implement ScrollTop event (key mapping: zt)

editor/key.go

@@ -34,6 +34,7 @@ func defaultKeyManagers() map[mode.Mode]*key.Manager {
 	km.Register(event.CursorEnd, "$")
 	km.Register(event.ScrollUp, "c-y")
 	km.Register(event.ScrollDown, "c-e")
+	km.Register(event.ScrollTop, "z", "t")
 	km.Register(event.PageUp, "c-b")
 	km.Register(event.PageDown, "c-f")
 	km.Register(event.PageUpHalf, "c-u")
@@ -153,6 +154,7 @@ func defaultKeyManagers() map[mode.Mode]*key.Manager {
 	km.Register(event.CursorEnd, "$")
 	km.Register(event.ScrollUp, "c-y")
 	km.Register(event.ScrollDown, "c-e")
+	km.Register(event.ScrollTop, "z", "t")
 	km.Register(event.PageUp, "c-b")
 	km.Register(event.PageDown, "c-f")
 	km.Register(event.PageUpHalf, "c-u")

event/event.go

@@ -37,6 +37,7 @@ const (
 	CursorGoto
 	ScrollUp
 	ScrollDown
+	ScrollTop
 	PageUp
 	PageDown
 	PageUpHalf

window/window.go

@@ -113,6 +113,8 @@ func (w *window) run() {
 			w.scrollUp(e.Count)
 		case event.ScrollDown:
 			w.scrollDown(e.Count)
+		case event.ScrollTop:
+			w.scrollTop(e.Count)
 		case event.PageUp:
 			w.pageUp()
 		case event.PageDown:
@@ -469,6 +471,19 @@ func (w *window) scrollDown(count int64) {
 	}
 }
 
+func (w *window) scrollTop(count int64) {
+	if count > 0 {
+		w.cursor = mathutil.MinInt64(
+			mathutil.MinInt64(
+				count*w.width+w.cursor%w.width,
+				(mathutil.MaxInt64(w.length, 1)-1)/w.width*w.width+w.cursor%w.width,
+			),
+			mathutil.MaxInt64(w.length, 1)-1,
+		)
+	}
+	w.offset = w.cursor / w.width * w.width
+}
+
 func (w *window) pageUp() {
 	w.offset = mathutil.MaxInt64(w.offset-(w.height-2)*w.width, 0)
 	if w.offset == 0 {

window/window_test.go

@@ -477,6 +477,25 @@ func TestWindowScreenMotions(t *testing.T) {
 	if s.Offset != 0 {
 		t.Errorf("s.Offset should be %d but got %d", 0, s.Offset)
 	}
+
+	window.scrollTop(5)
+	s, _ = window.state(width, height)
+	if s.Cursor != 80 {
+		t.Errorf("s.Cursor should be %d but got %d", 80, s.Cursor)
+	}
+	if s.Offset != 80 {
+		t.Errorf("s.Offset should be %d but got %d", 80, s.Offset)
+	}
+
+	window.cursorDown(4)
+	window.scrollTop(0)
+	s, _ = window.state(width, height)
+	if s.Cursor != 144 {
+		t.Errorf("s.Cursor should be %d but got %d", 144, s.Cursor)
+	}
+	if s.Offset != 144 {
+		t.Errorf("s.Offset should be %d but got %d", 144, s.Offset)
+	}
 }
 
 func TestWindowDeleteBytes(t *testing.T) {