Add helper classes to for managing shared buffers.

mojo/services/media/common/cpp/fifo_allocator.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/cpp/environment/logging.h"
+#include "mojo/services/media/common/cpp/fifo_allocator.h"
+
+namespace mojo {
+namespace media {
+
+FifoAllocator::FifoAllocator(uint64_t size) : front_(nullptr), free_(nullptr) {
+  // front_ and free_ need to be set to nullptr before calling reset.
+  Reset(size);
+}
+
+FifoAllocator::~FifoAllocator() {
+  DeleteFrontToBack(front_);
+  DeleteFrontToBack(free_);
+}
+
+void FifoAllocator::Reset(uint64_t size) {
+  MOJO_DCHECK(size < kNullOffset);
+  DeleteFrontToBack(front_);
+  DeleteFrontToBack(free_);
+  size_ = size;
+  free_ = nullptr;
+  front_ = back_ = active_ = get_free(false, size, 0);
+  active_->prev = nullptr;
+  active_->next = nullptr;
+}
+
+uint64_t FifoAllocator::AllocateRegion(uint64_t size) {
+  MOJO_DCHECK(size != 0);
+
+  if (active_->size < size) {
+    // The active region is too small. Look for one that's large enough.
+    if (!AdvanceActive(size)) {
+      // No unallocated regions are large enough. Can't do the allocation.
+      return kNullOffset;
+    }
+  }
+
+  if (active_->size == size) {
+    // The active region is exactly the right size. Use it for the allocation.
+    uint64_t result = active_->offset;
+    active_->allocated = true;
+    if (active_ == back_ && !front_->allocated) {
+      // active_ was the back region and the front region isn't allocated. Make
+      // the front region the new active region.
+      active_ = front_;
+    } else {
+      // The region after active_ is allocated, so make a zero-sized
+      // placeholder.
+      MakeActivePlaceholder();
+    }
+    return result;
+  }
+
+  // The active region can accommodate this allocation with room left over.
+  // Create a new region (allocated) of the requested size at the front of the
+  // active region, and adjust the active region to reflect the deficit.
+  MOJO_DCHECK(active_->size > size);
+  Region *allocated = get_free(true, size, active_->offset);
+  active_->size -= size;
+  active_->offset += size;
+  insert_before(allocated, active_);
+  return allocated->offset;
+}
+
+void FifoAllocator::ReleaseRegion(uint64_t size, uint64_t offset) {
+  // Start at active_->next. That's usually the region we're looking for.
+  bool released =
+      Release(size, offset, active_->next, nullptr) ||
+      Release(size, offset, front_, active_);
+  MOJO_DCHECK(released);
+}
+
+bool FifoAllocator::Release(
+    uint64_t size,
+    uint64_t offset,
+    Region* begin,
+    Region* end) {
+  MOJO_DCHECK(begin != nullptr || end == nullptr);
+  for (Region* region = begin; region != end; region = region->next) {
+    if (region->offset == offset) {
+      MOJO_DCHECK(region->allocated);
+      MOJO_DCHECK(region->size == size);
+      region->allocated = false;
+
+      Region *prev = region->prev;
+      if (prev != nullptr && !prev->allocated) {
+        // Coalesce wtih the previous region.
+        prev->size += region->size;
+        remove(region);
+        put_free(region);
+        region = prev;
+      }
+
+      Region *next = region->next;
+      if (next != nullptr && !next->allocated) {
+        // Coalesce wtih the next region.
+        next->offset = region->offset;
+        next->size += region->size;
+        if (active_ == region) {
+          // This can happen if we coalesced the previous region.
+          active_ = next;
+        }
+        remove(region);
+        put_free(region);
+      }
+
+      return true;
+    }
+  }
+
+  return false;
+}
+
+bool FifoAllocator::AdvanceActive(uint64_t size) {
+  MOJO_DCHECK(size != 0);
+  return
+      AdvanceActive(size, active_->next, nullptr) ||
+      AdvanceActive(size, front_, active_);
+}
+
+bool FifoAllocator::AdvanceActive(uint64_t size, Region* begin, Region* end) {
+  for (Region* region = begin; region != end; region = region->next) {
+    if (!region->allocated && region->size >= size) {
+      if (active_->size == 0) {
+        // The old active region is zero-sized. Get rid of it.
+        MOJO_DCHECK(!active_->allocated);
+        remove(active_);
+        put_free(active_);
+      }
+      active_ = region;
+      return true;
+    }
+  }
+  return false;
+}
+
+void FifoAllocator::MakeActivePlaceholder() {
+  // If the old active region was at the back of the list, we'll be inserting
+  // at the front, so make the offset zero. We insert at the front, because it's
+  // a bit more efficient and because we don't need to implement insert_after.
+  Region *new_active = get_free(
+      false,
+      0,
+      active_ == back_ ? 0 : active_->offset + active_->size);
+
+  MOJO_DCHECK((active_ == back_) == (active_->next == nullptr));
+  insert_before(new_active, active_ == back_ ? front_ : active_->next);
+  active_ = new_active;
+}
+
+void FifoAllocator::remove(Region* region) {
+  MOJO_DCHECK(region);
+
+  if (front_ == region) {
+    MOJO_DCHECK(region->prev == nullptr);
+    front_ = region->next;
+  } else {
+    MOJO_DCHECK(region->prev);
+    MOJO_DCHECK(region->prev->next ==region);
+    region->prev->next = region->next;
+  }
+
+  if (back_ == region) {
+    MOJO_DCHECK(region->next == nullptr);
+    back_ = region->prev;
+  } else {
+    MOJO_DCHECK(region->next);
+    MOJO_DCHECK(region->next->prev == region);
+    region->next->prev = region->prev;
+  }
+}
+
+void FifoAllocator::insert_before(Region* region, Region* before_this) {
+  MOJO_DCHECK(region);
+  MOJO_DCHECK(before_this);
+
+  region->prev = before_this->prev;
+  before_this->prev = region;
+  region->next = before_this;
+  if (front_ == before_this) {
+    MOJO_DCHECK(region->prev == nullptr);
+    front_ = region;
+  } else {
+    MOJO_DCHECK(region->prev);
+    region->prev->next = region;
+  }
+}
+
+FifoAllocator::Region* FifoAllocator::get_free(
+    bool allocated, uint64_t size, uint64_t offset) {
+  MOJO_DCHECK(size <= size_);
+  MOJO_DCHECK(offset <= size_ - size);
+
+  Region *result = free_;
+  if (result == nullptr) {
+    result = new Region();
+  } else {
+    free_ = free_->next;
+  }
+
+  result->allocated = allocated;
+  result->size = size;
+  result->offset = offset;
+
+  return result;
+}
+
+void FifoAllocator::DeleteFrontToBack(Region* region) {
+  while (region != nullptr) {
+    Region *to_delete = region;
+    region = region->next;
+    delete to_delete;
+  }
+}
+
+}  // namespace media
+}  // namespace mojo