Improve line editing

Correctly support moving around the text and placing glyphs at any
position.

1 files changed, 151 insertions, 0 deletions

diff --git a/ui/editor.go b/ui/editor.go
new file mode 100644
index 0000000..c735e48
--- /dev/null
+++ b/ui/editor.go
@@ -0,0 +1,151 @@
+package ui
+
+import (
+	"github.com/gdamore/tcell"
+	"github.com/mattn/go-runewidth"
+)
+
+// editor is the text field where the user writes messages and commands.
+type editor struct {
+	// text contains the written runes. An empty slice means no text is written.
+	text []rune
+
+	// textWidth[i] contains the width of string(text[:i]). Therefore
+	// len(textWidth) is always strictly greater than 0 and textWidth[0] is
+	// always 0.
+	textWidth []int
+
+	// cursorIdx is the index in text of the placement of the cursor, or is
+	// equal to len(text) if the cursor is at the end.
+	cursorIdx int
+
+	// offsetIdx is the number of elements of text that are skipped when
+	// rendering.
+	offsetIdx int
+
+	// width is the width of the screen.
+	width int
+}
+
+func newEditor(width int) editor {
+	return editor{
+		text:      []rune{},
+		textWidth: []int{0},
+		width:     width,
+	}
+}
+
+func (e *editor) Resize(width int) {
+	if width < e.width {
+		e.cursorIdx = 0
+		e.offsetIdx = 0
+	}
+	e.width = width
+}
+
+func (e *editor) IsCommand() bool {
+	return len(e.text) != 0 && e.text[0] == '/'
+}
+
+func (e *editor) TextLen() int {
+	return len(e.text)
+}
+
+func (e *editor) PutRune(r rune) {
+	e.text = append(e.text, ' ')
+	copy(e.text[e.cursorIdx+1:], e.text[e.cursorIdx:])
+	e.text[e.cursorIdx] = r
+
+	rw := runewidth.RuneWidth(r)
+	tw := e.textWidth[len(e.textWidth)-1]
+	e.textWidth = append(e.textWidth, tw+rw)
+	for i := e.cursorIdx + 1; i < len(e.textWidth); i++ {
+		e.textWidth[i] = rw + e.textWidth[i-1]
+	}
+
+	e.Right()
+}
+
+func (e *editor) RemRune() (ok bool) {
+	ok = 0 < e.cursorIdx
+	if !ok {
+		return
+	}
+
+	// TODO avoid looping twice
+	rw := e.textWidth[e.cursorIdx] - e.textWidth[e.cursorIdx-1]
+	for i := e.cursorIdx; i < len(e.textWidth); i++ {
+		e.textWidth[i] -= rw
+	}
+	copy(e.textWidth[e.cursorIdx:], e.textWidth[e.cursorIdx+1:])
+	e.textWidth = e.textWidth[:len(e.textWidth)-1]
+
+	copy(e.text[e.cursorIdx-1:], e.text[e.cursorIdx:])
+	e.text = e.text[:len(e.text)-1]
+	e.Left()
+	return
+}
+
+func (e *editor) Flush() (content string) {
+	content = string(e.text)
+	e.text = e.text[:0]
+	e.textWidth = e.textWidth[:1]
+	e.cursorIdx = 0
+	e.offsetIdx = 0
+	return
+}
+
+func (e *editor) Right() {
+	if e.cursorIdx == len(e.text) {
+		return
+	}
+	e.cursorIdx++
+	if e.width < e.textWidth[e.cursorIdx]-e.textWidth[e.offsetIdx] {
+		e.offsetIdx += 16
+		max := len(e.text) - 1
+		if max < e.offsetIdx {
+			e.offsetIdx = max
+		}
+	}
+}
+
+func (e *editor) Left() {
+	if e.cursorIdx == 0 {
+		return
+	}
+	e.cursorIdx--
+	if e.cursorIdx <= e.offsetIdx {
+		e.offsetIdx -= 16
+		if e.offsetIdx < 0 {
+			e.offsetIdx = 0
+		}
+	}
+}
+
+func (e *editor) Draw(screen tcell.Screen, y int) {
+	st := tcell.StyleDefault
+
+	x := 0
+	i := e.offsetIdx
+
+	for i < len(e.text) && x < e.width {
+		r := e.text[i]
+		screen.SetContent(x, y, r, nil, st)
+		x += runewidth.RuneWidth(r)
+		i++
+	}
+
+	for x < e.width {
+		screen.SetContent(x, y, ' ', nil, st)
+		x++
+	}
+
+	curStart := e.textWidth[e.cursorIdx] - e.textWidth[e.offsetIdx]
+	curEnd := curStart + 1
+	if e.cursorIdx+1 < len(e.textWidth) {
+		curEnd = e.textWidth[e.cursorIdx+1] - e.textWidth[e.offsetIdx]
+	}
+	for x := curStart; x < curEnd; x++ {
+		screen.ShowCursor(x, y)
+	}
+}