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// Copyright 2021 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_BASE_VLQ_H_
#define V8_BASE_VLQ_H_
#include <limits>
#include <vector>
#include "src/base/memory.h"
namespace v8 {
namespace base {
static constexpr uint32_t kContinueShift = 7;
static constexpr uint32_t kContinueBit = 1 << kContinueShift;
static constexpr uint32_t kDataMask = kContinueBit - 1;
// Encodes an unsigned value using variable-length encoding and stores it using
// the passed process_byte function. The function should return a pointer to
// the byte that was written, so that VLQEncodeUnsigned can mutate it after
// writing it.
template <typename Function>
inline typename std::enable_if<
std::is_same<decltype(std::declval<Function>()(0)), byte*>::value,
void>::type
VLQEncodeUnsigned(Function&& process_byte, uint32_t value) {
byte* written_byte = process_byte(value);
if (value <= kDataMask) {
// Value fits in first byte, early return.
return;
}
do {
// Turn on continuation bit in the byte we just wrote.
*written_byte |= kContinueBit;
value >>= kContinueShift;
written_byte = process_byte(value);
} while (value > kDataMask);
}
// Encodes value using variable-length encoding and stores it using the passed
// process_byte function.
template <typename Function>
inline typename std::enable_if<
std::is_same<decltype(std::declval<Function>()(0)), byte*>::value,
void>::type
VLQEncode(Function&& process_byte, int32_t value) {
// This wouldn't handle kMinInt correctly if it ever encountered it.
DCHECK_NE(value, std::numeric_limits<int32_t>::min());
bool is_negative = value < 0;
// Encode sign in least significant bit.
uint32_t bits = static_cast<uint32_t>((is_negative ? -value : value) << 1) |
static_cast<uint32_t>(is_negative);
VLQEncodeUnsigned(std::forward<Function>(process_byte), bits);
}
// Wrapper of VLQEncode for std::vector backed storage containers.
template <typename A>
inline void VLQEncode(std::vector<byte, A>* data, int32_t value) {
VLQEncode(
[data](byte value) {
data->push_back(value);
return &data->back();
},
value);
}
// Wrapper of VLQEncodeUnsigned for std::vector backed storage containers.
template <typename A>
inline void VLQEncodeUnsigned(std::vector<byte, A>* data, uint32_t value) {
VLQEncodeUnsigned(
[data](byte value) {
data->push_back(value);
return &data->back();
},
value);
}
// Decodes a variable-length encoded unsigned value from bytes returned by
// successive calls to the given function.
template <typename GetNextFunction>
inline typename std::enable_if<
std::is_same<decltype(std::declval<GetNextFunction>()()), byte>::value,
uint32_t>::type
VLQDecodeUnsigned(GetNextFunction&& get_next) {
byte cur_byte = get_next();
// Single byte fast path; no need to mask.
if (cur_byte <= kDataMask) {
return cur_byte;
}
uint32_t bits = cur_byte & kDataMask;
for (int shift = kContinueShift; shift <= 32; shift += kContinueShift) {
byte cur_byte = get_next();
bits |= (cur_byte & kDataMask) << shift;
if (cur_byte <= kDataMask) break;
}
return bits;
}
// Decodes a variable-length encoded unsigned value stored in contiguous memory
// starting at data_start + index, updating index to where the next encoded
// value starts.
inline uint32_t VLQDecodeUnsigned(byte* data_start, int* index) {
return VLQDecodeUnsigned([&] { return data_start[(*index)++]; });
}
// Decodes a variable-length encoded value stored in contiguous memory starting
// at data_start + index, updating index to where the next encoded value starts.
inline int32_t VLQDecode(byte* data_start, int* index) {
uint32_t bits = VLQDecodeUnsigned(data_start, index);
bool is_negative = (bits & 1) == 1;
int32_t result = bits >> 1;
return is_negative ? -result : result;
}
} // namespace base
} // namespace v8
#endif // V8_BASE_VLQ_H_
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