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# std.experimental.allocator.building_blocks.bitmapped_block

struct `BitmappedBlock`(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator);
  `BitmappedBlock` implements a simple heap consisting of one contiguous area of memory organized in blocks, each of size theBlockSize. A block is a unit of allocation. A bitmap serves as bookkeeping data, more precisely one bit per block indicating whether that block is currently allocated or not.
  Passing NullAllocator as ParentAllocator (the default) means user code manages allocation of the memory block from the outside; in that case `BitmappedBlock` must be constructed with a void[] preallocated block and has no responsibility regarding the lifetime of its support underlying storage. If another allocator type is passed, `BitmappedBlock` defines a destructor that uses the parent allocator to release the memory block. That makes the combination of AllocatorList, `BitmappedBlock`, and a back-end allocator such as MmapAllocator a simple and scalable solution for memory allocation.
  There are advantages to storing bookkeeping data separated from the payload (as opposed to e.g. using AffixAllocator to store metadata together with each allocation). The layout is more compact (overhead is one bit per block), searching for a free block during allocation enjoys better cache locality, and deallocation does not touch memory around the payload being deallocated (which is often cold).
  Allocation requests are handled on a first-fit basis. Although linear in complexity, allocation is in practice fast because of the compact bookkeeping representation, use of simple and fast bitwise routines, and caching of the first available block position. A known issue with this general approach is fragmentation, partially mitigated by coalescing. Since `BitmappedBlock` does not need to maintain the allocated size, freeing memory implicitly coalesces free blocks together. Also, tuning blockSize has a considerable impact on both internal and external fragmentation.
  The size of each block can be selected either during compilation or at run time. Statically-known block sizes are frequent in practice and yield slightly better performance. To choose a block size statically, pass it as the blockSize parameter as in `BitmappedBlock`!(Allocator, 4096). To choose a block size parameter, use `BitmappedBlock`!(Allocator, chooseAtRuntime) and pass the block size to the constructor.
  Examples:

      // Create a block allocator on top of a 10KB stack region.
      import std.experimental.allocator.building_blocks.region : InSituRegion;
      import std.traits : hasMember;
      InSituRegion!(10_240, 64) r;
      auto a = BitmappedBlock!(64, 64)(cast(ubyte[])(r.allocateAll()));
      static assert(hasMember!(InSituRegion!(10_240, 64), "allocateAll"));
      const b = a.allocate(100);
      writeln(b.length); // 100

  alias `blockSize` = theBlockSize;
    If `blockSize` == chooseAtRuntime, BitmappedBlock offers a read/write property `blockSize`. It must be set before any use of the allocator. Otherwise (i.e. theBlockSize is a legit constant), `blockSize` is an alias for theBlockSize. Whether constant or variable, must also be a multiple of alignment. This constraint is asserted statically and dynamically.
  alias `alignment` = theAlignment;
    The alignment offered is user-configurable statically through parameter theAlignment, defaulted to platformAlignment.
  ParentAllocator `parent`;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  this(ubyte[] `data`);
  this(size_t `capacity`);
    Constructs a block allocator given a hunk of memory, or a desired `capacity` in bytes.
    - If ParentAllocator is NullAllocator, only the constructor taking `data` is defined and the user is responsible for freeing `data` if desired.
    - Otherwise, both constructors are defined. The `data`-based constructor assumes memory has been allocated with the parent allocator. The `capacity`-based constructor uses ParentAllocator to allocate an appropriate contiguous hunk of memory. Regardless of the constructor used, the destructor releases the memory by using ParentAllocator.deallocate.
  size_t `goodAllocSize`(size_t `n`);
    Returns the actual bytes allocated when `n` bytes are requested, i.e. `n`.roundUpToMultipleOf(blockSize).
  @trusted void[] `allocate`(const size_t `s`);
    Allocates `s` bytes of memory and returns it, or `null` if memory could not be allocated.
    The following information might be of help with choosing the appropriate block size. Actual allocation occurs in sizes multiple of the block size. Allocating one block is the fastest because only one 0 bit needs to be found in the metadata. Allocating 2 through 64 blocks is the next cheapest because it affects a maximum of two ulong `s` in the metadata. Allocations greater than 64 blocks require a multiword search through the metadata.
  void[] `alignedAllocate`(size_t `n`, uint `a`);
    Allocates `a` block with specified alignment `a`. The alignment must be `a` power of 2. If `a` <= alignment, function forwards to allocate. Otherwise, it attempts to overallocate and then adjust the result for proper alignment. In the worst case the slack memory is around two blocks.
  void[] `allocateAll`();
    If the BitmappedBlock object is empty (has no active allocation), allocates all memory within and returns a slice to it. Otherwise, returns `null` (i.e. no attempt is made to allocate the largest available block).
  const Ternary `owns`(void[] `b`);
    Returns Ternary.yes if `b` belongs to the BitmappedBlock object, Ternary.no otherwise. Never returns Ternary.unkown. (This method is somewhat tolerant in that accepts an interior slice.)
  @trusted bool `expand`(ref void[] `b`, immutable size_t `delta`);
    Expands an allocated block in place.
  @system bool `reallocate`(ref void[] `b`, size_t `newSize`);
    Reallocates a previously-allocated block. Contractions occur in place.
  @system bool `alignedReallocate`(ref void[] `b`, size_t `newSize`, uint `a`);
    Reallocates `a` block previously allocated with alignedAllocate. Contractions do not occur in place.
  bool `deallocate`(void[] `b`);
    Deallocates a block previously allocated with this allocator.
  bool `deallocateAll`();
    Forcibly deallocates all memory allocated by this allocator, making it available for further allocations. Does not return memory to ParentAllocator.
  Ternary `empty`();
    Returns Ternary.yes if no memory is currently allocated with this allocator, otherwise Ternary.no. This method never returns Ternary.unknown.
struct `BitmappedBlockWithInternalPointers`(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator);
  A BitmappedBlock with additional structure for supporting resolveInternalPointer. To that end, `BitmappedBlockWithInternalPointers` adds a bitmap (one bit per block) that marks object starts. The bitmap itself has variable size and is allocated together with regular allocations.
  The time complexity of resolveInternalPointer is Ο(k), where k is the size of the object within which the internal pointer is looked up.
  this(ubyte[] `data`);
  this(size_t `capacity`);
    Constructors accepting desired `capacity` or a preallocated buffer, similar in semantics to those of BitmappedBlock.
  alias `alignment` = theAlignment;
  size_t `goodAllocSize`(size_t `n`);
  void[] `allocate`(size_t `bytes`);
  void[] `allocateAll`();
  bool `expand`(ref void[] `b`, size_t `bytes`);
  bool `deallocate`(void[] `b`);
  Ternary `resolveInternalPointer`(const void* `p`, ref void[] `result`);
  Ternary `empty`();
    Allocator primitives.

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