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codec.go
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codec.go
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package json
import (
"fmt"
"reflect"
"unsafe"
)
const (
maxAcceptableTypeAddrRange = 1024 * 1024 * 2 // 2 Mib
)
var (
cachedDecoder []decoder
baseTypeAddr uintptr
maxTypeAddr uintptr
)
//go:linkname typelinks reflect.typelinks
func typelinks() ([]unsafe.Pointer, [][]int32)
//go:linkname rtypeOff reflect.rtypeOff
func rtypeOff(unsafe.Pointer, int32) unsafe.Pointer
func setupCodec() error {
sections, offsets := typelinks()
if len(sections) != 1 {
return fmt.Errorf("failed to get sections")
}
if len(offsets) != 1 {
return fmt.Errorf("failed to get offsets")
}
section := sections[0]
offset := offsets[0]
var (
min uintptr = uintptr(^uint(0))
max uintptr = 0
)
for i := 0; i < len(offset); i++ {
typ := (*rtype)(rtypeOff(section, offset[i]))
addr := uintptr(unsafe.Pointer(typ))
if min > addr {
min = addr
}
if max < addr {
max = addr
}
if typ.Kind() == reflect.Ptr {
addr = uintptr(unsafe.Pointer(typ.Elem()))
if min > addr {
min = addr
}
if max < addr {
max = addr
}
}
}
addrRange := max - min
if addrRange == 0 {
return fmt.Errorf("failed to get address range of types")
}
if addrRange > maxAcceptableTypeAddrRange {
return fmt.Errorf("too big address range %d", addrRange)
}
cachedDecoder = make([]decoder, addrRange)
baseTypeAddr = min
maxTypeAddr = max
return nil
}
func init() {
_ = setupCodec()
}