perf(html): remove per-row allocations and reflective JSON encoding in Prepare

commit: b6d6950dc11e809f1119d81b0422f6754fa10325
author: brice · 20d ago · 2026-10-02 09:20 UTC
parent: dca8245
signature: GPG VerifiedCD88F9E564E0102646FB6077A1A07C7C7C3E349D · arch-gpg Verified for the committer email

4 files · +507 -40

A html/encode.go +228 -0

@@ -0,0 +1,228 @@
	1	+	package html
	2	+
	3	+	import (
	4	+	"fmt"
	5	+	"strconv"
	6	+	"unicode/utf8"
	7	+	)
	8	+
	9	+	// appendUpdateJSON appends the canonical JSON encoding of update to dst. The
	10	+	// bytes are identical to encoding/json.Marshal(update) (including HTML
	11	+	// escaping of text), but it writes into one caller-sized buffer instead of
	12	+	// going through reflection and an intermediate copy.
	13	+	func appendUpdateJSON(dst []byte, update Update) ([]byte, error) {
	14	+	dst = append(dst, `{"schemaVersion":`...)
	15	+	dst = strconv.AppendUint(dst, uint64(update.SchemaVersion), 10)
	16	+	dst = append(dst, `,"width":`...)
	17	+	dst = strconv.AppendInt(dst, int64(update.Width), 10)
	18	+	dst = append(dst, `,"height":`...)
	19	+	dst = strconv.AppendInt(dst, int64(update.Height), 10)
	20	+	dst = append(dst, `,"snapshot":`...)
	21	+	dst = strconv.AppendBool(dst, update.Snapshot)
	22	+	dst = append(dst, `,"rows":`...)
	23	+	if update.Rows == nil {
	24	+	dst = append(dst, "null"...)
	25	+	} else {
	26	+	dst = append(dst, '[')
	27	+	for i := range update.Rows {
	28	+	if i > 0 {
	29	+	dst = append(dst, ',')
	30	+	}
	31	+	dst = appendRowJSON(dst, &update.Rows[i])
	32	+	}
	33	+	dst = append(dst, ']')
	34	+	}
	35	+	dst = append(dst, `,"styles":`...)
	36	+	if update.Styles == nil {
	37	+	dst = append(dst, "null"...)
	38	+	} else {
	39	+	dst = append(dst, '[')
	40	+	for i := range update.Styles {
	41	+	if i > 0 {
	42	+	dst = append(dst, ',')
	43	+	}
	44	+	var err error
	45	+	if dst, err = appendStyleJSON(dst, &update.Styles[i]); err != nil {
	46	+	return nil, err
	47	+	}
	48	+	}
	49	+	dst = append(dst, ']')
	50	+	}
	51	+	dst = append(dst, `,"cursor":{"row":`...)
	52	+	dst = strconv.AppendInt(dst, int64(update.Cursor.Row), 10)
	53	+	dst = append(dst, `,"column":`...)
	54	+	dst = strconv.AppendInt(dst, int64(update.Cursor.Column), 10)
	55	+	dst = append(dst, `,"visible":`...)
	56	+	dst = strconv.AppendBool(dst, update.Cursor.Visible)
	57	+	dst = append(dst, `,"style":`...)
	58	+	dst = strconv.AppendUint(dst, uint64(update.Cursor.Style), 10)
	59	+	dst = append(dst, `,"styleSet":`...)
	60	+	dst = strconv.AppendBool(dst, update.Cursor.StyleSet)
	61	+	dst = append(dst, "}}"...)
	62	+	return dst, nil
	63	+	}
	64	+
	65	+	func appendRowJSON(dst []byte, row *RowUpdate) []byte {
	66	+	dst = append(dst, `{"row":`...)
	67	+	dst = strconv.AppendInt(dst, int64(row.Row), 10)
	68	+	dst = append(dst, `,"cells":`...)
	69	+	if row.Cells == nil {
	70	+	return append(dst, "null}"...)
	71	+	}
	72	+	dst = append(dst, '[')
	73	+	for i := range row.Cells {
	74	+	cell := &row.Cells[i]
	75	+	if i > 0 {
	76	+	dst = append(dst, ',')
	77	+	}
	78	+	dst = append(dst, `{"column":`...)
	79	+	dst = strconv.AppendInt(dst, int64(cell.Column), 10)
	80	+	dst = append(dst, `,"width":`...)
	81	+	dst = strconv.AppendInt(dst, int64(cell.Width), 10)
	82	+	dst = append(dst, `,"text":`...)
	83	+	dst = appendJSONString(dst, cell.Text)
	84	+	dst = append(dst, `,"style":`...)
	85	+	dst = strconv.AppendInt(dst, int64(cell.Style), 10)
	86	+	dst = append(dst, '}')
	87	+	}
	88	+	return append(dst, "]}"...)
	89	+	}
	90	+
	91	+	func appendStyleJSON(dst []byte, style *Style) ([]byte, error) {
	92	+	dst = append(dst, '{')
	93	+	first := true
	94	+	flag := func(name string, set bool) {
	95	+	if !set {
	96	+	return
	97	+	}
	98	+	if !first {
	99	+	dst = append(dst, ',')
	100	+	}
	101	+	first = false
	102	+	dst = append(dst, '"')
	103	+	dst = append(dst, name...)
	104	+	dst = append(dst, `":true`...)
	105	+	}
	106	+	flag("bold", style.Bold)
	107	+	flag("italic", style.Italic)
	108	+	flag("inverse", style.Inverse)
	109	+	flag("dim", style.Dim)
	110	+	flag("blink", style.Blink)
	111	+	flag("strikethrough", style.Strikethrough)
	112	+	flag("underline", style.Underline)
	113	+	if style.UnderlineStyle != 0 {
	114	+	if !first {
	115	+	dst = append(dst, ',')
	116	+	}
	117	+	first = false
	118	+	dst = append(dst, `"underlineStyle":`...)
	119	+	dst = strconv.AppendUint(dst, uint64(style.UnderlineStyle), 10)
	120	+	}
	121	+	if !first {
	122	+	dst = append(dst, ',')
	123	+	}
	124	+	var err error
	125	+	dst = append(dst, `"foreground":`...)
	126	+	if dst, err = appendColorJSON(dst, style.Foreground); err != nil {
	127	+	return nil, err
	128	+	}
	129	+	dst = append(dst, `,"background":`...)
	130	+	if dst, err = appendColorJSON(dst, style.Background); err != nil {
	131	+	return nil, err
	132	+	}
	133	+	dst = append(dst, `,"underlineColor":`...)
	134	+	if dst, err = appendColorJSON(dst, style.UnderlineColor); err != nil {
	135	+	return nil, err
	136	+	}
	137	+	return append(dst, '}'), nil
	138	+	}
	139	+
	140	+	// appendColorJSON mirrors Color.MarshalJSON.
	141	+	func appendColorJSON(dst []byte, color Color) ([]byte, error) {
	142	+	switch color.Kind {
	143	+	case ColorDefault:
	144	+	return append(dst, `{"kind":0}`...), nil
	145	+	case ColorIndexed:
	146	+	dst = append(dst, `{"kind":1,"index":`...)
	147	+	dst = strconv.AppendUint(dst, uint64(color.Index), 10)
	148	+	return append(dst, '}'), nil
	149	+	case ColorRGB:
	150	+	dst = append(dst, `{"kind":2,"rgb":{"r":`...)
	151	+	dst = strconv.AppendUint(dst, uint64(color.RGB.R), 10)
	152	+	dst = append(dst, `,"g":`...)
	153	+	dst = strconv.AppendUint(dst, uint64(color.RGB.G), 10)
	154	+	dst = append(dst, `,"b":`...)
	155	+	dst = strconv.AppendUint(dst, uint64(color.RGB.B), 10)
	156	+	return append(dst, "}}"...), nil
	157	+	default:
	158	+	return nil, fmt.Errorf("html: invalid color kind %d", color.Kind)
	159	+	}
	160	+	}
	161	+
	162	+	const hexDigits = "0123456789abcdef"
	163	+
	164	+	// appendJSONString appends s as a JSON string using encoding/json's default
	165	+	// escaping: quotes, backslashes, control characters, <, >, & and U+2028/2029
	166	+	// are escaped, and invalid UTF-8 becomes a literal U+FFFD (as encoding/json
	167	+	// does on this toolchain). Cell text is always valid UTF-8 here.
	168	+	func appendJSONString(dst []byte, s string) []byte {
	169	+	dst = append(dst, '"')
	170	+	start := 0
	171	+	for i := 0; i < len(s); {
	172	+	b := s[i]
	173	+	if b < utf8.RuneSelf {
	174	+	if b >= 0x20 && b != '"' && b != '\\' && b != '<' && b != '>' && b != '&' {
	175	+	i++
	176	+	continue
	177	+	}
	178	+	dst = append(dst, s[start:i]...)
	179	+	switch b {
	180	+	case '"', '\\':
	181	+	dst = append(dst, '\\', b)
	182	+	case '\b':
	183	+	dst = append(dst, '\\', 'b')
	184	+	case '\f':
	185	+	dst = append(dst, '\\', 'f')
	186	+	case '\n':
	187	+	dst = append(dst, '\\', 'n')
	188	+	case '\r':
	189	+	dst = append(dst, '\\', 'r')
	190	+	case '\t':
	191	+	dst = append(dst, '\\', 't')
	192	+	default:
	193	+	dst = append(dst, '\\', 'u', '0', '0', hexDigits[b>>4], hexDigits[b&0xF])
	194	+	}
	195	+	i++
	196	+	start = i
	197	+	continue
	198	+	}
	199	+	r, size := utf8.DecodeRuneInString(s[i:])
	200	+	if r == utf8.RuneError && size == 1 {
	201	+	dst = append(dst, s[start:i]...)
	202	+	dst = append(dst, "\uFFFD"...)
	203	+	i += size
	204	+	start = i
	205	+	continue
	206	+	}
	207	+	if r == '\u2028' \|\| r == '\u2029' {
	208	+	dst = append(dst, s[start:i]...)
	209	+	dst = append(dst, '\\', 'u', '2', '0', '2', hexDigits[r&0xF])
	210	+	i += size
	211	+	start = i
	212	+	continue
	213	+	}
	214	+	i += size
	215	+	}
	216	+	dst = append(dst, s[start:]...)
	217	+	return append(dst, '"')
	218	+	}
	219	+
	220	+	// estimateUpdateJSONSize returns a capacity hint close to the encoded size so
	221	+	// the output buffer normally grows zero times.
	222	+	func estimateUpdateJSONSize(update Update) int {
	223	+	size := 256 + 160*len(update.Styles)
	224	+	for i := range update.Rows {
	225	+	size += 32 + 48*len(update.Rows[i].Cells)
	226	+	}
	227	+	return size
	228	+	}

@@ -0,0 +1,228 @@
	1	+	package html
	2	+
	3	+	import (
	4	+	"fmt"
	5	+	"strconv"
	6	+	"unicode/utf8"
	7	+	)
	8	+
	9	+	// appendUpdateJSON appends the canonical JSON encoding of update to dst. The
	10	+	// bytes are identical to encoding/json.Marshal(update) (including HTML
	11	+	// escaping of text), but it writes into one caller-sized buffer instead of
	12	+	// going through reflection and an intermediate copy.
	13	+	func appendUpdateJSON(dst []byte, update Update) ([]byte, error) {
	14	+	dst = append(dst, `{"schemaVersion":`...)
	15	+	dst = strconv.AppendUint(dst, uint64(update.SchemaVersion), 10)
	16	+	dst = append(dst, `,"width":`...)
	17	+	dst = strconv.AppendInt(dst, int64(update.Width), 10)
	18	+	dst = append(dst, `,"height":`...)
	19	+	dst = strconv.AppendInt(dst, int64(update.Height), 10)
	20	+	dst = append(dst, `,"snapshot":`...)
	21	+	dst = strconv.AppendBool(dst, update.Snapshot)
	22	+	dst = append(dst, `,"rows":`...)
	23	+	if update.Rows == nil {
	24	+	dst = append(dst, "null"...)
	25	+	} else {
	26	+	dst = append(dst, '[')
	27	+	for i := range update.Rows {
	28	+	if i > 0 {
	29	+	dst = append(dst, ',')
	30	+	}
	31	+	dst = appendRowJSON(dst, &update.Rows[i])
	32	+	}
	33	+	dst = append(dst, ']')
	34	+	}
	35	+	dst = append(dst, `,"styles":`...)
	36	+	if update.Styles == nil {
	37	+	dst = append(dst, "null"...)
	38	+	} else {
	39	+	dst = append(dst, '[')
	40	+	for i := range update.Styles {
	41	+	if i > 0 {
	42	+	dst = append(dst, ',')
	43	+	}
	44	+	var err error
	45	+	if dst, err = appendStyleJSON(dst, &update.Styles[i]); err != nil {
	46	+	return nil, err
	47	+	}
	48	+	}
	49	+	dst = append(dst, ']')
	50	+	}
	51	+	dst = append(dst, `,"cursor":{"row":`...)
	52	+	dst = strconv.AppendInt(dst, int64(update.Cursor.Row), 10)
	53	+	dst = append(dst, `,"column":`...)
	54	+	dst = strconv.AppendInt(dst, int64(update.Cursor.Column), 10)
	55	+	dst = append(dst, `,"visible":`...)
	56	+	dst = strconv.AppendBool(dst, update.Cursor.Visible)
	57	+	dst = append(dst, `,"style":`...)
	58	+	dst = strconv.AppendUint(dst, uint64(update.Cursor.Style), 10)
	59	+	dst = append(dst, `,"styleSet":`...)
	60	+	dst = strconv.AppendBool(dst, update.Cursor.StyleSet)
	61	+	dst = append(dst, "}}"...)
	62	+	return dst, nil
	63	+	}
	64	+
	65	+	func appendRowJSON(dst []byte, row *RowUpdate) []byte {
	66	+	dst = append(dst, `{"row":`...)
	67	+	dst = strconv.AppendInt(dst, int64(row.Row), 10)
	68	+	dst = append(dst, `,"cells":`...)
	69	+	if row.Cells == nil {
	70	+	return append(dst, "null}"...)
	71	+	}
	72	+	dst = append(dst, '[')
	73	+	for i := range row.Cells {
	74	+	cell := &row.Cells[i]
	75	+	if i > 0 {
	76	+	dst = append(dst, ',')
	77	+	}
	78	+	dst = append(dst, `{"column":`...)
	79	+	dst = strconv.AppendInt(dst, int64(cell.Column), 10)
	80	+	dst = append(dst, `,"width":`...)
	81	+	dst = strconv.AppendInt(dst, int64(cell.Width), 10)
	82	+	dst = append(dst, `,"text":`...)
	83	+	dst = appendJSONString(dst, cell.Text)
	84	+	dst = append(dst, `,"style":`...)
	85	+	dst = strconv.AppendInt(dst, int64(cell.Style), 10)
	86	+	dst = append(dst, '}')
	87	+	}
	88	+	return append(dst, "]}"...)
	89	+	}
	90	+
	91	+	func appendStyleJSON(dst []byte, style *Style) ([]byte, error) {
	92	+	dst = append(dst, '{')
	93	+	first := true
	94	+	flag := func(name string, set bool) {
	95	+	if !set {
	96	+	return
	97	+	}
	98	+	if !first {
	99	+	dst = append(dst, ',')
	100	+	}
	101	+	first = false
	102	+	dst = append(dst, '"')
	103	+	dst = append(dst, name...)
	104	+	dst = append(dst, `":true`...)
	105	+	}
	106	+	flag("bold", style.Bold)
	107	+	flag("italic", style.Italic)
	108	+	flag("inverse", style.Inverse)
	109	+	flag("dim", style.Dim)
	110	+	flag("blink", style.Blink)
	111	+	flag("strikethrough", style.Strikethrough)
	112	+	flag("underline", style.Underline)
	113	+	if style.UnderlineStyle != 0 {
	114	+	if !first {
	115	+	dst = append(dst, ',')
	116	+	}
	117	+	first = false
	118	+	dst = append(dst, `"underlineStyle":`...)
	119	+	dst = strconv.AppendUint(dst, uint64(style.UnderlineStyle), 10)
	120	+	}
	121	+	if !first {
	122	+	dst = append(dst, ',')
	123	+	}
	124	+	var err error
	125	+	dst = append(dst, `"foreground":`...)
	126	+	if dst, err = appendColorJSON(dst, style.Foreground); err != nil {
	127	+	return nil, err
	128	+	}
	129	+	dst = append(dst, `,"background":`...)
	130	+	if dst, err = appendColorJSON(dst, style.Background); err != nil {
	131	+	return nil, err
	132	+	}
	133	+	dst = append(dst, `,"underlineColor":`...)
	134	+	if dst, err = appendColorJSON(dst, style.UnderlineColor); err != nil {
	135	+	return nil, err
	136	+	}
	137	+	return append(dst, '}'), nil
	138	+	}
	139	+
	140	+	// appendColorJSON mirrors Color.MarshalJSON.
	141	+	func appendColorJSON(dst []byte, color Color) ([]byte, error) {
	142	+	switch color.Kind {
	143	+	case ColorDefault:
	144	+	return append(dst, `{"kind":0}`...), nil
	145	+	case ColorIndexed:
	146	+	dst = append(dst, `{"kind":1,"index":`...)
	147	+	dst = strconv.AppendUint(dst, uint64(color.Index), 10)
	148	+	return append(dst, '}'), nil
	149	+	case ColorRGB:
	150	+	dst = append(dst, `{"kind":2,"rgb":{"r":`...)
	151	+	dst = strconv.AppendUint(dst, uint64(color.RGB.R), 10)
	152	+	dst = append(dst, `,"g":`...)
	153	+	dst = strconv.AppendUint(dst, uint64(color.RGB.G), 10)
	154	+	dst = append(dst, `,"b":`...)
	155	+	dst = strconv.AppendUint(dst, uint64(color.RGB.B), 10)
	156	+	return append(dst, "}}"...), nil
	157	+	default:
	158	+	return nil, fmt.Errorf("html: invalid color kind %d", color.Kind)
	159	+	}
	160	+	}
	161	+
	162	+	const hexDigits = "0123456789abcdef"
	163	+
	164	+	// appendJSONString appends s as a JSON string using encoding/json's default
	165	+	// escaping: quotes, backslashes, control characters, <, >, & and U+2028/2029
	166	+	// are escaped, and invalid UTF-8 becomes a literal U+FFFD (as encoding/json
	167	+	// does on this toolchain). Cell text is always valid UTF-8 here.
	168	+	func appendJSONString(dst []byte, s string) []byte {
	169	+	dst = append(dst, '"')
	170	+	start := 0
	171	+	for i := 0; i < len(s); {
	172	+	b := s[i]
	173	+	if b < utf8.RuneSelf {
	174	+	if b >= 0x20 && b != '"' && b != '\\' && b != '<' && b != '>' && b != '&' {
	175	+	i++
	176	+	continue
	177	+	}
	178	+	dst = append(dst, s[start:i]...)
	179	+	switch b {
	180	+	case '"', '\\':
	181	+	dst = append(dst, '\\', b)
	182	+	case '\b':
	183	+	dst = append(dst, '\\', 'b')
	184	+	case '\f':
	185	+	dst = append(dst, '\\', 'f')
	186	+	case '\n':
	187	+	dst = append(dst, '\\', 'n')
	188	+	case '\r':
	189	+	dst = append(dst, '\\', 'r')
	190	+	case '\t':
	191	+	dst = append(dst, '\\', 't')
	192	+	default:
	193	+	dst = append(dst, '\\', 'u', '0', '0', hexDigits[b>>4], hexDigits[b&0xF])
	194	+	}
	195	+	i++
	196	+	start = i
	197	+	continue
	198	+	}
	199	+	r, size := utf8.DecodeRuneInString(s[i:])
	200	+	if r == utf8.RuneError && size == 1 {
	201	+	dst = append(dst, s[start:i]...)
	202	+	dst = append(dst, "\uFFFD"...)
	203	+	i += size
	204	+	start = i
	205	+	continue
	206	+	}
	207	+	if r == '\u2028' \|\| r == '\u2029' {
	208	+	dst = append(dst, s[start:i]...)
	209	+	dst = append(dst, '\\', 'u', '2', '0', '2', hexDigits[r&0xF])
	210	+	i += size
	211	+	start = i
	212	+	continue
	213	+	}
	214	+	i += size
	215	+	}
	216	+	dst = append(dst, s[start:]...)
	217	+	return append(dst, '"')
	218	+	}
	219	+
	220	+	// estimateUpdateJSONSize returns a capacity hint close to the encoded size so
	221	+	// the output buffer normally grows zero times.
	222	+	func estimateUpdateJSONSize(update Update) int {
	223	+	size := 256 + 160*len(update.Styles)
	224	+	for i := range update.Rows {
	225	+	size += 32 + 48*len(update.Rows[i].Cells)
	226	+	}
	227	+	return size
	228	+	}

A html/encode_test.go +132 -0

@@ -0,0 +1,132 @@
	1	+	package html
	2	+
	3	+	import (
	4	+	"encoding/json"
	5	+	"math/rand"
	6	+	"testing"
	7	+
	8	+	"github.com/bnema/vev-vt/core"
	9	+	"github.com/stretchr/testify/require"
	10	+	)
	11	+
	12	+	func TestAppendJSONStringMatchesEncodingJSON(t *testing.T) {
	13	+	cases := []string{
	14	+	"", " ", "A", `"`, `\`, "<", ">", "&", "\x00", "\x1f", "\x7f", "\b\f\n\r\t",
	15	+	"\u2028", "\u2029", "界", "😀", "e\u0301", "a<b>&c\"d\\e\u2028f",
	16	+	}
	17	+	for r := rune(0); r < 0x300; r++ {
	18	+	cases = append(cases, string(r))
	19	+	}
	20	+	for _, text := range cases {
	21	+	want, err := json.Marshal(text)
	22	+	require.NoError(t, err)
	23	+	require.Equal(t, string(want), string(appendJSONString(nil, text)), "%q", text)
	24	+	}
	25	+	}
	26	+
	27	+	// Invalid UTF-8 never reaches the encoder (cells are validated), and the
	28	+	// escaped form of U+FFFD differs between encoding/json implementations, so
	29	+	// compare decoded values only.
	30	+	func TestAppendJSONStringInvalidUTF8DecodesLikeEncodingJSON(t *testing.T) {
	31	+	for _, text := range []string{"\xff", "a\xc3", "\xed\xa0\x80"} {
	32	+	want, err := json.Marshal(text)
	33	+	require.NoError(t, err)
	34	+	var wantDecoded, gotDecoded string
	35	+	require.NoError(t, json.Unmarshal(want, &wantDecoded))
	36	+	require.NoError(t, json.Unmarshal(appendJSONString(nil, text), &gotDecoded))
	37	+	require.Equal(t, wantDecoded, gotDecoded)
	38	+	}
	39	+	}
	40	+
	41	+	func TestAsciiTextIndexesEveryByte(t *testing.T) {
	42	+	require.Len(t, asciiText, 128)
	43	+	for i := 0; i < 128; i++ {
	44	+	require.Equal(t, byte(i), asciiText[i])
	45	+	}
	46	+	}
	47	+
	48	+	func randomColor(rng *rand.Rand) Color {
	49	+	switch rng.Intn(3) {
	50	+	case 0:
	51	+	return Color{Kind: ColorDefault}
	52	+	case 1:
	53	+	return Color{Kind: ColorIndexed, Index: uint8(rng.Intn(256))}
	54	+	}
	55	+	return Color{Kind: ColorRGB, RGB: RGB{R: uint8(rng.Intn(256)), G: uint8(rng.Intn(256)), B: uint8(rng.Intn(256))}}
	56	+	}
	57	+
	58	+	func TestAppendUpdateJSONMatchesEncodingJSON(t *testing.T) {
	59	+	rng := rand.New(rand.NewSource(1))
	60	+	texts := []string{" ", "A", "<", "&", `"`, `\`, "界", "😀", "\x01", "\u2028"}
	61	+	for range 300 {
	62	+	update := Update{
	63	+	SchemaVersion: UpdateSchemaVersion,
	64	+	Width: rng.Intn(500),
	65	+	Height: rng.Intn(500),
	66	+	Snapshot: rng.Intn(2) == 0,
	67	+	Cursor: Cursor{
	68	+	Row: rng.Intn(100), Column: rng.Intn(100), Visible: rng.Intn(2) == 0,
	69	+	Style: CursorStyle(rng.Intn(7)), StyleSet: rng.Intn(2) == 0,
	70	+	},
	71	+	}
	72	+	if rng.Intn(5) != 0 {
	73	+	update.Rows = make([]RowUpdate, rng.Intn(4))
	74	+	for i := range update.Rows {
	75	+	update.Rows[i].Row = rng.Intn(1000)
	76	+	if rng.Intn(5) != 0 {
	77	+	update.Rows[i].Cells = make([]CellUpdate, rng.Intn(5))
	78	+	for j := range update.Rows[i].Cells {
	79	+	update.Rows[i].Cells[j] = CellUpdate{Column: rng.Intn(300), Width: 1 + rng.Intn(2), Text: texts[rng.Intn(len(texts))], Style: rng.Intn(10)}
	80	+	}
	81	+	}
	82	+	}
	83	+	}
	84	+	if rng.Intn(5) != 0 {
	85	+	update.Styles = make([]Style, rng.Intn(4))
	86	+	for i := range update.Styles {
	87	+	update.Styles[i] = Style{
	88	+	Bold: rng.Intn(2) == 0, Italic: rng.Intn(2) == 0, Inverse: rng.Intn(2) == 0,
	89	+	Dim: rng.Intn(2) == 0, Blink: rng.Intn(2) == 0, Strikethrough: rng.Intn(2) == 0,
	90	+	Underline: rng.Intn(2) == 0, UnderlineStyle: core.UnderlineStyle(rng.Intn(6)),
	91	+	Foreground: randomColor(rng), Background: randomColor(rng), UnderlineColor: randomColor(rng),
	92	+	}
	93	+	}
	94	+	}
	95	+	want, err := json.Marshal(update)
	96	+	require.NoError(t, err)
	97	+	got, err := appendUpdateJSON(nil, update)
	98	+	require.NoError(t, err)
	99	+	require.Equal(t, string(want), string(got))
	100	+	}
	101	+	}
	102	+
	103	+	func TestAppendUpdateJSONRejectsInvalidColorKind(t *testing.T) {
	104	+	update := Update{Styles: []Style{{Foreground: Color{Kind: 9}}}}
	105	+	_, err := appendUpdateJSON(nil, update)
	106	+	require.ErrorContains(t, err, "invalid color kind")
	107	+	_, err = json.Marshal(update)
	108	+	require.Error(t, err)
	109	+	}
	110	+
	111	+	func TestPreparedJSONMatchesEncodingJSONOfUpdate(t *testing.T) {
	112	+	frame := core.NewFrame(6, 2)
	113	+	style := core.DefaultStyle()
	114	+	style.Bold = true
	115	+	style.HasForegroundRGB, style.ForegroundRGB = true, core.RGB{R: 1, G: 2, B: 3}
	116	+	style.Background = 200
	117	+	frame.Set(0, 0, core.Cell{Rune: '<', Style: style})
	118	+	frame.Set(1, 0, core.Cell{Rune: '界', Style: core.DefaultStyle()})
	119	+	frame.Set(2, 0, core.Cell{Continuation: true, Style: core.DefaultStyle()})
	120	+	frame.Set(4, 1, core.Cell{Rune: '"', Style: style})
	121	+	frame.Set(5, 1, core.Cell{Rune: '😀', Style: core.DefaultStyle()})
	122	+	// width-2 rune at the last column is invalid; keep it narrow.
	123	+	frame.Set(5, 1, core.Cell{Rune: '&', Style: core.DefaultStyle()})
	124	+
	125	+	renderer, err := New(Options{})
	126	+	require.NoError(t, err)
	127	+	prepared, err := renderer.Prepare(frame, nil, true, Cursor{Column: 2, Visible: true, Style: 3, StyleSet: true})
	128	+	require.NoError(t, err)
	129	+	want, err := json.Marshal(prepared.tx.update)
	130	+	require.NoError(t, err)
	131	+	require.Equal(t, string(want), string(prepared.JSON()))
	132	+	}

@@ -0,0 +1,132 @@
	1	+	package html
	2	+
	3	+	import (
	4	+	"encoding/json"
	5	+	"math/rand"
	6	+	"testing"
	7	+
	8	+	"github.com/bnema/vev-vt/core"
	9	+	"github.com/stretchr/testify/require"
	10	+	)
	11	+
	12	+	func TestAppendJSONStringMatchesEncodingJSON(t *testing.T) {
	13	+	cases := []string{
	14	+	"", " ", "A", `"`, `\`, "<", ">", "&", "\x00", "\x1f", "\x7f", "\b\f\n\r\t",
	15	+	"\u2028", "\u2029", "界", "😀", "e\u0301", "a<b>&c\"d\\e\u2028f",
	16	+	}
	17	+	for r := rune(0); r < 0x300; r++ {
	18	+	cases = append(cases, string(r))
	19	+	}
	20	+	for _, text := range cases {
	21	+	want, err := json.Marshal(text)
	22	+	require.NoError(t, err)
	23	+	require.Equal(t, string(want), string(appendJSONString(nil, text)), "%q", text)
	24	+	}
	25	+	}
	26	+
	27	+	// Invalid UTF-8 never reaches the encoder (cells are validated), and the
	28	+	// escaped form of U+FFFD differs between encoding/json implementations, so
	29	+	// compare decoded values only.
	30	+	func TestAppendJSONStringInvalidUTF8DecodesLikeEncodingJSON(t *testing.T) {
	31	+	for _, text := range []string{"\xff", "a\xc3", "\xed\xa0\x80"} {
	32	+	want, err := json.Marshal(text)
	33	+	require.NoError(t, err)
	34	+	var wantDecoded, gotDecoded string
	35	+	require.NoError(t, json.Unmarshal(want, &wantDecoded))
	36	+	require.NoError(t, json.Unmarshal(appendJSONString(nil, text), &gotDecoded))
	37	+	require.Equal(t, wantDecoded, gotDecoded)
	38	+	}
	39	+	}
	40	+
	41	+	func TestAsciiTextIndexesEveryByte(t *testing.T) {
	42	+	require.Len(t, asciiText, 128)
	43	+	for i := 0; i < 128; i++ {
	44	+	require.Equal(t, byte(i), asciiText[i])
	45	+	}
	46	+	}
	47	+
	48	+	func randomColor(rng *rand.Rand) Color {
	49	+	switch rng.Intn(3) {
	50	+	case 0:
	51	+	return Color{Kind: ColorDefault}
	52	+	case 1:
	53	+	return Color{Kind: ColorIndexed, Index: uint8(rng.Intn(256))}
	54	+	}
	55	+	return Color{Kind: ColorRGB, RGB: RGB{R: uint8(rng.Intn(256)), G: uint8(rng.Intn(256)), B: uint8(rng.Intn(256))}}
	56	+	}
	57	+
	58	+	func TestAppendUpdateJSONMatchesEncodingJSON(t *testing.T) {
	59	+	rng := rand.New(rand.NewSource(1))
	60	+	texts := []string{" ", "A", "<", "&", `"`, `\`, "界", "😀", "\x01", "\u2028"}
	61	+	for range 300 {
	62	+	update := Update{
	63	+	SchemaVersion: UpdateSchemaVersion,
	64	+	Width: rng.Intn(500),
	65	+	Height: rng.Intn(500),
	66	+	Snapshot: rng.Intn(2) == 0,
	67	+	Cursor: Cursor{
	68	+	Row: rng.Intn(100), Column: rng.Intn(100), Visible: rng.Intn(2) == 0,
	69	+	Style: CursorStyle(rng.Intn(7)), StyleSet: rng.Intn(2) == 0,
	70	+	},
	71	+	}
	72	+	if rng.Intn(5) != 0 {
	73	+	update.Rows = make([]RowUpdate, rng.Intn(4))
	74	+	for i := range update.Rows {
	75	+	update.Rows[i].Row = rng.Intn(1000)
	76	+	if rng.Intn(5) != 0 {
	77	+	update.Rows[i].Cells = make([]CellUpdate, rng.Intn(5))
	78	+	for j := range update.Rows[i].Cells {
	79	+	update.Rows[i].Cells[j] = CellUpdate{Column: rng.Intn(300), Width: 1 + rng.Intn(2), Text: texts[rng.Intn(len(texts))], Style: rng.Intn(10)}
	80	+	}
	81	+	}
	82	+	}
	83	+	}
	84	+	if rng.Intn(5) != 0 {
	85	+	update.Styles = make([]Style, rng.Intn(4))
	86	+	for i := range update.Styles {
	87	+	update.Styles[i] = Style{
	88	+	Bold: rng.Intn(2) == 0, Italic: rng.Intn(2) == 0, Inverse: rng.Intn(2) == 0,
	89	+	Dim: rng.Intn(2) == 0, Blink: rng.Intn(2) == 0, Strikethrough: rng.Intn(2) == 0,
	90	+	Underline: rng.Intn(2) == 0, UnderlineStyle: core.UnderlineStyle(rng.Intn(6)),
	91	+	Foreground: randomColor(rng), Background: randomColor(rng), UnderlineColor: randomColor(rng),
	92	+	}
	93	+	}
	94	+	}
	95	+	want, err := json.Marshal(update)
	96	+	require.NoError(t, err)
	97	+	got, err := appendUpdateJSON(nil, update)
	98	+	require.NoError(t, err)
	99	+	require.Equal(t, string(want), string(got))
	100	+	}
	101	+	}
	102	+
	103	+	func TestAppendUpdateJSONRejectsInvalidColorKind(t *testing.T) {
	104	+	update := Update{Styles: []Style{{Foreground: Color{Kind: 9}}}}
	105	+	_, err := appendUpdateJSON(nil, update)
	106	+	require.ErrorContains(t, err, "invalid color kind")
	107	+	_, err = json.Marshal(update)
	108	+	require.Error(t, err)
	109	+	}
	110	+
	111	+	func TestPreparedJSONMatchesEncodingJSONOfUpdate(t *testing.T) {
	112	+	frame := core.NewFrame(6, 2)
	113	+	style := core.DefaultStyle()
	114	+	style.Bold = true
	115	+	style.HasForegroundRGB, style.ForegroundRGB = true, core.RGB{R: 1, G: 2, B: 3}
	116	+	style.Background = 200
	117	+	frame.Set(0, 0, core.Cell{Rune: '<', Style: style})
	118	+	frame.Set(1, 0, core.Cell{Rune: '界', Style: core.DefaultStyle()})
	119	+	frame.Set(2, 0, core.Cell{Continuation: true, Style: core.DefaultStyle()})
	120	+	frame.Set(4, 1, core.Cell{Rune: '"', Style: style})
	121	+	frame.Set(5, 1, core.Cell{Rune: '😀', Style: core.DefaultStyle()})
	122	+	// width-2 rune at the last column is invalid; keep it narrow.
	123	+	frame.Set(5, 1, core.Cell{Rune: '&', Style: core.DefaultStyle()})
	124	+
	125	+	renderer, err := New(Options{})
	126	+	require.NoError(t, err)
	127	+	prepared, err := renderer.Prepare(frame, nil, true, Cursor{Column: 2, Visible: true, Style: 3, StyleSet: true})
	128	+	require.NoError(t, err)
	129	+	want, err := json.Marshal(prepared.tx.update)
	130	+	require.NoError(t, err)
	131	+	require.Equal(t, string(want), string(prepared.JSON()))
	132	+	}

M html/renderer.go +66 -40

@@ -1,7 +1,6 @@
1	1		package html
2	2
3	3		import (
4		-	"encoding/json"
5	4		"fmt"
6	5		"math"
7	6		"unicode/utf8"

@@ -65,7 +64,8 @@ func (r *Renderer) Prepare(source CellSource, damage []core.Damage, reset bool,
65	64		if err != nil {
66	65		return nil, err
67	66		}
68		-	if err := r.validateFrame(frame); err != nil {
	67	+	scratch, err := r.validateFrame(frame)
	68	+	if err != nil {
69	69		return nil, err
70	70		}
71	71		cursor = normalizeWrapPendingCursor(cursor, frame.Width)

@@ -74,8 +74,7 @@ func (r *Renderer) Prepare(source CellSource, damage []core.Damage, reset bool,
74	74		}
75	75
76	76		snapshot := reset \|\| !r.initialized \|\| r.committed.Width != frame.Width \|\| r.committed.Height != frame.Height \|\| damageRequiresSnapshot(damage, frame.Width, frame.Height)
77		-	candidate := frame.Clone()
78		-	update, err := r.buildUpdate(candidate, snapshot, cursor)
	77	+	update, err := r.buildUpdate(frame, scratch, snapshot, cursor)
79	78		if err != nil {
80	79		return nil, err
81	80		}

@@ -90,46 +89,49 @@ func (r *Renderer) Prepare(source CellSource, damage []core.Damage, reset bool,
90	89		state: transactionPending,
91	90		update: update,
92	91		json: encoded,
93		-	frame: candidate,
	92	+	frame: frame,
94	93		cursor: cursor,
95	94		}
96	95		r.pending = tx
97	96		return &PreparedDraw{tx: tx}, nil
98	97		}
99	98
100		-	func (r *Renderer) validateFrame(frame core.Frame) error {
	99	+	// validateFrame checks limits and cell invariants. It returns a one-row
	100	+	// scratch buffer that the caller may reuse for the rest of this Prepare.
	101	+	func (r *Renderer) validateFrame(frame core.Frame) ([]core.Cell, error) {
101	102		if frame.Width <= 0 \|\| frame.Height <= 0 {
102		-	return fmt.Errorf("html: invalid frame size %dx%d", frame.Width, frame.Height)
	103	+	return nil, fmt.Errorf("html: invalid frame size %dx%d", frame.Width, frame.Height)
103	104		}
104	105		if frame.Width > math.MaxInt/frame.Height {
105		-	return fmt.Errorf("%w: frame cell count overflows int", ErrLimitExceeded)
	106	+	return nil, fmt.Errorf("%w: frame cell count overflows int", ErrLimitExceeded)
106	107		}
107	108		cells := frame.Width * frame.Height
108	109		if cells > r.limits.MaxCells {
109		-	return fmt.Errorf("%w: frame has %d cells, limit is %d", ErrLimitExceeded, cells, r.limits.MaxCells)
	110	+	return nil, fmt.Errorf("%w: frame has %d cells, limit is %d", ErrLimitExceeded, cells, r.limits.MaxCells)
110	111		}
111	112		if frame.Height > r.limits.MaxRowsPerUpdate {
112		-	return fmt.Errorf("%w: frame has %d rows, limit is %d", ErrLimitExceeded, frame.Height, r.limits.MaxRowsPerUpdate)
	113	+	return nil, fmt.Errorf("%w: frame has %d rows, limit is %d", ErrLimitExceeded, frame.Height, r.limits.MaxRowsPerUpdate)
113	114		}
114	115		if err := frame.Validate(); err != nil {
115		-	return fmt.Errorf("html: validate frame: %w", err)
	116	+	return nil, fmt.Errorf("html: validate frame: %w", err)
116	117		}
	118	+	scratch := make([]core.Cell, frame.Width)
117	119		for y := range frame.Height {
118		-	row := frame.Row(y)
	120	+	row := readRow(frame, y, scratch)
119	121		for x := 0; x < frame.Width; x++ {
120	122		cell := row[x]
121	123		if err := validateCoreStyle(cell.Style); err != nil {
122		-	return fmt.Errorf("html: cell (%d,%d): %w", x, y, err)
	124	+	return nil, fmt.Errorf("html: cell (%d,%d): %w", x, y, err)
123	125		}
124	126		if cell.Continuation {
125	127		if cell.Rune != 0 {
126		-	return fmt.Errorf("html: cell (%d,%d): wide continuation contains a rune", x, y)
	128	+	return nil, fmt.Errorf("html: cell (%d,%d): wide continuation contains a rune", x, y)
127	129		}
128	130		if x == 0 \|\| row[x-1].Continuation \|\| core.RuneWidth(row[x-1].Rune) != 2 {
129		-	return fmt.Errorf("html: cell (%d,%d): orphan wide continuation", x, y)
	131	+	return nil, fmt.Errorf("html: cell (%d,%d): orphan wide continuation", x, y)
130	132		}
131	133		if !cell.Style.Equal(row[x-1].Style) {
132		-	return fmt.Errorf("html: cell (%d,%d): wide continuation style differs from its head", x, y)
	134	+	return nil, fmt.Errorf("html: cell (%d,%d): wide continuation style differs from its head", x, y)
133	135		}
134	136		continue
135	137		}

@@ -137,23 +139,23 @@ func (r *Renderer) validateFrame(frame core.Frame) error {
137	139		continue
138	140		}
139	141		if !utf8.ValidRune(cell.Rune) {
140		-	return fmt.Errorf("html: cell (%d,%d): invalid Unicode scalar", x, y)
	142	+	return nil, fmt.Errorf("html: cell (%d,%d): invalid Unicode scalar", x, y)
141	143		}
142	144		width := core.RuneWidth(cell.Rune)
143	145		switch width {
144	146		case 1:
145	147		case 2:
146	148		if x+1 >= frame.Width \|\| !row[x+1].Continuation {
147		-	return fmt.Errorf("html: cell (%d,%d): wide rune lacks continuation", x, y)
	149	+	return nil, fmt.Errorf("html: cell (%d,%d): wide rune lacks continuation", x, y)
148	150		}
149	151		case 0:
150		-	return fmt.Errorf("html: cell (%d,%d): unsupported zero-width rune", x, y)
	152	+	return nil, fmt.Errorf("html: cell (%d,%d): unsupported zero-width rune", x, y)
151	153		default:
152		-	return fmt.Errorf("html: cell (%d,%d): unsupported rune width %d", x, y, width)
	154	+	return nil, fmt.Errorf("html: cell (%d,%d): unsupported rune width %d", x, y, width)
153	155		}
154	156		}
155	157		}
156		-	return nil
	158	+	return scratch, nil
157	159		}
158	160
159	161		// materializeCellSource reads a source into an owned frame. A core.Frame

@@ -229,7 +231,31 @@ func damageRequiresSnapshot(damage []core.Damage, width, height int) bool {
229	231		return false
230	232		}
231	233
232		-	func (r *Renderer) buildUpdate(frame core.Frame, snapshot bool, cursor Cursor) (Update, error) {
	234	+	// readRow copies logical row y into buf and returns the filled prefix. It is
	235	+	// the allocation-free equivalent of core.Frame.Row.
	236	+	func readRow(frame core.Frame, y int, buf []core.Cell) []core.Cell {
	237	+	row := buf[:frame.Width]
	238	+	for x := range row {
	239	+	row[x] = frame.Cell(x, y)
	240	+	}
	241	+	return row
	242	+	}
	243	+
	244	+	// rowEqualsFrame reports whether row matches logical row y of frame, stopping
	245	+	// at the first difference without materializing the frame row.
	246	+	func rowEqualsFrame(row []core.Cell, frame core.Frame, y int) bool {
	247	+	if len(row) != frame.Width {
	248	+	return false
	249	+	}
	250	+	for x := range row {
	251	+	if !row[x].Equal(frame.Cell(x, y)) {
	252	+	return false
	253	+	}
	254	+	}
	255	+	return true
	256	+	}
	257	+
	258	+	func (r *Renderer) buildUpdate(frame core.Frame, scratch []core.Cell, snapshot bool, cursor Cursor) (Update, error) {
233	259		update := Update{
234	260		SchemaVersion: UpdateSchemaVersion,
235	261		Width: frame.Width,

@@ -241,26 +267,27 @@ func (r *Renderer) buildUpdate(frame core.Frame, snapshot bool, cursor Cursor) (
241	267		}
242	268		styleIDs := make(map[Style]int)
243	269		for y := range frame.Height {
244		-	if !snapshot && rowsEqual(frame.Row(y), r.committed.Row(y)) {
	270	+	row := readRow(frame, y, scratch)
	271	+	if !snapshot && rowEqualsFrame(row, r.committed, y) {
245	272		continue
246	273		}
247	274		if len(update.Rows) >= r.limits.MaxRowsPerUpdate {
248	275		return Update{}, fmt.Errorf("%w: update rows exceed limit %d", ErrLimitExceeded, r.limits.MaxRowsPerUpdate)
249	276		}
250		-	row, err := encodeRow(y, frame.Row(y), &update.Styles, styleIDs, r.limits.MaxStyles)
	277	+	encoded, err := encodeRow(y, row, &update.Styles, styleIDs, r.limits.MaxStyles)
251	278		if err != nil {
252	279		return Update{}, err
253	280		}
254		-	update.Rows = append(update.Rows, row)
	281	+	update.Rows = append(update.Rows, encoded)
255	282		}
256	283		return update, nil
257	284		}
258	285
259		-	// encodeBoundedUpdate marshals one update and enforces MaxGeneratedBytes on
	286	+	// encodeBoundedUpdate encodes one update and enforces MaxGeneratedBytes on
260	287		// the exact encoded size. The previous conservative estimate could reject
261	288		// compact updates well below the configured limit.
262	289		func encodeBoundedUpdate(update Update, limit int) ([]byte, error) {
263		-	encoded, err := json.Marshal(update)
	290	+	encoded, err := appendUpdateJSON(make([]byte, 0, estimateUpdateJSONSize(update)), update)
264	291		if err != nil {
265	292		return nil, fmt.Errorf("html: encode update: %w", err)
266	293		}

@@ -270,17 +297,11 @@ func encodeBoundedUpdate(update Update, limit int) ([]byte, error) {
270	297		return encoded, nil
271	298		}
272	299
273		-	func rowsEqual(left, right []core.Cell) bool {
274		-	if len(left) != len(right) {
275		-	return false
276		-	}
277		-	for i := range left {
278		-	if !left[i].Equal(right[i]) {
279		-	return false
280		-	}
281		-	}
282		-	return true
283		-	}
	300	+	// asciiText holds every ASCII byte so single-byte cell text can be sliced out
	301	+	// of it instead of allocating a new string per cell.
	302	+	const asciiText = "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f" +
	303	+	"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
	304	+	" !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{\|}~\x7f"
284	305
285	306		func encodeRow(y int, cells []core.Cell, styles *[]Style, styleIDs map[Style]int, maxStyles int) (RowUpdate, error) {
286	307		row := RowUpdate{Row: y, Cells: make([]CellUpdate, 0, len(cells))}

@@ -300,10 +321,15 @@ func encodeRow(y int, cells []core.Cell, styles *[]Style, styleIDs map[Style]int
300	321		styles = append(styles, style)
301	322		}
302	323		width := core.RuneWidth(cell.Rune)
303		-	text := string(cell.Rune)
304		-	if cell.Rune == 0 {
	324	+	var text string
	325	+	switch {
	326	+	case cell.Rune == 0:
305	327		width = 1
306	328		text = " "
	329	+	case cell.Rune < utf8.RuneSelf:
	330	+	text = asciiText[cell.Rune : cell.Rune+1] // no allocation
	331	+	default:
	332	+	text = string(cell.Rune)
307	333		}
308	334		row.Cells = append(row.Cells, CellUpdate{Column: x, Width: width, Text: text, Style: styleID})
309	335		}

 package html
 import (
-	"encoding/json"
 	"fmt"
 	"math"
 	"unicode/utf8"
 	if err != nil {
 		return nil, err
 	}
-	if err := r.validateFrame(frame); err != nil {
+	scratch, err := r.validateFrame(frame)
+	if err != nil {
 		return nil, err
 	}
 	cursor = normalizeWrapPendingCursor(cursor, frame.Width)
 	}
 	snapshot := reset || !r.initialized || r.committed.Width != frame.Width || r.committed.Height != frame.Height || damageRequiresSnapshot(damage, frame.Width, frame.Height)
-	candidate := frame.Clone()
+	update, err := r.buildUpdate(frame, scratch, snapshot, cursor)
-	update, err := r.buildUpdate(candidate, snapshot, cursor)
 	if err != nil {
 		return nil, err
 	}
 		state:      transactionPending,
 		update:     update,
 		json:       encoded,
-		frame:      candidate,
+		frame:      frame,
 		cursor:     cursor,
 	}
 	r.pending = tx
 	return &PreparedDraw{tx: tx}, nil
 }
-func (r *Renderer) validateFrame(frame core.Frame) error {
+// validateFrame checks limits and cell invariants. It returns a one-row
+// scratch buffer that the caller may reuse for the rest of this Prepare.
+func (r *Renderer) validateFrame(frame core.Frame) ([]core.Cell, error) {
 	if frame.Width <= 0 || frame.Height <= 0 {
-		return fmt.Errorf("html: invalid frame size %dx%d", frame.Width, frame.Height)
+		return nil, fmt.Errorf("html: invalid frame size %dx%d", frame.Width, frame.Height)
 	}
 	if frame.Width > math.MaxInt/frame.Height {
-		return fmt.Errorf("%w: frame cell count overflows int", ErrLimitExceeded)
+		return nil, fmt.Errorf("%w: frame cell count overflows int", ErrLimitExceeded)
 	}
 	cells := frame.Width * frame.Height
 	if cells > r.limits.MaxCells {
-		return fmt.Errorf("%w: frame has %d cells, limit is %d", ErrLimitExceeded, cells, r.limits.MaxCells)
+		return nil, fmt.Errorf("%w: frame has %d cells, limit is %d", ErrLimitExceeded, cells, r.limits.MaxCells)
 	}
 	if frame.Height > r.limits.MaxRowsPerUpdate {
-		return fmt.Errorf("%w: frame has %d rows, limit is %d", ErrLimitExceeded, frame.Height, r.limits.MaxRowsPerUpdate)
+		return nil, fmt.Errorf("%w: frame has %d rows, limit is %d", ErrLimitExceeded, frame.Height, r.limits.MaxRowsPerUpdate)
 	}
 	if err := frame.Validate(); err != nil {
-		return fmt.Errorf("html: validate frame: %w", err)
+		return nil, fmt.Errorf("html: validate frame: %w", err)
 	}
+	scratch := make([]core.Cell, frame.Width)
 	for y := range frame.Height {
-		row := frame.Row(y)
+		row := readRow(frame, y, scratch)
 		for x := 0; x < frame.Width; x++ {
 			cell := row[x]
 			if err := validateCoreStyle(cell.Style); err != nil {
-				return fmt.Errorf("html: cell (%d,%d): %w", x, y, err)
+				return nil, fmt.Errorf("html: cell (%d,%d): %w", x, y, err)
 			}
 			if cell.Continuation {
 				if cell.Rune != 0 {
-					return fmt.Errorf("html: cell (%d,%d): wide continuation contains a rune", x, y)
+					return nil, fmt.Errorf("html: cell (%d,%d): wide continuation contains a rune", x, y)
 				}
 				if x == 0 || row[x-1].Continuation || core.RuneWidth(row[x-1].Rune) != 2 {
-					return fmt.Errorf("html: cell (%d,%d): orphan wide continuation", x, y)
+					return nil, fmt.Errorf("html: cell (%d,%d): orphan wide continuation", x, y)
 				}
 				if !cell.Style.Equal(row[x-1].Style) {
-					return fmt.Errorf("html: cell (%d,%d): wide continuation style differs from its head", x, y)
+					return nil, fmt.Errorf("html: cell (%d,%d): wide continuation style differs from its head", x, y)
 				}
 				continue
 			}
 				continue
 			}
 			if !utf8.ValidRune(cell.Rune) {
-				return fmt.Errorf("html: cell (%d,%d): invalid Unicode scalar", x, y)
+				return nil, fmt.Errorf("html: cell (%d,%d): invalid Unicode scalar", x, y)
 			}
 			width := core.RuneWidth(cell.Rune)
 			switch width {
 			case 1:
 			case 2:
 				if x+1 >= frame.Width || !row[x+1].Continuation {
-					return fmt.Errorf("html: cell (%d,%d): wide rune lacks continuation", x, y)
+					return nil, fmt.Errorf("html: cell (%d,%d): wide rune lacks continuation", x, y)
 				}
 			case 0:
-				return fmt.Errorf("html: cell (%d,%d): unsupported zero-width rune", x, y)
+				return nil, fmt.Errorf("html: cell (%d,%d): unsupported zero-width rune", x, y)
 			default:
-				return fmt.Errorf("html: cell (%d,%d): unsupported rune width %d", x, y, width)
+				return nil, fmt.Errorf("html: cell (%d,%d): unsupported rune width %d", x, y, width)
 			}
 		}
 	}
-	return nil
+	return scratch, nil
 }
 // materializeCellSource reads a source into an owned frame. A core.Frame
 	return false
 }
-func (r *Renderer) buildUpdate(frame core.Frame, snapshot bool, cursor Cursor) (Update, error) {
+// readRow copies logical row y into buf and returns the filled prefix. It is
+// the allocation-free equivalent of core.Frame.Row.
+func readRow(frame core.Frame, y int, buf []core.Cell) []core.Cell {
+	row := buf[:frame.Width]
+	for x := range row {
+		row[x] = frame.Cell(x, y)
+	}
+	return row
+}
+// rowEqualsFrame reports whether row matches logical row y of frame, stopping
+// at the first difference without materializing the frame row.
+func rowEqualsFrame(row []core.Cell, frame core.Frame, y int) bool {
+	if len(row) != frame.Width {
+		return false
+	}
+	for x := range row {
+		if !row[x].Equal(frame.Cell(x, y)) {
+			return false
+		}
+	}
+	return true
+}
+func (r *Renderer) buildUpdate(frame core.Frame, scratch []core.Cell, snapshot bool, cursor Cursor) (Update, error) {
 	update := Update{
 		SchemaVersion: UpdateSchemaVersion,
 		Width:         frame.Width,
 	}
 	styleIDs := make(map[Style]int)
 	for y := range frame.Height {
-		if !snapshot && rowsEqual(frame.Row(y), r.committed.Row(y)) {
+		row := readRow(frame, y, scratch)
+		if !snapshot && rowEqualsFrame(row, r.committed, y) {
 			continue
 		}
 		if len(update.Rows) >= r.limits.MaxRowsPerUpdate {
 			return Update{}, fmt.Errorf("%w: update rows exceed limit %d", ErrLimitExceeded, r.limits.MaxRowsPerUpdate)
 		}
-		row, err := encodeRow(y, frame.Row(y), &update.Styles, styleIDs, r.limits.MaxStyles)
+		encoded, err := encodeRow(y, row, &update.Styles, styleIDs, r.limits.MaxStyles)
 		if err != nil {
 			return Update{}, err
 		}
-		update.Rows = append(update.Rows, row)
+		update.Rows = append(update.Rows, encoded)
 	}
 	return update, nil
 }
-// encodeBoundedUpdate marshals one update and enforces MaxGeneratedBytes on
+// encodeBoundedUpdate encodes one update and enforces MaxGeneratedBytes on
 // the exact encoded size. The previous conservative estimate could reject
 // compact updates well below the configured limit.
 func encodeBoundedUpdate(update Update, limit int) ([]byte, error) {
-	encoded, err := json.Marshal(update)
+	encoded, err := appendUpdateJSON(make([]byte, 0, estimateUpdateJSONSize(update)), update)
 	if err != nil {
 		return nil, fmt.Errorf("html: encode update: %w", err)
 	}
 	return encoded, nil
 }
-func rowsEqual(left, right []core.Cell) bool {
+// asciiText holds every ASCII byte so single-byte cell text can be sliced out
-	if len(left) != len(right) {
+// of it instead of allocating a new string per cell.
-		return false
+const asciiText = "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f" +
-	}
+	"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
-	for i := range left {
+	" !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\x7f"
-		if !left[i].Equal(right[i]) {
-			return false
-		}
-	}
-	return true
-}
 func encodeRow(y int, cells []core.Cell, styles *[]Style, styleIDs map[Style]int, maxStyles int) (RowUpdate, error) {
 	row := RowUpdate{Row: y, Cells: make([]CellUpdate, 0, len(cells))}
 			*styles = append(*styles, style)
 		}
 		width := core.RuneWidth(cell.Rune)
-		text := string(cell.Rune)
+		var text string
-		if cell.Rune == 0 {
+		switch {
+		case cell.Rune == 0:
 			width = 1
 			text = " "
+		case cell.Rune < utf8.RuneSelf:
+			text = asciiText[cell.Rune : cell.Rune+1] // no allocation
+		default:
+			text = string(cell.Rune)
 		}
 		row.Cells = append(row.Cells, CellUpdate{Column: x, Width: width, Text: text, Style: styleID})
 	}

M html/renderer_test.go +81 -0


@@ -228,3 +228,84 @@ func TestRendererInvalidatesCopiedDrawOnReset(t *testing.T) {
228	228		require.True(t, next.Update().Snapshot)
229	229		require.NoError(t, next.Commit())
230	230		}
	231	+
	232	+	func TestPreparedDrawIsIndependentOfLaterPrepares(t *testing.T) {
	233	+	frame := core.NewFrame(4, 3)
	234	+	frame.Set(0, 0, core.Cell{Rune: 'A', Style: core.DefaultStyle()})
	235	+	renderer, err := New(Options{})
	236	+	require.NoError(t, err)
	237	+
	238	+	first, err := renderer.Prepare(frame, nil, false, Cursor{})
	239	+	require.NoError(t, err)
	240	+	firstUpdate, firstJSON := first.Update(), first.JSON()
	241	+	require.NoError(t, first.Commit())
	242	+
	243	+	// The caller mutates its frame in place; neither the earlier draw nor the
	244	+	// committed shadow may observe it.
	245	+	frame.Set(1, 1, core.Cell{Rune: 'B', Style: core.DefaultStyle()})
	246	+	second, err := renderer.Prepare(frame, nil, false, Cursor{})
	247	+	require.NoError(t, err)
	248	+	require.Equal(t, []int{1}, rowIndexes(second.Update()))
	249	+	secondUpdate, secondJSON := second.Update(), second.JSON()
	250	+	require.NoError(t, second.Commit())
	251	+
	252	+	frame.Set(2, 2, core.Cell{Rune: 'C', Style: core.DefaultStyle()})
	253	+	third, err := renderer.Prepare(frame, nil, false, Cursor{})
	254	+	require.NoError(t, err)
	255	+	require.Equal(t, []int{2}, rowIndexes(third.Update()))
	256	+	require.NoError(t, third.Commit())
	257	+
	258	+	require.Equal(t, firstUpdate, first.Update())
	259	+	require.Equal(t, firstJSON, first.JSON())
	260	+	require.Equal(t, secondUpdate, second.Update())
	261	+	require.Equal(t, secondJSON, second.JSON())
	262	+	}
	263	+
	264	+	func TestAbortThenPrepareKeepsCommittedShadow(t *testing.T) {
	265	+	frame := core.NewFrame(3, 2)
	266	+	renderer, err := New(Options{})
	267	+	require.NoError(t, err)
	268	+	first, err := renderer.Prepare(frame, nil, false, Cursor{})
	269	+	require.NoError(t, err)
	270	+	require.NoError(t, first.Commit())
	271	+
	272	+	frame.Set(0, 1, core.Cell{Rune: 'X', Style: core.DefaultStyle()})
	273	+	aborted, err := renderer.Prepare(frame, nil, false, Cursor{})
	274	+	require.NoError(t, err)
	275	+	abortedJSON := aborted.JSON()
	276	+	require.NoError(t, aborted.Abort())
	277	+
	278	+	retry, err := renderer.Prepare(frame, nil, false, Cursor{})
	279	+	require.NoError(t, err)
	280	+	require.False(t, retry.Update().Snapshot)
	281	+	require.Equal(t, []int{1}, rowIndexes(retry.Update()))
	282	+	require.Equal(t, abortedJSON, retry.JSON())
	283	+	require.NoError(t, retry.Commit())
	284	+
	285	+	steady, err := renderer.Prepare(frame, nil, false, Cursor{})
	286	+	require.NoError(t, err)
	287	+	require.Empty(t, steady.Update().Rows)
	288	+	}
	289	+
	290	+	func TestPrepareDoesNotAliasCallerFrame(t *testing.T) {
	291	+	frame := core.NewFrame(2, 1)
	292	+	renderer, err := New(Options{})
	293	+	require.NoError(t, err)
	294	+	prepared, err := renderer.Prepare(frame, nil, false, Cursor{})
	295	+	require.NoError(t, err)
	296	+	require.NoError(t, prepared.Commit())
	297	+
	298	+	// Mutating the caller frame after Commit must be seen as a change.
	299	+	frame.Set(0, 0, core.Cell{Rune: 'Q', Style: core.DefaultStyle()})
	300	+	next, err := renderer.Prepare(frame, nil, false, Cursor{})
	301	+	require.NoError(t, err)
	302	+	require.Equal(t, []int{0}, rowIndexes(next.Update()))
	303	+	}
	304	+
	305	+	func rowIndexes(update Update) []int {
	306	+	rows := make([]int, 0, len(update.Rows))
	307	+	for _, row := range update.Rows {
	308	+	rows = append(rows, row.Row)
	309	+	}
	310	+	return rows
	311	+	}


@@ -228,3 +228,84 @@ func TestRendererInvalidatesCopiedDrawOnReset(t *testing.T) {
228	require.True(t, next.Update().Snapshot)	228		require.True(t, next.Update().Snapshot)
229	require.NoError(t, next.Commit())	229		require.NoError(t, next.Commit())
230	}	230		}
		231	+
		232	+	func TestPreparedDrawIsIndependentOfLaterPrepares(t *testing.T) {
		233	+	frame := core.NewFrame(4, 3)
		234	+	frame.Set(0, 0, core.Cell{Rune: 'A', Style: core.DefaultStyle()})
		235	+	renderer, err := New(Options{})
		236	+	require.NoError(t, err)
		237	+
		238	+	first, err := renderer.Prepare(frame, nil, false, Cursor{})
		239	+	require.NoError(t, err)
		240	+	firstUpdate, firstJSON := first.Update(), first.JSON()
		241	+	require.NoError(t, first.Commit())
		242	+
		243	+	// The caller mutates its frame in place; neither the earlier draw nor the
		244	+	// committed shadow may observe it.
		245	+	frame.Set(1, 1, core.Cell{Rune: 'B', Style: core.DefaultStyle()})
		246	+	second, err := renderer.Prepare(frame, nil, false, Cursor{})
		247	+	require.NoError(t, err)
		248	+	require.Equal(t, []int{1}, rowIndexes(second.Update()))
		249	+	secondUpdate, secondJSON := second.Update(), second.JSON()
		250	+	require.NoError(t, second.Commit())
		251	+
		252	+	frame.Set(2, 2, core.Cell{Rune: 'C', Style: core.DefaultStyle()})
		253	+	third, err := renderer.Prepare(frame, nil, false, Cursor{})
		254	+	require.NoError(t, err)
		255	+	require.Equal(t, []int{2}, rowIndexes(third.Update()))
		256	+	require.NoError(t, third.Commit())
		257	+
		258	+	require.Equal(t, firstUpdate, first.Update())
		259	+	require.Equal(t, firstJSON, first.JSON())
		260	+	require.Equal(t, secondUpdate, second.Update())
		261	+	require.Equal(t, secondJSON, second.JSON())
		262	+	}
		263	+
		264	+	func TestAbortThenPrepareKeepsCommittedShadow(t *testing.T) {
		265	+	frame := core.NewFrame(3, 2)
		266	+	renderer, err := New(Options{})
		267	+	require.NoError(t, err)
		268	+	first, err := renderer.Prepare(frame, nil, false, Cursor{})
		269	+	require.NoError(t, err)
		270	+	require.NoError(t, first.Commit())
		271	+
		272	+	frame.Set(0, 1, core.Cell{Rune: 'X', Style: core.DefaultStyle()})
		273	+	aborted, err := renderer.Prepare(frame, nil, false, Cursor{})
		274	+	require.NoError(t, err)
		275	+	abortedJSON := aborted.JSON()
		276	+	require.NoError(t, aborted.Abort())
		277	+
		278	+	retry, err := renderer.Prepare(frame, nil, false, Cursor{})
		279	+	require.NoError(t, err)
		280	+	require.False(t, retry.Update().Snapshot)
		281	+	require.Equal(t, []int{1}, rowIndexes(retry.Update()))
		282	+	require.Equal(t, abortedJSON, retry.JSON())
		283	+	require.NoError(t, retry.Commit())
		284	+
		285	+	steady, err := renderer.Prepare(frame, nil, false, Cursor{})
		286	+	require.NoError(t, err)
		287	+	require.Empty(t, steady.Update().Rows)
		288	+	}
		289	+
		290	+	func TestPrepareDoesNotAliasCallerFrame(t *testing.T) {
		291	+	frame := core.NewFrame(2, 1)
		292	+	renderer, err := New(Options{})
		293	+	require.NoError(t, err)
		294	+	prepared, err := renderer.Prepare(frame, nil, false, Cursor{})
		295	+	require.NoError(t, err)
		296	+	require.NoError(t, prepared.Commit())
		297	+
		298	+	// Mutating the caller frame after Commit must be seen as a change.
		299	+	frame.Set(0, 0, core.Cell{Rune: 'Q', Style: core.DefaultStyle()})
		300	+	next, err := renderer.Prepare(frame, nil, false, Cursor{})
		301	+	require.NoError(t, err)
		302	+	require.Equal(t, []int{0}, rowIndexes(next.Update()))
		303	+	}
		304	+
		305	+	func rowIndexes(update Update) []int {
		306	+	rows := make([]int, 0, len(update.Rows))
		307	+	for _, row := range update.Rows {
		308	+	rows = append(rows, row.Row)
		309	+	}
		310	+	return rows
		311	+	}