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 // AppendUvarint appends the varint-encoded form of x, 40 // as generated by [PutUvarint], to buf and returns the extended buffer. 41 func AppendUvarint(buf []byte, x uint64) []byte { 42 for x >= 0x80 { 43 buf = append(buf, byte(x)|0x80) 44 x >>= 7 45 } 46 return append(buf, byte(x)) 47 } 48 49 // PutUvarint encodes a uint64 into buf and returns the number of bytes written. 50 // If the buffer is too small, PutUvarint will panic. 51 func PutUvarint(buf []byte, x uint64) int { 52 i := 0 53 for x >= 0x80 { 54 buf[i] = byte(x) | 0x80 55 x >>= 7 56 i++ 57 } 58 buf[i] = byte(x) 59 return i + 1 60 } 61 62 // Uvarint decodes a uint64 from buf and returns that value and the 63 // number of bytes read (> 0). If an error occurred, the value is 0 64 // and the number of bytes n is <= 0 meaning: 65 // 66 // n == 0: buf too small 67 // n < 0: value larger than 64 bits (overflow) 68 // and -n is the number of bytes read 69 func Uvarint(buf []byte) (uint64, int) { 70 var x uint64 71 var s uint 72 for i, b := range buf { 73 if i == MaxVarintLen64 { 74 // Catch byte reads past MaxVarintLen64. 75 // See issue https://golang.org/issues/41185 76 return 0, -(i + 1) // overflow 77 } 78 if b < 0x80 { 79 if i == MaxVarintLen64-1 && b > 1 { 80 return 0, -(i + 1) // overflow 81 } 82 return x | uint64(b)<<s, i + 1 83 } 84 x |= uint64(b&0x7f) << s 85 s += 7 86 } 87 return 0, 0 88 } 89 90 // AppendVarint appends the varint-encoded form of x, 91 // as generated by [PutVarint], to buf and returns the extended buffer. 92 func AppendVarint(buf []byte, x int64) []byte { 93 ux := uint64(x) << 1 94 if x < 0 { 95 ux = ^ux 96 } 97 return AppendUvarint(buf, ux) 98 } 99 100 // PutVarint encodes an int64 into buf and returns the number of bytes written. 101 // If the buffer is too small, PutVarint will panic. 102 func PutVarint(buf []byte, x int64) int { 103 ux := uint64(x) << 1 104 if x < 0 { 105 ux = ^ux 106 } 107 return PutUvarint(buf, ux) 108 } 109 110 // Varint decodes an int64 from buf and returns that value and the 111 // number of bytes read (> 0). If an error occurred, the value is 0 112 // and the number of bytes n is <= 0 with the following meaning: 113 // 114 // n == 0: buf too small 115 // n < 0: value larger than 64 bits (overflow) 116 // and -n is the number of bytes read 117 func Varint(buf []byte) (int64, int) { 118 ux, n := Uvarint(buf) // ok to continue in presence of error 119 x := int64(ux >> 1) 120 if ux&1 != 0 { 121 x = ^x 122 } 123 return x, n 124 } 125 126 var errOverflow = errors.New("binary: varint overflows a 64-bit integer") 127 128 // ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64. 129 // The error is [io.EOF] only if no bytes were read. 130 // If an [io.EOF] happens after reading some but not all the bytes, 131 // ReadUvarint returns [io.ErrUnexpectedEOF]. 132 func ReadUvarint(r io.ByteReader) (uint64, error) { 133 var x uint64 134 var s uint 135 for i := 0; i < MaxVarintLen64; i++ { 136 b, err := r.ReadByte() 137 if err != nil { 138 if i > 0 && err == io.EOF { 139 err = io.ErrUnexpectedEOF 140 } 141 return x, err 142 } 143 if b < 0x80 { 144 if i == MaxVarintLen64-1 && b > 1 { 145 return x, errOverflow 146 } 147 return x | uint64(b)<<s, nil 148 } 149 x |= uint64(b&0x7f) << s 150 s += 7 151 } 152 return x, errOverflow 153 } 154 155 // ReadVarint reads an encoded signed integer from r and returns it as an int64. 156 // The error is [io.EOF] only if no bytes were read. 157 // If an [io.EOF] happens after reading some but not all the bytes, 158 // ReadVarint returns [io.ErrUnexpectedEOF]. 159 func ReadVarint(r io.ByteReader) (int64, error) { 160 ux, err := ReadUvarint(r) // ok to continue in presence of error 161 x := int64(ux >> 1) 162 if ux&1 != 0 { 163 x = ^x 164 } 165 return x, err 166 } 167