Lesson 17 · Backspace - Build a Text Editor

Backspace

Backspace deletes the character before the cursor - and at the start of a line, it pulls that line up onto the one above. Today you handle both cases in one operation.

The goal

Delete the character before the cursor; at column 0, delete the preceding newline so the line joins the one above.

Start here - the target

TO DO

Scenario: Backspace within a line and across a line

Givenan editor over "abc" with the cursor at Row 0, Col 2

WhenBackspace is called

ThenText() is "ac" and the cursor is at Row 0, Col 1

Andover "ab\ncd" at Row 1, Col 0, Backspace gives Text() "abcd" with the cursor at Row 0, Col 2, and at Row 0, Col 0 Backspace does nothing

Background

Backspace has two faces. In the middle of a line it is the plain case: delete the character just before the cursor (at Offset() - 1) and step the column back one. But at column 0 there is no character to the left on this line - the thing immediately before the cursor is the newline that ended the previous line. Deleting that newline joins this line onto the previous one, and the cursor lands where the join happened: the previous line’s old end.

That landing column is the detail to get right - it is the length of the previous line before the join, which is why you read it first. And at the very top of the file, column 0 of row 0, there is nothing before the cursor at all, so backspace must do nothing rather than delete off the front of the buffer. Pinning all three - a normal delete, a line join, and the no-op at the origin - is what makes backspace safe on any input.

Make it work

func (e *Editor) Backspace() {
  if e.Col > 0 {
    e.Buf.Delete(e.Offset()-1, 1); e.Col--
  } else if e.Row > 0 {
    prev := e.Buf.LineLen(e.Row - 1)     // where the join lands
    e.Buf.Delete(e.Offset()-1, 1)        // delete the newline above
    e.Row--; e.Col = prev
  } // at (0,0): nothing to delete
}

CheckpointDONE

Backspace deletes and joins lines correctly. Commit and stop here.