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vector_downward.h
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vector_downward.h
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/*
* Copyright 2021 Google Inc. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef FLATBUFFERS_VECTOR_DOWNWARD_H_
#define FLATBUFFERS_VECTOR_DOWNWARD_H_
#include <algorithm>
#include "flatbuffers/base.h"
#include "flatbuffers/default_allocator.h"
#include "flatbuffers/detached_buffer.h"
namespace flatbuffers {
// This is a minimal replication of std::vector<uint8_t> functionality,
// except growing from higher to lower addresses. i.e. push_back() inserts data
// in the lowest address in the vector.
// Since this vector leaves the lower part unused, we support a "scratch-pad"
// that can be stored there for temporary data, to share the allocated space.
// Essentially, this supports 2 std::vectors in a single buffer.
class vector_downward {
public:
explicit vector_downward(size_t initial_size, Allocator *allocator,
bool own_allocator, size_t buffer_minalign)
: allocator_(allocator),
own_allocator_(own_allocator),
initial_size_(initial_size),
buffer_minalign_(buffer_minalign),
reserved_(0),
size_(0),
buf_(nullptr),
cur_(nullptr),
scratch_(nullptr) {}
vector_downward(vector_downward &&other)
// clang-format on
: allocator_(other.allocator_),
own_allocator_(other.own_allocator_),
initial_size_(other.initial_size_),
buffer_minalign_(other.buffer_minalign_),
reserved_(other.reserved_),
size_(other.size_),
buf_(other.buf_),
cur_(other.cur_),
scratch_(other.scratch_) {
// No change in other.allocator_
// No change in other.initial_size_
// No change in other.buffer_minalign_
other.own_allocator_ = false;
other.reserved_ = 0;
other.buf_ = nullptr;
other.cur_ = nullptr;
other.scratch_ = nullptr;
}
vector_downward &operator=(vector_downward &&other) {
// Move construct a temporary and swap idiom
vector_downward temp(std::move(other));
swap(temp);
return *this;
}
~vector_downward() {
clear_buffer();
clear_allocator();
}
void reset() {
clear_buffer();
clear();
}
void clear() {
if (buf_) {
cur_ = buf_ + reserved_;
} else {
reserved_ = 0;
cur_ = nullptr;
}
size_ = 0;
clear_scratch();
}
void clear_scratch() { scratch_ = buf_; }
void clear_allocator() {
if (own_allocator_ && allocator_) { delete allocator_; }
allocator_ = nullptr;
own_allocator_ = false;
}
void clear_buffer() {
if (buf_) Deallocate(allocator_, buf_, reserved_);
buf_ = nullptr;
}
// Relinquish the pointer to the caller.
uint8_t *release_raw(size_t &allocated_bytes, size_t &offset) {
auto *buf = buf_;
allocated_bytes = reserved_;
offset = static_cast<size_t>(cur_ - buf_);
// release_raw only relinquishes the buffer ownership.
// Does not deallocate or reset the allocator. Destructor will do that.
buf_ = nullptr;
clear();
return buf;
}
// Relinquish the pointer to the caller.
DetachedBuffer release() {
// allocator ownership (if any) is transferred to DetachedBuffer.
DetachedBuffer fb(allocator_, own_allocator_, buf_, reserved_, cur_,
size());
if (own_allocator_) {
allocator_ = nullptr;
own_allocator_ = false;
}
buf_ = nullptr;
clear();
return fb;
}
size_t ensure_space(size_t len) {
FLATBUFFERS_ASSERT(cur_ >= scratch_ && scratch_ >= buf_);
if (len > static_cast<size_t>(cur_ - scratch_)) { reallocate(len); }
// Beyond this, signed offsets may not have enough range:
// (FlatBuffers > 2GB not supported).
FLATBUFFERS_ASSERT(size() < FLATBUFFERS_MAX_BUFFER_SIZE);
return len;
}
inline uint8_t *make_space(size_t len) {
if (len) {
ensure_space(len);
cur_ -= len;
size_ += static_cast<uoffset_t>(len);
}
return cur_;
}
// Returns nullptr if using the DefaultAllocator.
Allocator *get_custom_allocator() { return allocator_; }
inline uoffset_t size() const { return size_; }
uoffset_t scratch_size() const {
return static_cast<uoffset_t>(scratch_ - buf_);
}
size_t capacity() const { return reserved_; }
uint8_t *data() const {
FLATBUFFERS_ASSERT(cur_);
return cur_;
}
uint8_t *scratch_data() const {
FLATBUFFERS_ASSERT(buf_);
return buf_;
}
uint8_t *scratch_end() const {
FLATBUFFERS_ASSERT(scratch_);
return scratch_;
}
uint8_t *data_at(size_t offset) const { return buf_ + reserved_ - offset; }
void push(const uint8_t *bytes, size_t num) {
if (num > 0) { memcpy(make_space(num), bytes, num); }
}
// Specialized version of push() that avoids memcpy call for small data.
template<typename T> void push_small(const T &little_endian_t) {
make_space(sizeof(T));
*reinterpret_cast<T *>(cur_) = little_endian_t;
}
template<typename T> void scratch_push_small(const T &t) {
ensure_space(sizeof(T));
*reinterpret_cast<T *>(scratch_) = t;
scratch_ += sizeof(T);
}
// fill() is most frequently called with small byte counts (<= 4),
// which is why we're using loops rather than calling memset.
void fill(size_t zero_pad_bytes) {
make_space(zero_pad_bytes);
for (size_t i = 0; i < zero_pad_bytes; i++) cur_[i] = 0;
}
// Version for when we know the size is larger.
// Precondition: zero_pad_bytes > 0
void fill_big(size_t zero_pad_bytes) {
memset(make_space(zero_pad_bytes), 0, zero_pad_bytes);
}
void pop(size_t bytes_to_remove) {
cur_ += bytes_to_remove;
size_ -= static_cast<uoffset_t>(bytes_to_remove);
}
void scratch_pop(size_t bytes_to_remove) { scratch_ -= bytes_to_remove; }
void swap(vector_downward &other) {
using std::swap;
swap(allocator_, other.allocator_);
swap(own_allocator_, other.own_allocator_);
swap(initial_size_, other.initial_size_);
swap(buffer_minalign_, other.buffer_minalign_);
swap(reserved_, other.reserved_);
swap(size_, other.size_);
swap(buf_, other.buf_);
swap(cur_, other.cur_);
swap(scratch_, other.scratch_);
}
void swap_allocator(vector_downward &other) {
using std::swap;
swap(allocator_, other.allocator_);
swap(own_allocator_, other.own_allocator_);
}
private:
// You shouldn't really be copying instances of this class.
FLATBUFFERS_DELETE_FUNC(vector_downward(const vector_downward &));
FLATBUFFERS_DELETE_FUNC(vector_downward &operator=(const vector_downward &));
Allocator *allocator_;
bool own_allocator_;
size_t initial_size_;
size_t buffer_minalign_;
size_t reserved_;
uoffset_t size_;
uint8_t *buf_;
uint8_t *cur_; // Points at location between empty (below) and used (above).
uint8_t *scratch_; // Points to the end of the scratchpad in use.
void reallocate(size_t len) {
auto old_reserved = reserved_;
auto old_size = size();
auto old_scratch_size = scratch_size();
reserved_ +=
(std::max)(len, old_reserved ? old_reserved / 2 : initial_size_);
reserved_ = (reserved_ + buffer_minalign_ - 1) & ~(buffer_minalign_ - 1);
if (buf_) {
buf_ = ReallocateDownward(allocator_, buf_, old_reserved, reserved_,
old_size, old_scratch_size);
} else {
buf_ = Allocate(allocator_, reserved_);
}
cur_ = buf_ + reserved_ - old_size;
scratch_ = buf_ + old_scratch_size;
}
};
} // namespace flatbuffers
#endif // FLATBUFFERS_VECTOR_DOWNWARD_H_