x/term: handle transpose

The behavior implemented here matches readline and libedit.

Updates golang/go#76826

Change-Id: I893677f9bceaf75aa1dada7d893845728e07057e
Reviewed-on: https://go-review.googlesource.com/c/term/+/730441
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>

terminal.go

@@ -162,6 +162,7 @@ const (
 	keyDeleteLine
 	keyDelete
 	keyClearScreen
+	keyTranspose
 	keyPasteStart
 	keyPasteEnd
 )
@@ -195,6 +196,8 @@ func bytesToKey(b []byte, pasteActive bool) (rune, []byte) {
 			return keyDeleteLine, b[1:]
 		case 12: // ^L
 			return keyClearScreen, b[1:]
+		case 20: // ^T
+			return keyTranspose, b[1:]
 		case 23: // ^W
 			return keyDeleteWord, b[1:]
 		case 14: // ^N
@@ -605,6 +608,24 @@ func (t *Terminal) handleKey(key rune) (line string, ok bool) {
 		}
 	case keyCtrlU:
 		t.eraseNPreviousChars(t.pos)
+	case keyTranspose:
+		// This transposes the two characters around the cursor and advances the cursor. Best-effort.
+		if len(t.line) < 2 || t.pos < 1 {
+			return
+		}
+		swap := t.pos
+		if swap == len(t.line) {
+			swap-- // special: at end of line, swap previous two chars
+		}
+		t.line[swap-1], t.line[swap] = t.line[swap], t.line[swap-1]
+		if t.pos < len(t.line) {
+			t.pos++
+		}
+		if t.echo {
+			t.moveCursorToPos(swap - 1)
+			t.writeLine(t.line[swap-1:])
+			t.moveCursorToPos(t.pos)
+		}
 	case keyClearScreen:
 		// Erases the screen and moves the cursor to the home position.
 		t.queue([]rune("\x1b[2J\x1b[H"))

terminal_test.go

@@ -263,6 +263,66 @@ var keyPressTests = []struct {
 		in:   "abc\x1b[H\x1b[3~\x1b[3~\r",
 		line: "c",
 	},
+	{
+		// Ctrl-T at end of line: transpose last two chars
+		in:   "abc\x14\r",
+		line: "acb",
+	},
+	{
+		// Ctrl-T at end then type: cursor stays at end
+		in:   "abc\x14N\r",
+		line: "acbN",
+	},
+	{
+		// Ctrl-T in middle: transpose chars before cursor, move cursor forward
+		in:   "abc\x1b[D\x14\r",
+		line: "acb",
+	},
+	{
+		// Ctrl-T in middle then type: cursor moved past swapped char
+		in:   "abcd\x1b[D\x1b[D\x14N\r",
+		line: "acbNd",
+	},
+	{
+		// Ctrl-T at pos 1 then type: cursor moves to pos 2
+		in:   "abc\x1b[H\x1b[C\x14N\r",
+		line: "baNc",
+	},
+	{
+		// Ctrl-T with one char: do nothing
+		in:   "a\x14\r",
+		line: "a",
+	},
+	{
+		// Ctrl-T with one char then type: cursor unchanged
+		in:   "a\x14N\r",
+		line: "aN",
+	},
+	{
+		// Ctrl-T at beginning: do nothing
+		in:   "ab\x1b[H\x14\r",
+		line: "ab",
+	},
+	{
+		// Ctrl-T at beginning then type: cursor unchanged, inserts at start
+		in:   "ab\x1b[H\x14N\r",
+		line: "Nab",
+	},
+	{
+		// Ctrl-T on empty line: do nothing
+		in:   "\x14\r",
+		line: "",
+	},
+	{
+		// Multiple Ctrl-T at end: keeps swapping last two
+		in:   "abc\x14\x14\r",
+		line: "abc",
+	},
+	{
+		// Multiple Ctrl-T in middle: progresses through line
+		in:   "abcd\x1b[D\x1b[D\x1b[D\x14\x14\x14\r",
+		line: "bcda",
+	},
 }
 
 func TestKeyPresses(t *testing.T) {