Source file src/encoding/binary/varint.go

Documentation: encoding/binary

     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  }
   134  

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