go-json/decode_slice.go

package json

import (
	"reflect"
	"sync"
	"unsafe"
)

type sliceDecoder struct {
	elemType     *rtype
	valueDecoder decoder
	size         uintptr
	arrayPool    sync.Pool
}

// If use reflect.SliceHeader, data type is uintptr.
// In this case, Go compiler cannot trace reference created by newArray().
// So, define using unsafe.Pointer as data type
type sliceHeader struct {
	data unsafe.Pointer
	len  int
	cap  int
}

func newSliceDecoder(dec decoder, elemType *rtype, size uintptr) *sliceDecoder {
	return &sliceDecoder{
		valueDecoder: dec,
		elemType:     elemType,
		size:         size,
		arrayPool: sync.Pool{
			New: func() interface{} {
				cap := 2
				return &sliceHeader{
					data: newArray(elemType, cap),
					len:  0,
					cap:  cap,
				}
			},
		},
	}
}

func (d *sliceDecoder) newSlice() *sliceHeader {
	slice := d.arrayPool.Get().(*sliceHeader)
	slice.len = 0
	return slice
}

func (d *sliceDecoder) releaseSlice(p *sliceHeader) {
	d.arrayPool.Put(p)
}

//go:linkname copySlice reflect.typedslicecopy
func copySlice(elemType *rtype, dst, src reflect.SliceHeader) int

//go:linkname newArray reflect.unsafe_NewArray
func newArray(*rtype, int) unsafe.Pointer

func (d *sliceDecoder) decode(buf []byte, cursor int64, p uintptr) (int64, error) {
	buflen := int64(len(buf))
	for ; cursor < buflen; cursor++ {
		switch buf[cursor] {
		case ' ', '\n', '\t', '\r':
			continue
		case '[':
			idx := 0
			slice := d.newSlice()
			cap := slice.cap
			data := slice.data
			for {
				cursor++
				if cap <= idx {
					src := reflect.SliceHeader{Data: uintptr(data), Len: idx, Cap: cap}
					cap *= 2
					data = newArray(d.elemType, cap)
					dst := reflect.SliceHeader{Data: uintptr(data), Len: idx, Cap: cap}
					copySlice(d.elemType, dst, src)
				}
				c, err := d.valueDecoder.decode(buf, cursor, uintptr(data)+uintptr(idx)*d.size)
				if err != nil {
					return 0, err
				}
				cursor = c
				cursor = skipWhiteSpace(buf, cursor)
				switch buf[cursor] {
				case ']':
					slice.cap = cap
					slice.len = idx + 1
					slice.data = data
					dstCap := idx + 1
					dst := reflect.SliceHeader{
						Data: uintptr(newArray(d.elemType, dstCap)),
						Len:  idx + 1,
						Cap:  dstCap,
					}
					copySlice(d.elemType, dst, reflect.SliceHeader{
						Data: uintptr(slice.data),
						Len:  slice.len,
						Cap:  slice.cap,
					})
					*(*reflect.SliceHeader)(unsafe.Pointer(p)) = dst
					d.releaseSlice(slice)
					cursor++
					return cursor, nil
				case ',':
					idx++
					continue
				default:
					slice.cap = cap
					slice.data = data
					d.releaseSlice(slice)
					return 0, errInvalidCharacter(buf[cursor], "slice", cursor)
				}
			}
		}
	}
	return 0, errUnexpectedEndOfJSON("slice", cursor)
}