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/*
* 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(const uint8_t* bytes, size_t length, size_t& offset, T& result)
{
static_assert(std::is_unsigned_v<T>);
if (length <= offset)
return false;
result = 0;
unsigned shift = 0;
size_t last = std::min(maxByteLength<T>(), length - 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(const uint8_t* bytes, size_t length, size_t& offset, T& result)
{
static_assert(std::is_signed_v<T>);
if (length <= offset)
return false;
using UnsignedT = typename std::make_unsigned<T>::type;
result = 0;
unsigned shift = 0;
size_t last = std::min(maxByteLength<T>(), length - 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(const uint8_t* bytes, size_t length, size_t& offset, uint32_t& result)
{
return decodeUInt<uint32_t>(bytes, length, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeUInt64(const uint8_t* bytes, size_t length, size_t& offset, uint64_t& result)
{
return decodeUInt<uint64_t>(bytes, length, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeInt32(const uint8_t* bytes, size_t length, size_t& offset, int32_t& result)
{
return decodeInt<int32_t>(bytes, length, offset, result);
}
inline bool WARN_UNUSED_RETURN decodeInt64(const uint8_t* bytes, size_t length, size_t& offset, int64_t& result)
{
return decodeInt<int64_t>(bytes, length, offset, result);
}
} } // WTF::LEBDecoder