Lesson 14 · Jumping to a line - Build a Text Editor

Jumping to a line

The widest jumps go straight to a line number or to the ends of the whole file. Today you add a clamped go-to-line, with jumps to the document's start and end riding on top, completing the cursor.

The goal

Jump to a given line number, clamped into range, and provide jumps to the document's start and end.

Start here - the target

TO DO

Scenario: Jumping to a line and to the document ends

Givenan editor over "ab\ncde\nfgh" (three lines) with the cursor anywhere

WhenGoToLine, DocStart, and DocEnd are called

ThenGoToLine(1) moves to Row 1, Col 0; GoToLine(9) clamps to the last line, Row 2, Col 0; GoToLine(-3) clamps to Row 0, Col 0

AndDocStart moves to Row 0, Col 0, and DocEnd moves to Row 2, Col 3 (the last line "fgh", at its end)

Background

The broadest navigation jumps to a line number - the :42 you type to leap across a file - and the one bit of logic it needs is a clamp. A go-to-line has to survive being handed a number past the end of the file or a negative one, folding either back to a real line rather than dropping the cursor into nowhere. That clamp is what makes it safe to wire up to user input later, and it is the same guard-the-edges discipline every cursor move in this chapter has followed.

The two ends of the document then fall out as special cases: DocStart is go-to-line 0, and DocEnd is go-to-line at the last line followed by a jump to that line’s end column. Building them on top of the clamped GoToLine means there is one place that reasons about line bounds, and the shortcuts just name the positions you reach for most. That rounds out the cursor: you can move by character, line, and word, snap to a line’s ends, jump to any line, and reach both ends of the file, every move clamped so the cursor never lands where the buffer cannot describe it. With a place to stand fully established, the next chapter changes the text at that place.

Make it work

func (e *Editor) GoToLine(n int) {
  if n < 0 { n = 0 }                                   // clamp low
  if n > e.Buf.LineCount()-1 { n = e.Buf.LineCount()-1 } // clamp high
  e.Row, e.Col = n, 0
}
func (e *Editor) DocStart() { e.GoToLine(0) }
func (e *Editor) DocEnd() {
  e.GoToLine(e.Buf.LineCount() - 1); e.Col = e.Buf.LineLen(e.Row)
}

CheckpointDONE

The cursor can jump to any line and to either end of the document - the cursor is complete. Commit and stop here.