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/*
* Copyright 2003-2019 The Music Player Daemon Project
* http://www.musicpd.org
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#ifndef MPD_SLICE_BUFFER_HXX
#define MPD_SLICE_BUFFER_HXX
#include "HugeAllocator.hxx"
#include "Compiler.h"
#include <utility>
#include <new>
#include <assert.h>
#include <stddef.h>
/**
* This class pre-allocates a certain number of objects, and allows
* callers to allocate and free these objects ("slices").
*/
template<typename T>
class SliceBuffer {
union Slice {
Slice *next;
T value;
};
HugeArray<Slice> buffer;
/**
* The number of slices that are initialized. This is used to
* avoid page faulting on the new allocation, so the kernel
* does not need to reserve physical memory pages.
*/
unsigned n_initialized = 0;
/**
* The number of slices currently allocated.
*/
unsigned n_allocated = 0;
/**
* Pointer to the first free element in the chain.
*/
Slice *available = nullptr;
public:
SliceBuffer(unsigned _count)
:buffer(_count) {
buffer.ForkCow(false);
}
~SliceBuffer() noexcept {
/* all slices must be freed explicitly, and this
assertion checks for leaks */
assert(n_allocated == 0);
}
SliceBuffer(const SliceBuffer &other) = delete;
SliceBuffer &operator=(const SliceBuffer &other) = delete;
unsigned GetCapacity() const noexcept {
return buffer.size();
}
bool empty() const noexcept {
return n_allocated == 0;
}
bool IsFull() const noexcept {
return n_allocated == buffer.size();
}
void DiscardMemory() noexcept {
assert(empty());
n_initialized = 0;
buffer.Discard();
available = nullptr;
}
template<typename... Args>
T *Allocate(Args&&... args) {
assert(n_initialized <= buffer.size());
assert(n_allocated <= n_initialized);
if (available == nullptr) {
if (n_initialized == buffer.size()) {
/* out of (internal) memory, buffer is full */
assert(n_allocated == buffer.size());
return nullptr;
}
available = &buffer[n_initialized++];
available->next = nullptr;
}
/* allocate a slice */
T *value = &available->value;
available = available->next;
++n_allocated;
/* construct the object */
return ::new((void *)value) T(std::forward<Args>(args)...);
}
void Free(T *value) noexcept {
assert(n_initialized <= buffer.size());
assert(n_allocated > 0);
assert(n_allocated <= n_initialized);
Slice *slice = reinterpret_cast<Slice *>(value);
assert(slice >= &buffer.front() && slice <= &buffer.back());
/* destruct the object */
value->~T();
/* insert the slice in the "available" linked list */
slice->next = available;
available = slice;
--n_allocated;
/* give memory back to the kernel when the last slice
was freed */
if (n_allocated == 0) {
DiscardMemory();
}
}
};
#endif
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