Source file src/pkg/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 panic("unreachable") 124 } 125 126 // ReadVarint reads an encoded unsigned integer from r and returns it as a uint64. 127 func ReadVarint(r io.ByteReader) (int64, error) { 128 ux, err := ReadUvarint(r) // ok to continue in presence of error 129 x := int64(ux >> 1) 130 if ux&1 != 0 { 131 x = ^x 132 } 133 return x, err 134 }