the next one.

The goal

Search backward from the cursor for a query and move the cursor to the previous match.

Start here - the target

TO DO

Scenario: Finding the previous occurrence

Givenan editor over "foo bar foo" with the cursor at Row 0, Col 8 (the second "foo")

WhenFindPrev("foo") is called

Thenit returns true and moves the cursor to Row 0, Col 0 (the first "foo", before the cursor)

Andsearching for a missing string returns false and leaves the cursor put

Background

Finding the previous match is the natural counterpart to finding the next one, and it flips two things. First, you search only the text before the cursor, the region [:offset], because a previous match must lie behind you. Second, within that region you want the match nearest the cursor, which is the last occurrence in that slice - so where forward search takes the first index it finds, backward search takes the last.

Everything else is shared: a hit still comes back as a byte offset that PositionAt turns into a cursor, and a miss still returns false with the cursor untouched. With both directions in place, you can walk a query’s matches forward and backward through the file. The one behavior still missing is what happens at the ends - when there is no next match ahead, or no previous match behind - which is where wrap-around comes in next.

Make it work

// search the text BEFORE the current offset, and take the LAST match
// there (the one nearest the cursor):
//   idx := strings.LastIndex(text[:e.Offset()], q)
// on a hit, move via PositionAt(idx); otherwise return false

CheckpointDONE

The editor can jump to the previous match of a query. Commit and stop here.