1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
|
/*
* Copyright (C) 2016 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include <algorithm>
#include <limits.h>
#include <wtf/Compiler.h>
// This file contains a bunch of helper functions for decoding LEB numbers.
// See https://en.wikipedia.org/wiki/LEB128 for more information about the
// LEB format.
namespace WTF { namespace LEBDecoder {
template<typename T>
constexpr size_t maxByteLength()
{
constexpr size_t numBits = sizeof(T) * CHAR_BIT;
return (numBits - 1) / 7 + 1; // numBits / 7 rounding up.
}
template<typename T>
constexpr unsigned lastByteMask()
{
constexpr size_t numBits = sizeof(T) * CHAR_BIT;
static_assert(numBits % 7);
return ~((1U << (numBits % 7)) - 1);
}
template<typename T>
inline bool WARN_UNUSED_RETURN decodeUInt(std::span<const uint8_t> bytes, size_t& offset, T& result)
{
static_assert(std::is_unsigned_v<T>);
if (bytes.size() <= offset)
return false;
result = 0;
unsigned shift = 0;
size_t last = std::min(maxByteLength<T>(), bytes.size() - offset) - 1;
for (unsigned i = 0; true; ++i) {
uint8_t byte = bytes[offset++];
result |= static_cast<T>(byte & 0x7f) << shift;
shift += 7;
if (!(byte & 0x80))
return !(((maxByteLength<T>() - 1) == i && (byte & lastByteMask<T>())));
if (i == last)
return false;
}
RELEASE_ASSERT_NOT_REACHED();
return true;
}
template<typename T>
inline bool WARN_UNUSED_RETURN decodeInt(std::span<const uint8_t> bytes, size_t& offset, T& result)
{
static_assert(std::is_signed_v<T>);
if (bytes.size() <= offset)
return false;
using UnsignedT = typename std::make_unsigned<T>::type;
result = 0;
unsigned shift = 0;
size_t last = std::min(maxByteLength<T>(), bytes.size() - offset) - 1;
uint8_t byte;
for (unsigned i = 0; true; ++i) {
byte = bytes[offset++];
result |= static_cast<T>(static_cast<UnsignedT>(byte & 0x7f) << shift);
shift += 7;
if (!(byte & 0x80)) {
if ((maxByteLength<T>() - 1) == i) {
if (!(byte & 0x40)) {
// This is a non-sign-extended, positive number. Then, the remaining bits should be (lastByteMask<T>() >> 1).
// For example, in the int32_t case, the last byte should be less than 0b00000111, since 7 * 4 + 3 = 31.
if (byte & (lastByteMask<T>() >> 1))
return false;
} else {
// This is sign-extended, negative number. Then, zero should not exists in (lastByteMask<T>() >> 1) bits except for the top bit.
// For example, in the int32_t case, the last byte should be 0b01111XXX and 1 part must be 1. Since we already checked 0x40 is 1,
// middle [3,5] bits must be zero (e.g. 0b01000111 is invalid). We convert 0b01111XXX =(| 0x80)=> 0b11111XXX =(~)=> 0b00000YYY.
// And check that we do not have 1 in upper 5 bits.
if (static_cast<uint8_t>(~(byte | 0x80)) & (lastByteMask<T>() >> 1))
return false;
}
}
break;
}
if (i == last)
return false;
}
const size_t numBits = sizeof(T) * CHAR_BIT;
if (shift < numBits && (byte & 0x40))
result = static_cast<T>(static_cast<UnsignedT>(result) | (static_cast<UnsignedT>(-1) << shift));
return true;
}
inline bool WARN_UNUSED_RETURN decodeUInt32(std::span<const uint8_t> bytes, size_t& offset, uint32_t& result)
{
return decodeUInt<uint32_t>(bytes, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeUInt64(std::span<const uint8_t> bytes, size_t& offset, uint64_t& result)
{
return decodeUInt<uint64_t>(bytes, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeInt32(std::span<const uint8_t> bytes, size_t& offset, int32_t& result)
{
return decodeInt<int32_t>(bytes, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeInt64(std::span<const uint8_t> bytes, size_t& offset, int64_t& result)
{
return decodeInt<int64_t>(bytes, offset, result);
}
} } // WTF::LEBDecoder
|
