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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package binary
// This file implements "varint" encoding of 64-bit integers.
// The encoding is:
// - unsigned integers are serialized 7 bits at a time, starting with the
// least significant bits
// - the most significant bit (msb) in each output byte indicates if there
// is a continuation byte (msb = 1)
// - signed integers are mapped to unsigned integers using "zig-zag"
// encoding: Positive values x are written as 2*x + 0, negative values
// are written as 2*(^x) + 1; that is, negative numbers are complemented
// and whether to complement is encoded in bit 0.
//
// Design note:
// At most 10 bytes are needed for 64-bit values. The encoding could
// be more dense: a full 64-bit value needs an extra byte just to hold bit 63.
// Instead, the msb of the previous byte could be used to hold bit 63 since we
// know there can't be more than 64 bits. This is a trivial improvement and
// would reduce the maximum encoding length to 9 bytes. However, it breaks the
// invariant that the msb is always the "continuation bit" and thus makes the
// format incompatible with a varint encoding for larger numbers (say 128-bit).
import (
"errors"
"io"
)
// MaxVarintLenN is the maximum length of a varint-encoded N-bit integer.
const (
MaxVarintLen16 = 3
MaxVarintLen32 = 5
MaxVarintLen64 = 10
)
// PutUvarint encodes a uint64 into buf and returns the number of bytes written.
func PutUvarint(buf []byte, x uint64) int {
i := 0
for x >= 0x80 {
buf[i] = byte(x) | 0x80
x >>= 7
i++
}
buf[i] = byte(x)
return i + 1
}
// Uvarint decodes a uint64 from buf and returns that value and the
// number of bytes read (> 0). If an error occurred, the value is 0
// and the number of bytes n is <= 0 meaning:
//
// n == 0: buf too small
// n < 0: value larger than 64 bits (overflow)
// and -n is the number of bytes read
//
func Uvarint(buf []byte) (uint64, int) {
var x uint64
var s uint
for i, b := range buf {
if b < 0x80 {
if i > 9 || i == 9 && b > 1 {
return 0, -(i + 1) // overflow
}
return x | uint64(b)<<s, i + 1
}
x |= uint64(b&0x7f) << s
s += 7
}
return 0, 0
}
// PutVarint encodes an int64 into buf and returns the number of bytes written.
func PutVarint(buf []byte, x int64) int {
ux := uint64(x) << 1
if x < 0 {
ux = ^ux
}
return PutUvarint(buf, ux)
}
// Varint decodes an int64 from buf and returns that value and the
// number of bytes read (> 0). If an error occurred, the value is 0
// and the number of bytes n is <= 0 with the following meaning:
//
// n == 0: buf too small
// n < 0: value larger than 64 bits (overflow)
// and -n is the number of bytes read
//
func Varint(buf []byte) (int64, int) {
ux, n := Uvarint(buf) // ok to continue in presence of error
x := int64(ux >> 1)
if ux&1 != 0 {
x = ^x
}
return x, n
}
// WriteUvarint encodes x and writes the result to w.
func WriteUvarint(w io.Writer, x uint64) error {
var buf [MaxVarintLen64]byte
n := PutUvarint(buf[:], x)
_, err := w.Write(buf[0:n])
return err
}
var overflow = errors.New("binary: varint overflows a 64-bit integer")
// ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
func ReadUvarint(r io.ByteReader) (uint64, error) {
var x uint64
var s uint
for i := 0; ; i++ {
b, err := r.ReadByte()
if err != nil {
return x, err
}
if b < 0x80 {
if i > 9 || i == 9 && b > 1 {
return x, overflow
}
return x | uint64(b)<<s, nil
}
x |= uint64(b&0x7f) << s
s += 7
}
panic("unreachable")
}
// WriteVarint encodes x and writes the result to w.
func WriteVarint(w io.Writer, x int64) error {
ux := uint64(x) << 1
if x < 0 {
ux = ^ux
}
return WriteUvarint(w, ux)
}
// ReadVarint reads an encoded unsigned integer from r and returns it as a uint64.
func ReadVarint(r io.ByteReader) (int64, error) {
ux, err := ReadUvarint(r) // ok to continue in presence of error
x := int64(ux >> 1)
if ux&1 != 0 {
x = ^x
}
return x, err
}
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