Lesson 28 · Finding the next match

Finding the next match

Search is how you move through a real file. Today you find the next occurrence of a query after the cursor and move there, converting a flat buffer offset back into a row and column.

The goal

Search forward from the cursor for a query and move the cursor to the next match, reporting whether one was found.

Start here - the target

TO DO

Scenario: Finding the next occurrence

Givenan editor over "foo bar foo" with the cursor at Row 0, Col 0

WhenFindNext("bar") is called, then FindNext("foo")

ThenFindNext("bar") returns true and moves the cursor to Row 0, Col 4; FindNext("foo") then moves it to Row 0, Col 8 (the match after the cursor)

Andover "ab\nfoo" FindNext("foo") moves the cursor to Row 1, Col 0, and searching for a missing string returns false and leaves the cursor put

Background

Search is the other way, besides the arrow keys, that you get around a file. Finding the next match means scanning the text from just after the cursor for the query and moving there. Starting the scan one past the current offset is what lets you press “find” repeatedly and walk through every occurrence instead of sticking on the one you are already on.

A match comes back as a flat byte offset, but the cursor lives in (row, col), so you need the inverse of chapter one’s offset formula: PositionAt finds which line’s span contains the offset and subtracts that line’s start to get the column. It is the mirror of Offset(), and search is the first feature that needs to go backwards from a buffer position to a cursor - the next lessons lean on it too. When nothing matches, the cursor stays where it is and the call reports false, so the caller can tell the difference between “found it” and “not here”.

Make it work

// search Text() starting just AFTER the current offset, so repeated
// finds advance. On a hit, turn the byte offset back into a cursor:
func (b *Buffer) PositionAt(off int) (row, col int) {
  // find the line whose start <= off, then col = off - LineStart(row)
}
// FindNext: idx := strings.Index(text[e.Offset()+1:], q); adjust, move

CheckpointDONE

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