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encoding/json: simplify folded name logic
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The folded name logic (despite all attempts to optimize it)
was fundamentally an O(n) operation where every field in a struct
needed to be linearly scanned in order to find a match.
This made unmashaling of unknown fields always O(n).
Instead of optimizing the comparison for each field,
make it such that we can look up a name in O(1).

We accomplish this by maintaining a map keyed by pre-folded names,
which we can pre-calculate when processing the struct type.
Using a stack-allocated buffer, we can fold the input name and
look up its presence in the map.

Also, instead of mapping from names to indexes,
map directly to a pointer to the field information.
The memory cost of this is the same and avoids an extra slice index.

The new logic is both simpler and faster.

Performance:

	name                   old time/op    new time/op    delta
	CodeDecoder           2.47ms ± 4%    2.42ms ± 2%  -1.83%  (p=0.022 n=10+9)
	UnicodeDecoder         259ns ± 2%     248ns ± 1%  -4.32%  (p=0.000 n=10+10)
	DecoderStream          150ns ± 1%     149ns ± 1%    ~     (p=0.516 n=10+10)
	CodeUnmarshal         3.13ms ± 2%    3.09ms ± 2%  -1.37%  (p=0.022 n=10+9)
	CodeUnmarshalReuse    2.50ms ± 1%    2.45ms ± 1%  -1.96%  (p=0.001 n=8+9)
	UnmarshalString       67.1ns ± 5%    64.5ns ± 5%  -3.90%  (p=0.005 n=10+10)
	UnmarshalFloat64      60.1ns ± 4%    58.4ns ± 2%  -2.89%  (p=0.002 n=10+8)
	UnmarshalInt64        51.0ns ± 4%    49.2ns ± 1%  -3.53%  (p=0.001 n=10+8)
	Issue10335            80.7ns ± 2%    79.2ns ± 1%  -1.82%  (p=0.016 n=10+8)
	Issue34127            28.6ns ± 3%    28.8ns ± 3%    ~     (p=0.388 n=9+10)
	Unmapped               177ns ± 2%     177ns ± 2%    ~     (p=0.956 n=10+10)

Change-Id: I478b2b958f5a63a69c9a991a39cd5ffb43244a2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/471196
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Johan Brandhorst-Satzkorn <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
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@dsnet @gopherbot
dsnet authored and gopherbot committed Feb 27, 2023
1 parent 2de406b commit b9b8cec
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Showing 4 changed files with 78 additions and 238 deletions.
17 changes: 3 additions & 14 deletions src/encoding/json/decode.go
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Original file line number Diff line number Diff line change
Expand Up @@ -690,20 +690,9 @@ func (d *decodeState) object(v reflect.Value) error {
}
subv = mapElem
} else {
var f *field
if i, ok := fields.nameIndex[string(key)]; ok {
// Found an exact name match.
f = &fields.list[i]
} else {
// Fall back to the expensive case-insensitive
// linear search.
for i := range fields.list {
ff := &fields.list[i]
if ff.equalFold(ff.nameBytes, key) {
f = ff
break
}
}
f := fields.byExactName[string(key)]
if f == nil {
f = fields.byFoldedName[string(foldName(key))]
}
if f != nil {
subv = v
Expand Down
20 changes: 12 additions & 8 deletions src/encoding/json/encode.go
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Original file line number Diff line number Diff line change
Expand Up @@ -672,8 +672,9 @@ type structEncoder struct {
}

type structFields struct {
list []field
nameIndex map[string]int
list []field
byExactName map[string]*field
byFoldedName map[string]*field
}

func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
Expand Down Expand Up @@ -1033,8 +1034,7 @@ func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool)
// A field represents a single field found in a struct.
type field struct {
name string
nameBytes []byte // []byte(name)
equalFold func(s, t []byte) bool // bytes.EqualFold or equivalent
nameBytes []byte // []byte(name)

nameNonEsc string // `"` + name + `":`
nameEscHTML string // `"` + HTMLEscape(name) + `":`
Expand Down Expand Up @@ -1161,7 +1161,6 @@ func typeFields(t reflect.Type) structFields {
quoted: quoted,
}
field.nameBytes = []byte(field.name)
field.equalFold = foldFunc(field.nameBytes)

// Build nameEscHTML and nameNonEsc ahead of time.
nameEscBuf = appendHTMLEscape(nameEscBuf[:0], field.nameBytes)
Expand Down Expand Up @@ -1240,11 +1239,16 @@ func typeFields(t reflect.Type) structFields {
f := &fields[i]
f.encoder = typeEncoder(typeByIndex(t, f.index))
}
nameIndex := make(map[string]int, len(fields))
exactNameIndex := make(map[string]*field, len(fields))
foldedNameIndex := make(map[string]*field, len(fields))
for i, field := range fields {
nameIndex[field.name] = i
exactNameIndex[field.name] = &fields[i]
// For historical reasons, first folded match takes precedence.
if _, ok := foldedNameIndex[string(foldName(field.nameBytes))]; !ok {
foldedNameIndex[string(foldName(field.nameBytes))] = &fields[i]
}
}
return structFields{fields, nameIndex}
return structFields{fields, exactNameIndex, foldedNameIndex}
}

// dominantField looks through the fields, all of which are known to
Expand Down
147 changes: 27 additions & 120 deletions src/encoding/json/fold.go
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Original file line number Diff line number Diff line change
Expand Up @@ -5,137 +5,44 @@
package json

import (
"bytes"
"unicode"
"unicode/utf8"
)

const (
caseMask = ^byte(0x20) // Mask to ignore case in ASCII.
kelvin = '\u212a'
smallLongEss = '\u017f'
)

// foldFunc returns one of four different case folding equivalence
// functions, from most general (and slow) to fastest:
//
// 1) bytes.EqualFold, if the key s contains any non-ASCII UTF-8
// 2) equalFoldRight, if s contains special folding ASCII ('k', 'K', 's', 'S')
// 3) asciiEqualFold, no special, but includes non-letters (including _)
// 4) simpleLetterEqualFold, no specials, no non-letters.
//
// The letters S and K are special because they map to 3 runes, not just 2:
// - S maps to s and to U+017F 'ſ' Latin small letter long s
// - k maps to K and to U+212A 'K' Kelvin sign
//
// See https://play.golang.org/p/tTxjOc0OGo
//
// The returned function is specialized for matching against s and
// should only be given s. It's not curried for performance reasons.
func foldFunc(s []byte) func(s, t []byte) bool {
nonLetter := false
special := false // special letter
for _, b := range s {
if b >= utf8.RuneSelf {
return bytes.EqualFold
}
upper := b & caseMask
if upper < 'A' || upper > 'Z' {
nonLetter = true
} else if upper == 'K' || upper == 'S' {
// See above for why these letters are special.
special = true
}
}
if special {
return equalFoldRight
}
if nonLetter {
return asciiEqualFold
}
return simpleLetterEqualFold
// foldName returns a folded string such that foldName(x) == foldName(y)
// is identical to bytes.EqualFold(x, y).
func foldName(in []byte) []byte {
// This is inlinable to take advantage of "function outlining".
var arr [32]byte // large enough for most JSON names
return appendFoldedName(arr[:0], in)
}

// equalFoldRight is a specialization of bytes.EqualFold when s is
// known to be all ASCII (including punctuation), but contains an 's',
// 'S', 'k', or 'K', requiring a Unicode fold on the bytes in t.
// See comments on foldFunc.
func equalFoldRight(s, t []byte) bool {
for _, sb := range s {
if len(t) == 0 {
return false
}
tb := t[0]
if tb < utf8.RuneSelf {
if sb != tb {
sbUpper := sb & caseMask
if 'A' <= sbUpper && sbUpper <= 'Z' {
if sbUpper != tb&caseMask {
return false
}
} else {
return false
}
func appendFoldedName(out, in []byte) []byte {
for i := 0; i < len(in); {
// Handle single-byte ASCII.
if c := in[i]; c < utf8.RuneSelf {
if 'a' <= c && c <= 'z' {
c -= 'a' - 'A'
}
t = t[1:]
out = append(out, c)
i++
continue
}
// sb is ASCII and t is not. t must be either kelvin
// sign or long s; sb must be s, S, k, or K.
tr, size := utf8.DecodeRune(t)
switch sb {
case 's', 'S':
if tr != smallLongEss {
return false
}
case 'k', 'K':
if tr != kelvin {
return false
}
default:
return false
}
t = t[size:]

}
return len(t) == 0
}

// asciiEqualFold is a specialization of bytes.EqualFold for use when
// s is all ASCII (but may contain non-letters) and contains no
// special-folding letters.
// See comments on foldFunc.
func asciiEqualFold(s, t []byte) bool {
if len(s) != len(t) {
return false
}
for i, sb := range s {
tb := t[i]
if sb == tb {
continue
}
if ('a' <= sb && sb <= 'z') || ('A' <= sb && sb <= 'Z') {
if sb&caseMask != tb&caseMask {
return false
}
} else {
return false
}
// Handle multi-byte Unicode.
r, n := utf8.DecodeRune(in[i:])
out = utf8.AppendRune(out, foldRune(r))
i += n
}
return true
return out
}

// simpleLetterEqualFold is a specialization of bytes.EqualFold for
// use when s is all ASCII letters (no underscores, etc) and also
// doesn't contain 'k', 'K', 's', or 'S'.
// See comments on foldFunc.
func simpleLetterEqualFold(s, t []byte) bool {
if len(s) != len(t) {
return false
}
for i, b := range s {
if b&caseMask != t[i]&caseMask {
return false
// foldRune is returns the smallest rune for all runes in the same fold set.
func foldRune(r rune) rune {
for {
r2 := unicode.SimpleFold(r)
if r2 <= r {
return r2
}
r = r2
}
return true
}
132 changes: 36 additions & 96 deletions src/encoding/json/fold_test.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -6,105 +6,45 @@ package json

import (
"bytes"
"strings"
"testing"
"unicode/utf8"
)

var foldTests = []struct {
fn func(s, t []byte) bool
s, t string
want bool
}{
{equalFoldRight, "", "", true},
{equalFoldRight, "a", "a", true},
{equalFoldRight, "", "a", false},
{equalFoldRight, "a", "", false},
{equalFoldRight, "a", "A", true},
{equalFoldRight, "AB", "ab", true},
{equalFoldRight, "AB", "ac", false},
{equalFoldRight, "sbkKc", "ſbKKc", true},
{equalFoldRight, "SbKkc", "ſbKKc", true},
{equalFoldRight, "SbKkc", "ſbKK", false},
{equalFoldRight, "e", "é", false},
{equalFoldRight, "s", "S", true},

{simpleLetterEqualFold, "", "", true},
{simpleLetterEqualFold, "abc", "abc", true},
{simpleLetterEqualFold, "abc", "ABC", true},
{simpleLetterEqualFold, "abc", "ABCD", false},
{simpleLetterEqualFold, "abc", "xxx", false},

{asciiEqualFold, "a_B", "A_b", true},
{asciiEqualFold, "aa@", "aa`", false}, // verify 0x40 and 0x60 aren't case-equivalent
}

func TestFold(t *testing.T) {
for i, tt := range foldTests {
if got := tt.fn([]byte(tt.s), []byte(tt.t)); got != tt.want {
t.Errorf("%d. %q, %q = %v; want %v", i, tt.s, tt.t, got, tt.want)
}
truth := strings.EqualFold(tt.s, tt.t)
if truth != tt.want {
t.Errorf("strings.EqualFold doesn't agree with case %d", i)
}
func FuzzEqualFold(f *testing.F) {
for _, ss := range [][2]string{
{"", ""},
{"123abc", "123ABC"},
{"αβδ", "ΑΒΔ"},
{"abc", "xyz"},
{"abc", "XYZ"},
{"1", "2"},
{"hello, world!", "hello, world!"},
{"hello, world!", "Hello, World!"},
{"hello, world!", "HELLO, WORLD!"},
{"hello, world!", "jello, world!"},
{"γειά, κόσμε!", "γειά, κόσμε!"},
{"γειά, κόσμε!", "Γειά, Κόσμε!"},
{"γειά, κόσμε!", "ΓΕΙΆ, ΚΌΣΜΕ!"},
{"γειά, κόσμε!", "ΛΕΙΆ, ΚΌΣΜΕ!"},
{"AESKey", "aesKey"},
{"AESKEY", "aes_key"},
{"aes_key", "AES_KEY"},
{"AES_KEY", "aes-key"},
{"aes-key", "AES-KEY"},
{"AES-KEY", "aesKey"},
{"aesKey", "AesKey"},
{"AesKey", "AESKey"},
{"AESKey", "aeskey"},
{"DESKey", "aeskey"},
{"AES Key", "aeskey"},
} {
f.Add([]byte(ss[0]), []byte(ss[1]))
}
}

func TestFoldAgainstUnicode(t *testing.T) {
var buf1, buf2 []byte
var runes []rune
for i := 0x20; i <= 0x7f; i++ {
runes = append(runes, rune(i))
}
runes = append(runes, kelvin, smallLongEss)

funcs := []struct {
name string
fold func(s, t []byte) bool
letter bool // must be ASCII letter
simple bool // must be simple ASCII letter (not 'S' or 'K')
}{
{
name: "equalFoldRight",
fold: equalFoldRight,
},
{
name: "asciiEqualFold",
fold: asciiEqualFold,
simple: true,
},
{
name: "simpleLetterEqualFold",
fold: simpleLetterEqualFold,
simple: true,
letter: true,
},
}

for _, ff := range funcs {
for _, r := range runes {
if r >= utf8.RuneSelf {
continue
}
if ff.letter && !isASCIILetter(byte(r)) {
continue
}
if ff.simple && (r == 's' || r == 'S' || r == 'k' || r == 'K') {
continue
}
for _, r2 := range runes {
buf1 = append(utf8.AppendRune(append(buf1[:0], 'x'), r), 'x')
buf2 = append(utf8.AppendRune(append(buf2[:0], 'x'), r2), 'x')
want := bytes.EqualFold(buf1, buf2)
if got := ff.fold(buf1, buf2); got != want {
t.Errorf("%s(%q, %q) = %v; want %v", ff.name, buf1, buf2, got, want)
}
}
equalFold := func(x, y []byte) bool { return string(foldName(x)) == string(foldName(y)) }
f.Fuzz(func(t *testing.T, x, y []byte) {
got := equalFold(x, y)
want := bytes.EqualFold(x, y)
if got != want {
t.Errorf("equalFold(%q, %q) = %v, want %v", x, y, got, want)
}
}
}

func isASCIILetter(b byte) bool {
return ('A' <= b && b <= 'Z') || ('a' <= b && b <= 'z')
})
}

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