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Merged
jensneuse merged 1 commit into from
Sep 29, 2025
Merged

feat: reduce alloc when parsing large objects and arrays #11

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147 changes: 121 additions & 26 deletions parser.go
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Original file line number Diff line number Diff line change
Expand Up @@ -97,26 +97,55 @@ func (p *Parser) ParseBytesWithoutCache(b []byte) (*Value, error) {

type cache struct {
vs []Value
nx *cache // next
lt *cache // last
}

func (c *cache) reset() {
c.vs = c.vs[:0]
c.lt = nil
if c.nx != nil {
c.nx.reset()
}
}

const (
preAllocatedCacheSize = 341 // 32kb class size
maxAllocatedCacheSize = 10922 // 1MB
)

func (c *cache) getValue() *Value {
if c == nil {
return &Value{}
}
if cap(c.vs) > len(c.vs) {
c.vs = c.vs[:len(c.vs)+1]
} else {
if len(c.vs) == 0 {
c.vs = make([]Value, 4)
} else {
c.vs = make([]Value, 1, len(c.vs)*2)
readSrc := c
if readSrc.lt != nil {
readSrc = readSrc.lt
}
switch {
case cap(readSrc.vs) == 0:
// initial state
readSrc.vs = make([]Value, 1, preAllocatedCacheSize)

case cap(readSrc.vs) > len(readSrc.vs):
readSrc.vs = readSrc.vs[:len(readSrc.vs)+1]

default:
if readSrc.nx == nil {
nextLen := len(readSrc.vs) * 2
if nextLen > maxAllocatedCacheSize {
nextLen = maxAllocatedCacheSize
}
readSrc.nx = &cache{
vs: make([]Value, 0, nextLen),
}
}
c.lt = readSrc.nx
readSrc = readSrc.nx
readSrc.vs = readSrc.vs[:len(readSrc.vs)+1]
}
return &c.vs[len(c.vs)-1]
// Do not reset the value, since the caller must properly init it.
return &readSrc.vs[len(readSrc.vs)-1]
}

func skipWS(s string) string {
Expand Down Expand Up @@ -540,29 +569,41 @@ func parseRawNumber(s string) (string, string, error) {
type Object struct {
kvs []kv
keysUnescaped bool
nx *Object
lt *Object
}

func (o *Object) reset() {
o.kvs = o.kvs[:0]
o.keysUnescaped = false
o.lt = nil
if o.nx != nil {
o.nx.reset()
}
}

// MarshalTo appends marshaled o to dst and returns the result.
func (o *Object) MarshalTo(dst []byte) []byte {
dst = append(dst, '{')
for i, kv := range o.kvs {
if o.keysUnescaped {
dst = escapeString(dst, kv.k)
} else {
dst = append(dst, '"')
dst = append(dst, kv.k...)
dst = append(dst, '"')
}
dst = append(dst, ':')
dst = kv.v.MarshalTo(dst)
if i != len(o.kvs)-1 {
dst = append(dst, ',')
srcKV := o
lastN := o.Len()
n := 0
for srcKV != nil {
for _, kv := range srcKV.kvs {
if srcKV.keysUnescaped {
dst = escapeString(dst, kv.k)
} else {
dst = append(dst, '"')
dst = append(dst, kv.k...)
dst = append(dst, '"')
}
dst = append(dst, ':')
dst = kv.v.MarshalTo(dst)
if n++; n != lastN {
dst = append(dst, ',')
}
}
srcKV = srcKV.nx
}
dst = append(dst, '}')
return dst
Expand All @@ -579,13 +620,45 @@ func (o *Object) String() string {
return b2s(b)
}

const (
preAllocatedObjectKVs = 170 // 8kb class
maxAllocatedObjectKVS = 21845 // 1MB class
)

func (o *Object) getKV() *kv {
if cap(o.kvs) > len(o.kvs) {
o.kvs = o.kvs[:len(o.kvs)+1]
} else {
o.kvs = append(o.kvs, kv{})
kvSrc := o
if kvSrc.lt != nil {
kvSrc = kvSrc.lt
}
return &o.kvs[len(o.kvs)-1]
switch {
case cap(kvSrc.kvs) == 0:
// initial state
kvSrc.kvs = append(kvSrc.kvs, kv{})

case cap(kvSrc.kvs) > len(kvSrc.kvs):
kvSrc.kvs = kvSrc.kvs[:len(kvSrc.kvs)+1]

default:
if cap(kvSrc.kvs) < preAllocatedObjectKVs {
kvSrc.kvs = append(kvSrc.kvs, kv{})
break
}
// new chain
if kvSrc.nx == nil {
nextLen := len(kvSrc.kvs) * 2
if nextLen > maxAllocatedObjectKVS {
nextLen = maxAllocatedObjectKVS
}
kvSrc.nx = &Object{
kvs: make([]kv, 0, nextLen),
}
}
kvSrc = kvSrc.nx
o.lt = kvSrc
kvSrc.kvs = kvSrc.kvs[:len(kvSrc.kvs)+1]
}

return &kvSrc.kvs[len(kvSrc.kvs)-1]
}

func (o *Object) unescapeKeys() {
Expand All @@ -597,12 +670,18 @@ func (o *Object) unescapeKeys() {
kv := &kvs[i]
kv.k = unescapeStringBestEffort(kv.k)
}
if o.nx != nil {
o.nx.unescapeKeys()
}
o.keysUnescaped = true
}

// Len returns the number of items in the o.
func (o *Object) Len() int {
return len(o.kvs)
if o.nx == nil {
return len(o.kvs)
}
return len(o.kvs) + o.nx.Len()
}

// Get returns the value for the given key in the o.
Expand All @@ -618,6 +697,11 @@ func (o *Object) Get(key string) *Value {
return kv.v
}
}
if o.nx != nil {
if v := o.nx.Get(key); v != nil {
return v
}
}
}

// Slow path - unescape object keys.
Expand All @@ -628,6 +712,13 @@ func (o *Object) Get(key string) *Value {
return kv.v
}
}

if o.nx != nil {
if v := o.nx.Get(key); v != nil {
return v
}
}

return nil
}

Expand All @@ -645,6 +736,10 @@ func (o *Object) Visit(f func(key []byte, v *Value)) {
for _, kv := range o.kvs {
f(s2b(kv.k), kv.v)
}

if o.nx != nil {
o.nx.Visit(f)
}
}

// Value represents any JSON value.
Expand Down
24 changes: 24 additions & 0 deletions parser_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1288,3 +1288,27 @@ func TestParseWithoutCache(t *testing.T) {
t.Fatalf("unexpected value for key=%q; got %q; want %q", "foo", sb, "bar")
}
}

func TestMarshalTo(t *testing.T) {
fileData := getFromFile("testdata/bunchFields.json")
var p Parser
v, err := p.Parse(fileData)
if err != nil {
t.Fatalf("cannot parse json: %s", err)
}
data := make([]byte, 0, len(fileData))
data = v.MarshalTo(data)
// check
var p2 Parser
v, err = p2.ParseBytes(data)
if err != nil {
t.Fatalf("cannot parse json: %s", err)
}
o, err := v.Object()
if err != nil {
t.Fatalf("expected object, got: %s", o.String())
}
if o.Len() != 871 {
t.Fatalf("expected 871 fields, got %d", o.Len())
}
}
17 changes: 13 additions & 4 deletions parser_timing_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,7 @@ func BenchmarkObjectGet(b *testing.B) {
}

func benchmarkObjectGet(b *testing.B, itemsCount, lookupsCount int) {
var benchPool ParserPool
b.StopTimer()
var ss []string
for i := 0; i < itemsCount; i++ {
Expand Down Expand Up @@ -127,10 +128,15 @@ func BenchmarkMarshalTo(b *testing.B) {
b.Run("twitter", func(b *testing.B) {
benchmarkMarshalTo(b, twitterFixture)
})
b.Run("20mb", func(b *testing.B) {
benchmarkMarshalTo(b, huge20MbFixture)
})
}

var benchPoolMarshalTo ParserPool

func benchmarkMarshalTo(b *testing.B, s string) {
p := benchPool.Get()
p := benchPoolMarshalTo.Get()
v, err := p.Parse(s)
if err != nil {
panic(fmt.Errorf("unexpected error: %s", err))
Expand All @@ -146,7 +152,7 @@ func benchmarkMarshalTo(b *testing.B, s string) {
b = v.MarshalTo(b[:0])
}
})
benchPool.Put(p)
benchPoolMarshalTo.Put(p)
}

func BenchmarkParse(b *testing.B) {
Expand Down Expand Up @@ -180,6 +186,9 @@ var (
canadaFixture = getFromFile("testdata/canada.json")
citmFixture = getFromFile("testdata/citm_catalog.json")
twitterFixture = getFromFile("testdata/twitter.json")

// 20mb is a huge (stressful) fixture from https://examplefile.com/code/json/20-mb-json
huge20MbFixture = getFromFile("testdata/20mb.json")
)

func getFromFile(filename string) string {
Expand Down Expand Up @@ -209,6 +218,7 @@ func benchmarkParse(b *testing.B, s string) {
}

func benchmarkFastJSONParse(b *testing.B, s string) {
var benchPool ParserPool
b.ReportAllocs()
b.SetBytes(int64(len(s)))
b.RunParallel(func(pb *testing.PB) {
Expand All @@ -227,6 +237,7 @@ func benchmarkFastJSONParse(b *testing.B, s string) {
}

func benchmarkFastJSONParseGet(b *testing.B, s string) {
var benchPool ParserPool
b.ReportAllocs()
b.SetBytes(int64(len(s)))
b.RunParallel(func(pb *testing.PB) {
Expand Down Expand Up @@ -264,8 +275,6 @@ func benchmarkFastJSONParseGet(b *testing.B, s string) {
})
}

var benchPool ParserPool

func benchmarkStdJSONParseMap(b *testing.B, s string) {
b.ReportAllocs()
b.SetBytes(int64(len(s)))
Expand Down
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