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# std.experimental.allocator.building_blocks.ascending_page_allocator

struct `AscendingPageAllocator`;
  `AscendingPageAllocator` is a fast and safe allocator that rounds all allocations to multiples of the system's page size. It reserves a range of virtual addresses (using mmap on Posix and VirtualAlloc on Windows) and allocates memory at consecutive virtual addresses.
  When a chunk of memory is requested, the allocator finds a range of virtual pages that satisfy the requested size, changing their protection to read/write using OS primitives (mprotect and VirtualProtect, respectively). The physical memory is allocated on demand, when the pages are accessed.
  Deallocation removes any read/write permissions from the target pages and notifies the OS to reclaim the physical memory, while keeping the virtual memory.
  Because the allocator does not reuse memory, any dangling references to deallocated memory will always result in deterministically crashing the process.
  See Also:
  Project Snoflake for the general approach.
  this(size_t `n`);
    The allocator receives as a parameter the size in pages of the virtual address range
  void[] `allocate`(size_t `n`);
    Rounds the allocation size to the next multiple of the page size. The allocation only reserves a range of virtual pages but the actual physical memory is allocated on demand, when accessing the memory.
    Parameters:

    size_t `n` | Bytes to `allocate`

    Returns:
    null on failure or if the requested size exceeds the remaining capacity.
  void[] `alignedAllocate`(size_t `n`, uint `a`);
    Rounds the allocation size to the next multiple of the page size. The allocation only reserves `a` range of virtual pages but the actual physical memory is allocated on demand, when accessing the memory.
    The allocated memory is aligned to the specified alignment `a`.
    Parameters:

    size_t `n` | Bytes to allocate
    uint `a` | Alignment

    Returns:
    null on failure or if the requested size exceeds the remaining capacity.
  size_t `goodAllocSize`(size_t `n`);
    Rounds the requested size to the next multiple of the page size.
  bool `deallocate`(void[] `buf`);
    Decommit all physical memory associated with the buffer given as parameter, but keep the range of virtual addresses.
    On POSIX systems `deallocate` calls mmap with `MAP_FIXED' a second time to decommit the memory. On Windows, it uses VirtualFree with MEM_DECOMMIT.
  Ternary `owns`(void[] `buf`);
    Returns Ternary.yes if the passed buffer is inside the range of virtual adresses. Does not guarantee that the passed buffer is still valid.
  bool `deallocateAll`();
    Removes the memory mapping causing all physical memory to be decommited and the virtual address space to be reclaimed.
  size_t `getAvailableSize`();
    Returns the available size for further allocations in bytes.
  bool `expand`(ref void[] `b`, size_t `delta`);
    If the passed buffer is not the last allocation, then `delta` can be at most the number of bytes left on the last page. Otherwise, we can `expand` the last allocation until the end of the virtual address range.
  Ternary `empty`();
    Returns Ternary.yes if the allocator does not contain any alive objects and Ternary.no otherwise.

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