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|
package leb128
import (
"errors"
"fmt"
"io"
)
const (
maxVarintLen32 = 5
maxVarintLen33 = maxVarintLen32
maxVarintLen64 = 10
int33Mask int64 = 1 << 7
int33Mask2 = ^int33Mask
int33Mask3 = 1 << 6
int33Mask4 = 8589934591 // 2^33-1
int33Mask5 = 1 << 32
int33Mask6 = int33Mask4 + 1 // 2^33
int64Mask3 = 1 << 6
int64Mask4 = ^0
)
var (
errOverflow32 = errors.New("overflows a 32-bit integer")
errOverflow33 = errors.New("overflows a 33-bit integer")
errOverflow64 = errors.New("overflows a 64-bit integer")
)
// EncodeInt32 encodes the signed value into a buffer in LEB128 format
//
// See https://en.wikipedia.org/wiki/LEB128#Encode_signed_integer
func EncodeInt32(value int32) []byte {
return EncodeInt64(int64(value))
}
// EncodeInt64 encodes the signed value into a buffer in LEB128 format
//
// See https://en.wikipedia.org/wiki/LEB128#Encode_signed_integer
func EncodeInt64(value int64) (buf []byte) {
for {
// Take 7 remaining low-order bits from the value into b.
b := uint8(value & 0x7f)
// Extract the sign bit.
s := uint8(value & 0x40)
value >>= 7
// The encoding unsigned numbers is simpler as it only needs to check if the value is non-zero to tell if there
// are more bits to encode. Signed is a little more complicated as you have to double-check the sign bit.
// If either case, set the high-order bit to tell the reader there are more bytes in this int.
if (value != -1 || s == 0) && (value != 0 || s != 0) {
b |= 0x80
}
// Append b into the buffer
buf = append(buf, b)
if b&0x80 == 0 {
break
}
}
return buf
}
// EncodeUint32 encodes the value into a buffer in LEB128 format
//
// See https://en.wikipedia.org/wiki/LEB128#Encode_unsigned_integer
func EncodeUint32(value uint32) []byte {
return EncodeUint64(uint64(value))
}
// EncodeUint64 encodes the value into a buffer in LEB128 format
//
// See https://en.wikipedia.org/wiki/LEB128#Encode_unsigned_integer
func EncodeUint64(value uint64) (buf []byte) {
// This is effectively a do/while loop where we take 7 bits of the value and encode them until it is zero.
for {
// Take 7 remaining low-order bits from the value into b.
b := uint8(value & 0x7f)
value = value >> 7
// If there are remaining bits, the value won't be zero: Set the high-
// order bit to tell the reader there are more bytes in this uint.
if value != 0 {
b |= 0x80
}
// Append b into the buffer
buf = append(buf, b)
if b&0x80 == 0 {
return buf
}
}
}
type nextByte func(i int) (byte, error)
func DecodeUint32(r io.ByteReader) (ret uint32, bytesRead uint64, err error) {
return decodeUint32(func(_ int) (byte, error) { return r.ReadByte() })
}
func LoadUint32(buf []byte) (ret uint32, bytesRead uint64, err error) {
return decodeUint32(func(i int) (byte, error) {
if i >= len(buf) {
return 0, io.EOF
}
return buf[i], nil
})
}
func decodeUint32(next nextByte) (ret uint32, bytesRead uint64, err error) {
// Derived from https://github.com/golang/go/blob/go1.20/src/encoding/binary/varint.go
// with the modification on the overflow handling tailored for 32-bits.
var s uint32
for i := 0; i < maxVarintLen32; i++ {
b, err := next(i)
if err != nil {
return 0, 0, err
}
if b < 0x80 {
// Unused bits must be all zero.
if i == maxVarintLen32-1 && (b&0xf0) > 0 {
return 0, 0, errOverflow32
}
return ret | uint32(b)<<s, uint64(i) + 1, nil
}
ret |= (uint32(b) & 0x7f) << s
s += 7
}
return 0, 0, errOverflow32
}
func LoadUint64(buf []byte) (ret uint64, bytesRead uint64, err error) {
bufLen := len(buf)
if bufLen == 0 {
return 0, 0, io.EOF
}
// Derived from https://github.com/golang/go/blob/go1.20/src/encoding/binary/varint.go
var s uint64
for i := 0; i < maxVarintLen64; i++ {
if i >= bufLen {
return 0, 0, io.EOF
}
b := buf[i]
if b < 0x80 {
// Unused bits (non first bit) must all be zero.
if i == maxVarintLen64-1 && b > 1 {
return 0, 0, errOverflow64
}
return ret | uint64(b)<<s, uint64(i) + 1, nil
}
ret |= (uint64(b) & 0x7f) << s
s += 7
}
return 0, 0, errOverflow64
}
func DecodeInt32(r io.ByteReader) (ret int32, bytesRead uint64, err error) {
return decodeInt32(func(_ int) (byte, error) { return r.ReadByte() })
}
func LoadInt32(buf []byte) (ret int32, bytesRead uint64, err error) {
return decodeInt32(func(i int) (byte, error) {
if i >= len(buf) {
return 0, io.EOF
}
return buf[i], nil
})
}
func decodeInt32(next nextByte) (ret int32, bytesRead uint64, err error) {
var shift int
var b byte
for {
b, err = next(int(bytesRead))
if err != nil {
return 0, 0, fmt.Errorf("readByte failed: %w", err)
}
ret |= (int32(b) & 0x7f) << shift
shift += 7
bytesRead++
if b&0x80 == 0 {
if shift < 32 && (b&0x40) != 0 {
ret |= ^0 << shift
}
// Over flow checks.
// fixme: can be optimized.
if bytesRead > maxVarintLen32 {
return 0, 0, errOverflow32
} else if unused := b & 0b00110000; bytesRead == maxVarintLen32 && ret < 0 && unused != 0b00110000 {
return 0, 0, errOverflow32
} else if bytesRead == maxVarintLen32 && ret >= 0 && unused != 0x00 {
return 0, 0, errOverflow32
}
return
}
}
}
// DecodeInt33AsInt64 is a special cased decoder for wasm.BlockType which is encoded as a positive signed integer, yet
// still needs to fit the 32-bit range of allowed indices. Hence, this is 33, not 32-bit!
//
// See https://webassembly.github.io/spec/core/binary/instructions.html#control-instructions
func DecodeInt33AsInt64(r io.ByteReader) (ret int64, bytesRead uint64, err error) {
var shift int
var b int64
var rb byte
for shift < 35 {
rb, err = r.ReadByte()
if err != nil {
return 0, 0, fmt.Errorf("readByte failed: %w", err)
}
b = int64(rb)
ret |= (b & int33Mask2) << shift
shift += 7
bytesRead++
if b&int33Mask == 0 {
break
}
}
// fixme: can be optimized
if shift < 33 && (b&int33Mask3) == int33Mask3 {
ret |= int33Mask4 << shift
}
ret = ret & int33Mask4
// if 33rd bit == 1, we translate it as a corresponding signed-33bit minus value
if ret&int33Mask5 > 0 {
ret = ret - int33Mask6
}
// Over flow checks.
// fixme: can be optimized.
if bytesRead > maxVarintLen33 {
return 0, 0, errOverflow33
} else if unused := b & 0b00100000; bytesRead == maxVarintLen33 && ret < 0 && unused != 0b00100000 {
return 0, 0, errOverflow33
} else if bytesRead == maxVarintLen33 && ret >= 0 && unused != 0x00 {
return 0, 0, errOverflow33
}
return ret, bytesRead, nil
}
func DecodeInt64(r io.ByteReader) (ret int64, bytesRead uint64, err error) {
return decodeInt64(func(_ int) (byte, error) { return r.ReadByte() })
}
func LoadInt64(buf []byte) (ret int64, bytesRead uint64, err error) {
return decodeInt64(func(i int) (byte, error) {
if i >= len(buf) {
return 0, io.EOF
}
return buf[i], nil
})
}
func decodeInt64(next nextByte) (ret int64, bytesRead uint64, err error) {
var shift int
var b byte
for {
b, err = next(int(bytesRead))
if err != nil {
return 0, 0, fmt.Errorf("readByte failed: %w", err)
}
ret |= (int64(b) & 0x7f) << shift
shift += 7
bytesRead++
if b&0x80 == 0 {
if shift < 64 && (b&int64Mask3) == int64Mask3 {
ret |= int64Mask4 << shift
}
// Over flow checks.
// fixme: can be optimized.
if bytesRead > maxVarintLen64 {
return 0, 0, errOverflow64
} else if unused := b & 0b00111110; bytesRead == maxVarintLen64 && ret < 0 && unused != 0b00111110 {
return 0, 0, errOverflow64
} else if bytesRead == maxVarintLen64 && ret >= 0 && unused != 0x00 {
return 0, 0, errOverflow64
}
return
}
}
}
|
