feat(cellbuf): optimize for backspace characters in cursor movements

This adds a new option to the Screen struct, Backspace, which when enabled
will use backspace characters to move the cursor instead of using escape
sequences since it can be more efficient in some cases.

cellbuf/screen.go

@@ -36,7 +36,7 @@ func notLocal(cols, fx, fy, tx, ty int) bool {
 // [ansi.VPA], [ansi.HPA].
 // When overwrite is true, this will try to optimize the sequence by using the
 // screen cells values to move the cursor instead of using escape sequences.
-func relativeCursorMove(s *Screen, fx, fy, tx, ty int, overwrite, useTabs bool) string {
+func relativeCursorMove(s *Screen, fx, fy, tx, ty int, overwrite, useTabs, useBackspace bool) string {
 	var seq strings.Builder
 
 	width, height := s.newbuf.Width(), s.newbuf.Height()
@@ -80,7 +80,7 @@ func relativeCursorMove(s *Screen, fx, fy, tx, ty int, overwrite, useTabs bool)
 
 		if tx > fx {
 			n := tx - fx
-			if useTabs && s.opts.HardTabs {
+			if useTabs {
 				var tabs int
 				var col int
 				for col = fx; s.tabs.Next(col) <= tx; col = s.tabs.Next(col) {
@@ -144,7 +144,7 @@ func relativeCursorMove(s *Screen, fx, fy, tx, ty int, overwrite, useTabs bool)
 			}
 		} else if tx < fx {
 			n := fx - tx
-			if useTabs && s.opts.HardTabs && s.xtermLike {
+			if useTabs && s.xtermLike {
 				// VT100 does not support backward tabs [ansi.CBT].
 
 				col := fx
@@ -164,12 +164,12 @@ func relativeCursorMove(s *Screen, fx, fy, tx, ty int, overwrite, useTabs bool)
 				}
 			}
 
-			if bs := strings.Repeat("\b", n); xseq == "" || len(bs) < len(xseq) {
-				xseq = bs
+			if cub := ansi.CursorBackward(n); xseq == "" || len(cub) < len(xseq) {
+				xseq = cub
 			}
 
-			if cub := ansi.CursorBackward(n); len(cub) < len(xseq) {
-				xseq = cub
+			if useBackspace && n < len(xseq) {
+				xseq = strings.Repeat("\b", n)
 			}
 		}
 
@@ -194,42 +194,40 @@ func moveCursor(s *Screen, x, y int, overwrite bool) (seq string) {
 		}
 	}
 
-	// Method #1: Use local movement sequences.
-	nseq := relativeCursorMove(s, fx, fy, x, y, overwrite, false)
-	if len(seq) == 0 || len(nseq) < len(seq) {
-		seq = nseq
+	// Optimize based on options.
+	trials := 0
+	if s.opts.HardTabs {
+		trials |= 2 // 0b10 in binary
 	}
-
-	// Method #2: Use [ansi.CR] and local movement sequences.
-	nseq = "\r" + relativeCursorMove(s, 0, fy, x, y, overwrite, false)
-	if len(nseq) < len(seq) {
-		seq = nseq
+	if s.opts.Backspace {
+		trials |= 1 // 0b01 in binary
 	}
 
-	if !s.opts.RelativeCursor {
-		// Method #3: Use [ansi.CursorHomePosition] and local movement sequences.
-		nseq = ansi.CursorHomePosition + relativeCursorMove(s, 0, 0, x, y, overwrite, false)
-		if len(nseq) < len(seq) {
-			seq = nseq
+	// Try all possible combinations of hard tabs and backspace optimizations.
+	for i := 0; i <= trials; i++ {
+		// Skip combinations that are not enabled.
+		if i & ^trials != 0 {
+			continue
 		}
-	}
 
-	if s.opts.HardTabs {
-		// Method #4: Use tab optimized local movement sequences.
-		nseq := relativeCursorMove(s, fx, fy, x, y, overwrite, true)
-		if len(nseq) < len(seq) {
+		useHardTabs := i&2 != 0
+		useBackspace := i&1 != 0
+
+		// Method #1: Use local movement sequences.
+		nseq := relativeCursorMove(s, fx, fy, x, y, overwrite, useHardTabs, useBackspace)
+		if (i == 0 && len(seq) == 0) || len(nseq) < len(seq) {
 			seq = nseq
 		}
 
-		// Method #5: Use [ansi.CR] and tab optimized local movement sequences.
-		nseq = "\r" + relativeCursorMove(s, 0, fy, x, y, overwrite, true)
+		// Method #2: Use [ansi.CR] and local movement sequences.
+		nseq = "\r" + relativeCursorMove(s, 0, fy, x, y, overwrite, useHardTabs, useBackspace)
 		if len(nseq) < len(seq) {
 			seq = nseq
 		}
 
 		if !s.opts.RelativeCursor {
-			// Method #6: Use [ansi.CursorHomePosition] and tab optimized local movement sequences.
-			nseq = ansi.CursorHomePosition + relativeCursorMove(s, 0, 0, x, y, overwrite, true)
+			// Method #3: Use [ansi.CursorHomePosition] and local movement sequences.
+			nseq = ansi.CursorHomePosition + relativeCursorMove(s, 0, 0, x, y, overwrite, useHardTabs, useBackspace)
 			if len(nseq) < len(seq) {
 				seq = nseq
 			}
@@ -344,6 +342,8 @@ type ScreenOptions struct {
 	ShowCursor bool
 	// HardTabs is whether to use hard tabs to optimize cursor movements.
 	HardTabs bool
+	// Backspace is whether to use backspace characters to move the cursor.
+	Backspace bool
 }
 
 // lineData represents the metadata for a line.