widget{,/material}: rebuild label and editor with textView

This commit rebuilds the editor and label types on the common
foundation provided by textView. This enables labels to have
optional state that makes them selectable, and allows the
two widgets to share the code for managing cursor positions,
displaying selections, and soforth. Labels now have an additional
Layout function which can be invoked if they have a Selectable.
It accepts a layout.Widget used to paint their contents. Stateless
labels should still use the old Layout method.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/buffer.go

@@ -4,7 +4,6 @@ package widget
 
 import (
 	"io"
-	"strings"
 	"unicode/utf8"
 
 	"golang.org/x/text/runes"
@@ -24,6 +23,8 @@ type editBuffer struct {
 	changed bool
 }
 
+var _ textSource = (*editBuffer)(nil)
+
 const minSpace = 5
 
 func (e *editBuffer) Changed() bool {
@@ -56,10 +57,10 @@ func (e *editBuffer) moveGap(caret, space int) {
 		if space < minSpace {
 			space = minSpace
 		}
-		txt := make([]byte, e.len()+space)
+		txt := make([]byte, int(e.Size())+space)
 		// Expand to capacity.
 		txt = txt[:cap(txt)]
-		gaplen := len(txt) - e.len()
+		gaplen := len(txt) - int(e.Size())
 		if caret > e.gapstart {
 			copy(txt, e.text[:e.gapstart])
 			copy(txt[caret+gaplen:], e.text[caret:])
@@ -84,83 +85,38 @@ func (e *editBuffer) moveGap(caret, space int) {
 	}
 }
 
-func (e *editBuffer) len() int {
-	return len(e.text) - e.gapLen()
+func (e *editBuffer) Size() int64 {
+	return int64(len(e.text) - e.gapLen())
 }
 
 func (e *editBuffer) gapLen() int {
 	return e.gapend - e.gapstart
 }
 
-func (e *editBuffer) Reset() {
-	e.Seek(0, io.SeekStart)
-}
-
-// Seek implements io.Seeker
-func (e *editBuffer) Seek(offset int64, whence int) (ret int64, err error) {
-	switch whence {
-	case io.SeekStart:
-		e.pos = int(offset)
-	case io.SeekCurrent:
-		e.pos += int(offset)
-	case io.SeekEnd:
-		e.pos = e.len() - int(offset)
-	}
-	if e.pos < 0 {
-		e.pos = 0
-	} else if e.pos > e.len() {
-		e.pos = e.len()
-	}
-	return int64(e.pos), nil
-}
-
-func (e *editBuffer) Read(p []byte) (int, error) {
+func (e *editBuffer) ReadAt(p []byte, offset int64) (int, error) {
 	if len(p) == 0 {
 		return 0, nil
 	}
-	if e.pos == e.len() {
+	if offset == e.Size() {
 		return 0, io.EOF
 	}
 	var total int
-	if e.pos < e.gapstart {
-		n := copy(p, e.text[e.pos:e.gapstart])
+	if offset < int64(e.gapstart) {
+		n := copy(p, e.text[offset:e.gapstart])
 		p = p[n:]
 		total += n
-		e.pos += n
+		offset += int64(n)
 	}
-	if e.pos >= e.gapstart {
-		n := copy(p, e.text[e.pos+e.gapLen():])
+	if offset >= int64(e.gapstart) {
+		n := copy(p, e.text[offset+int64(e.gapLen()):])
 		total += n
-		e.pos += n
 	}
 	return total, nil
 }
 
-func (e *editBuffer) ReadRune() (rune, int, error) {
-	if e.pos == e.len() {
-		return 0, 0, io.EOF
-	}
-	r, s := e.runeAt(e.pos)
-	e.pos += s
-	return r, s, nil
-}
-
-// WriteTo implements io.WriterTo.
-func (e *editBuffer) WriteTo(w io.Writer) (int64, error) {
-	n1, err := w.Write(e.text[:e.gapstart])
-	if err != nil || n1 < e.gapstart {
-		return int64(n1), err
-	}
-	n2, err := w.Write(e.text[e.gapend:])
-	return int64(n1 + n2), err
-}
-
-func (e *editBuffer) String() string {
-	var b strings.Builder
-	b.Grow(e.len())
-	b.Write(e.text[:e.gapstart])
-	b.Write(e.text[e.gapend:])
-	return b.String()
+func (e *editBuffer) ReplaceRunes(byteOffset, runeCount int64, s string) {
+	e.deleteRunes(int(byteOffset), int(runeCount))
+	e.prepend(int(byteOffset), s)
 }
 
 func (e *editBuffer) prepend(caret int, s string) {
@@ -173,17 +129,3 @@ func (e *editBuffer) prepend(caret int, s string) {
 	e.gapstart += len(s)
 	e.changed = e.changed || len(s) > 0
 }
-
-func (e *editBuffer) runeBefore(idx int) (rune, int) {
-	if idx > e.gapstart {
-		idx += e.gapLen()
-	}
-	return utf8.DecodeLastRune(e.text[:idx])
-}
-
-func (e *editBuffer) runeAt(idx int) (rune, int) {
-	if idx >= e.gapstart {
-		idx += e.gapLen()
-	}
-	return utf8.DecodeRune(e.text[idx:])
-}