perf(screen): write each printable cell once

putPrintable blanked every target cell before writing it, interning the
default style twice per character. Only the orphaned wide-pair halves
outside the written span are blanked now, continuation checks read the
stored flag without materializing a Cell, and RuneWidth returns early for
printable ASCII. The differential oracle stays byte-identical.

ScreenPrintableASCII 15.8 -> 44.2 MiB/s, ScrollbackFlood/history-0
15.7 -> 36.4 MiB/s, history-10000 4.9 -> 13.0 MiB/s

commit: 61dc24930674f861fd4db8642d43ad23e397a8ef
author: brice · 15d ago · 2026-10-07 16:26 UTC
parent: 92e0c36
signature: GPG VerifiedCD88F9E564E0102646FB6077A1A07C7C7C3E349D · arch-gpg Verified for the committer email

3 files · +16 -6

M core/frame.go +6 -0


@@ -267,6 +267,12 @@ func (f Frame) Cell(x, y int) Cell {
267	267		}
268	268		func (f Frame) At(x, y int) Cell { return f.Cell(x, y) }
269	269
	270	+	// IsContinuation reports whether (x,y) is the right half of a wide pair
	271	+	// without materializing the cell.
	272	+	func (f Frame) IsContinuation(x, y int) bool {
	273	+	return f.page.cells[f.offset(x, y)].flags&continuationFlag != 0
	274	+	}
	275	+
270	276		func (f Frame) Set(x, y int, cell Cell) {
271	277		index := f.offset(x, y)
272	278		payloadID := f.internPayload(cell.Payload)


@@ -267,6 +267,12 @@ func (f Frame) Cell(x, y int) Cell {
267	}	267		}
268	func (f Frame) At(x, y int) Cell { return f.Cell(x, y) }	268		func (f Frame) At(x, y int) Cell { return f.Cell(x, y) }
269		269
		270	+	// IsContinuation reports whether (x,y) is the right half of a wide pair
		271	+	// without materializing the cell.
		272	+	func (f Frame) IsContinuation(x, y int) bool {
		273	+	return f.page.cells[f.offset(x, y)].flags&continuationFlag != 0
		274	+	}
		275	+
270	func (f Frame) Set(x, y int, cell Cell) {	276		func (f Frame) Set(x, y int, cell Cell) {
271	index := f.offset(x, y)	277		index := f.offset(x, y)
272	payloadID := f.internPayload(cell.Payload)	278		payloadID := f.internPayload(cell.Payload)

M core/rune_width.go +3 -0


@@ -13,6 +13,9 @@ import "unicode"
13	13		// sequences are out of scope: combining marks report width 0 (current
14	14		// behavior), ZWJ sequences are not coalesced (each rune is measured on its own).
15	15		func RuneWidth(r rune) int {
	16	+	if r >= 0x20 && r < 0x7F { // printable ASCII, the common case
	17	+	return 1
	18	+	}
16	19		switch {
17	20		case r < 0x20 \|\| r == 0x7F \|\| (r >= 0x80 && r <= 0x9F):
18	21		return 0


@@ -13,6 +13,9 @@ import "unicode"
13	// sequences are out of scope: combining marks report width 0 (current	13		// sequences are out of scope: combining marks report width 0 (current
14	// behavior), ZWJ sequences are not coalesced (each rune is measured on its own).	14		// behavior), ZWJ sequences are not coalesced (each rune is measured on its own).
15	func RuneWidth(r rune) int {	15		func RuneWidth(r rune) int {
		16	+	if r >= 0x20 && r < 0x7F { // printable ASCII, the common case
		17	+	return 1
		18	+	}
16	switch {	19		switch {
17	case r < 0x20 \|\| r == 0x7F \|\| (r >= 0x80 && r <= 0x9F):	20		case r < 0x20 \|\| r == 0x7F \|\| (r >= 0x80 && r <= 0x9F):
18	return 0	21		return 0

M screen_write.go +7 -6


@@ -107,7 +107,7 @@ func (s *Screen) putPrintable(r rune) {
107	107		insertDamageX := s.Col
108	108		insertDamageWidth := 0
109	109		if s.insertMode {
110		-	leftSplit := s.Col > 0 && s.frame.At(s.Col, s.Row).Continuation
	110	+	leftSplit := s.Col > 0 && s.frame.IsContinuation(s.Col, s.Row)
111	111		s.frame.CopyRow(s.Row, s.Col+w, s.Col, s.frame.Width-s.Col-w)
112	112		s.frame.FillRow(s.Row, s.Col, s.Col+w, renderer.BlankCell())
113	113		s.repairRow(s.Row)

@@ -120,15 +120,16 @@ func (s *Screen) putPrintable(r rune) {
120	120
121	121		// Determine the range of cells actually modified, extending over any wide
122	122		// pair the write lands on so no orphaned half is left behind.
	123	+	// Cells inside [Col, Col+w) are overwritten below, so only the orphaned
	124	+	// halves just outside it need blanking.
123	125		lo, hi := s.Col, s.Col+w-1
124		-	if s.frame.At(s.Col, s.Row).Continuation {
	126	+	if s.frame.IsContinuation(s.Col, s.Row) {
125	127		lo = s.Col - 1
	128	+	s.frame.Set(lo, s.Row, renderer.BlankCell())
126	129		}
127		-	if right := s.Col + w; right < s.frame.Width && s.frame.At(right, s.Row).Continuation {
	130	+	if right := s.Col + w; right < s.frame.Width && s.frame.IsContinuation(right, s.Row) {
128	131		hi = right
129		-	}
130		-	for x := lo; x <= hi; x++ {
131		-	s.frame.Set(x, s.Row, renderer.BlankCell())
	132	+	s.frame.Set(right, s.Row, renderer.BlankCell())
132	133		}
133	134		s.frame.Set(s.Col, s.Row, renderer.Cell{Rune: r, Style: s.Style})
134	135		s.buffer.content(s.Row, s.Col+w)

 	insertDamageX := s.Col
 	insertDamageWidth := 0
 	if s.insertMode {
-		leftSplit := s.Col > 0 && s.frame.At(s.Col, s.Row).Continuation
+		leftSplit := s.Col > 0 && s.frame.IsContinuation(s.Col, s.Row)
 		s.frame.CopyRow(s.Row, s.Col+w, s.Col, s.frame.Width-s.Col-w)
 		s.frame.FillRow(s.Row, s.Col, s.Col+w, renderer.BlankCell())
 		s.repairRow(s.Row)
 	// Determine the range of cells actually modified, extending over any wide
 	// pair the write lands on so no orphaned half is left behind.
+	// Cells inside [Col, Col+w) are overwritten below, so only the orphaned
+	// halves just outside it need blanking.
 	lo, hi := s.Col, s.Col+w-1
-	if s.frame.At(s.Col, s.Row).Continuation {
+	if s.frame.IsContinuation(s.Col, s.Row) {
 		lo = s.Col - 1
+		s.frame.Set(lo, s.Row, renderer.BlankCell())
 	}
-	if right := s.Col + w; right < s.frame.Width && s.frame.At(right, s.Row).Continuation {
+	if right := s.Col + w; right < s.frame.Width && s.frame.IsContinuation(right, s.Row) {
 		hi = right
-	}
+		s.frame.Set(right, s.Row, renderer.BlankCell())
-	for x := lo; x <= hi; x++ {
-		s.frame.Set(x, s.Row, renderer.BlankCell())
 	}
 	s.frame.Set(s.Col, s.Row, renderer.Cell{Rune: r, Style: s.Style})
 	s.buffer.content(s.Row, s.Col+w)