Lesson 10 · Moving left and right

Moving left and right

The most basic navigation is one column at a time - but the cursor must never fall off the ends of a line. Today you add horizontal movement that clamps at column zero and the line's end.

The goal

Move the cursor left and right within a line, clamped so it never goes past either end.

Start here - the target

TO DO

Scenario: Horizontal movement with clamping

Givenan editor over "abc" (a single line of length 3) with the cursor at Col 0

WhenMoveLeft and MoveRight are called

ThenMoveLeft at Col 0 stays at Col 0, and three MoveRight calls give Col 1, 2, 3

Anda fourth MoveRight stays at Col 3 (the line length), and MoveLeft from there goes to Col 2

Background

Moving one column left or right is the simplest navigation there is, and the whole lesson is the clamp. A cursor that walks off the left edge into a negative column, or off the right edge past the last character, would poison the offset formula and every edit that follows. So MoveRight advances only while the column is short of the line’s length, and MoveLeft retreats only while the column is above zero.

Note the column can legally equal the line length, one past the last character: that is where you stand to append to the end of a line, so it is a valid position, not an overshoot. Crossing between lines (right at the end of a line wrapping to the next) is deliberately left out - that is a separate behavior. Today’s single job is horizontal motion that respects the two edges of the current line, and pinning both clamps proves it.

Make it work

func (e *Editor) MoveRight() {
  if e.Col < e.Buf.LineLen(e.Row) { e.Col++ } // stop at end of line
}
func (e *Editor) MoveLeft() {
  if e.Col > 0 { e.Col-- } // stop at column 0
}

CheckpointDONE

The cursor moves horizontally and clamps at both ends. Commit and stop here.