libgo/go/encoding/binary/varint.go

   1 // Copyright 2011 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package binary
   6
   7 // This file implements "varint" encoding of 64-bit integers.
   8 // The encoding is:
   9 // - unsigned integers are serialized 7 bits at a time, starting with the
  10 //   least significant bits
  11 // - the most significant bit (msb) in each output byte indicates if there
  12 //   is a continuation byte (msb = 1)
  13 // - signed integers are mapped to unsigned integers using "zig-zag"
  14 //   encoding: Positive values x are written as 2*x + 0, negative values
  15 //   are written as 2*(^x) + 1; that is, negative numbers are complemented
  16 //   and whether to complement is encoded in bit 0.
  17 //
  18 // Design note:
  19 // At most 10 bytes are needed for 64-bit values. The encoding could
  20 // be more dense: a full 64-bit value needs an extra byte just to hold bit 63.
  21 // Instead, the msb of the previous byte could be used to hold bit 63 since we
  22 // know there can't be more than 64 bits. This is a trivial improvement and
  23 // would reduce the maximum encoding length to 9 bytes. However, it breaks the
  24 // invariant that the msb is always the "continuation bit" and thus makes the
  25 // format incompatible with a varint encoding for larger numbers (say 128-bit).
  26
  27 import (
  28         "errors"
  29         "io"
  30 )
  31
  32 // MaxVarintLenN is the maximum length of a varint-encoded N-bit integer.
  33 const (
  34         MaxVarintLen16 = 3
  35         MaxVarintLen32 = 5
  36         MaxVarintLen64 = 10
  37 )
  38
  39 // PutUvarint encodes a uint64 into buf and returns the number of bytes written.
  40 // If the buffer is too small, PutUvarint will panic.
  41 func PutUvarint(buf []byte, x uint64) int {
  42         i := 0
  43         for x >= 0x80 {
  44                 buf[i] = byte(x) | 0x80
  45                 x >>= 7
  46                 i++
  47         }
  48         buf[i] = byte(x)
  49         return i + 1
  50 }
  51
  52 // Uvarint decodes a uint64 from buf and returns that value and the
  53 // number of bytes read (> 0). If an error occurred, the value is 0
  54 // and the number of bytes n is <= 0 meaning:
  55 //
  56 //      n == 0: buf too small
  57 //      n  < 0: value larger than 64 bits (overflow)
  58 //              and -n is the number of bytes read
  59 //
  60 func Uvarint(buf []byte) (uint64, int) {
  61         var x uint64
  62         var s uint
  63         for i, b := range buf {
  64                 if b < 0x80 {
  65                         if i > 9 || i == 9 && b > 1 {
  66                                 return 0, -(i + 1) // overflow
  67                         }
  68                         return x | uint64(b)<<s, i + 1
  69                 }
  70                 x |= uint64(b&0x7f) << s
  71                 s += 7
  72         }
  73         return 0, 0
  74 }
  75
  76 // PutVarint encodes an int64 into buf and returns the number of bytes written.
  77 // If the buffer is too small, PutVarint will panic.
  78 func PutVarint(buf []byte, x int64) int {
  79         ux := uint64(x) << 1
  80         if x < 0 {
  81                 ux = ^ux
  82         }
  83         return PutUvarint(buf, ux)
  84 }
  85
  86 // Varint decodes an int64 from buf and returns that value and the
  87 // number of bytes read (> 0). If an error occurred, the value is 0
  88 // and the number of bytes n is <= 0 with the following meaning:
  89 //
  90 //      n == 0: buf too small
  91 //      n  < 0: value larger than 64 bits (overflow)
  92 //              and -n is the number of bytes read
  93 //
  94 func Varint(buf []byte) (int64, int) {
  95         ux, n := Uvarint(buf) // ok to continue in presence of error
  96         x := int64(ux >> 1)
  97         if ux&1 != 0 {
  98                 x = ^x
  99         }
 100         return x, n
 101 }
 102
 103 var overflow = errors.New("binary: varint overflows a 64-bit integer")
 104
 105 // ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
 106 func ReadUvarint(r io.ByteReader) (uint64, error) {
 107         var x uint64
 108         var s uint
 109         for i := 0; ; i++ {
 110                 b, err := r.ReadByte()
 111                 if err != nil {
 112                         return x, err
 113                 }
 114                 if b < 0x80 {
 115                         if i > 9 || i == 9 && b > 1 {
 116                                 return x, overflow
 117                         }
 118                         return x | uint64(b)<<s, nil
 119                 }
 120                 x |= uint64(b&0x7f) << s
 121                 s += 7
 122         }
 123 }
 124
 125 // ReadVarint reads an encoded signed integer from r and returns it as an int64.
 126 func ReadVarint(r io.ByteReader) (int64, error) {
 127         ux, err := ReadUvarint(r) // ok to continue in presence of error
 128         x := int64(ux >> 1)
 129         if ux&1 != 0 {
 130                 x = ^x
 131         }
 132         return x, err
 133 }