textedit/indent.go

package textedit

type indentState struct {
	started, lineStarted bool
}

func indentEdit(first, rest []rune) func(rune, indentState) ([]rune, indentState) {
	return func(r rune, s indentState) ([]rune, indentState) {
		if r == '\n' {
			s.started = true
			s.lineStarted = false
			return []rune{'\n'}, s
		}

		var ret []rune
		if !s.started {
			ret = append(ret, first...)
		} else if !s.lineStarted {
			ret = append(ret, rest...)
		}

		ret = append(ret, r)
		s.started = true
		s.lineStarted = true
		return ret, s
	}
}

func indent(first, rest []rune) Editor {
	return Func(
		indentEdit(first, rest),
		func(indentState) []rune { return nil },
	)
}