Lesson 16 · Splitting a line with Enter

Splitting a line with Enter

Pressing Enter breaks one line into two. Today you insert a newline at the cursor and move to the start of the freshly opened line - the operation that gives text its shape.

The goal

Insert a newline at the cursor, splitting the line, and place the cursor at the start of the new line.

Start here - the target

TO DO

Scenario: Enter splits the current line

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

WhenEnter is called

ThenText() is "ab\ncd" and the cursor is at Row 1, Col 0

Andthe buffer now reports LineCount() 2

Background

Enter is just inserting a "\n" at the cursor - but because the buffer treats a newline as an ordinary byte that happens to end a line, dropping one into the middle of "abcd" splits it cleanly into "ab" and "cd". Everything before the cursor stays on the current line; everything after it becomes the next line. No line list to update, no re-indexing: the line structure re-derives itself from where the newlines now sit.

The cursor move is what makes it feel like Enter rather than a stray character. After the split you belong at the start of the new line - row one greater, column back to zero - which is exactly where you continue typing. This is the mirror of the line-joining delete from chapter one: inserting a newline makes two lines, deleting one makes them one again, and both fall straight out of the buffer treating newlines as content.

Make it work

func (e *Editor) Enter() {
  e.Buf.Insert(e.Offset(), "\n") // a newline right at the cursor
  e.Row++                        // drop onto the new line...
  e.Col = 0                      // ...at its very start
}

CheckpointDONE

Enter splits a line and moves the cursor down. Commit and stop here.