bug 606080 - add class to generate unique 32 bit ids r=froydnj

accessible/windows/msaa/IDSet.h

@@ -0,0 +1,126 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * A class to generate unique IDs in the range [ - 2^31, 0 )
+ */
+
+#ifndef MOZILLA_A11Y_IDSet_h_
+#define MOZILLA_A11Y_IDSet_h_
+
+#include "mozilla/Attributes.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/SplayTree.h"
+
+namespace mozilla {
+namespace a11y {
+
+/**
+ * On windows an accessible's id must be a negative 32 bit integer.  It is
+ * important to support recycling arbitrary IDs because accessibles can be
+ * created and destroyed at any time in the life of a page.  IDSet provides 2
+ * operations: generate an ID in the range [ - 2^31, 0 ), and release an ID so
+ * it can be allocated again.  Allocated ID are tracked by a sparse bitmap
+ * implemented with a splay tree.  Nodes in the tree are keyed by the upper N
+ * bits of the bitwise negation of the ID, and the node contains a bitmap
+ * tracking the allocation of 2^(32 - N) IDs.
+ */
+class IDSet
+{
+public:
+  MOZ_CONSTEXPR IDSet() : mBitSet(), mIdx(0) {}
+
+  /**
+   * Return a new unique id.
+   */
+  uint32_t GetID()
+  {
+    uint32_t idx = mIdx;
+    while (true) {
+      BitSetElt* elt = mBitSet.findOrInsert(BitSetElt(idx));
+      if (elt->mBitvec[0] != UINT64_MAX) {
+        uint32_t i = CountTrailingZeroes64(~elt->mBitvec[0]);
+
+        elt->mBitvec[0] |= (1ull << i);
+        mIdx = idx;
+        return ~(elt->mIdx * bitsPerElt + i);
+      }
+
+      if (elt->mBitvec[1] != UINT64_MAX) {
+        uint32_t i = CountTrailingZeroes64(~elt->mBitvec[1]);
+
+        elt->mBitvec[1] |= (1ull << i);
+        mIdx = idx;
+        return ~(elt->mIdx * bitsPerElt + bitsPerWord + i);
+      }
+
+      idx++;
+      if (idx > sMaxIdx) {
+        idx = 0;
+      }
+
+      if (idx == mIdx) {
+        MOZ_CRASH("used up all the available ids");
+      }
+    }
+  }
+
+  /**
+   * Free a no longer required id so it may be allocated again.
+   */
+  void ReleaseID(uint32_t aID)
+  {
+    aID = ~aID;
+    MOZ_ASSERT(aID < static_cast<uint32_t>(INT32_MAX));
+
+    uint32_t idx = aID / bitsPerElt;
+    mIdx = idx;
+    BitSetElt* elt = mBitSet.find(BitSetElt(idx));
+    MOZ_ASSERT(elt);
+
+    uint32_t vecIdx = (aID % bitsPerElt) / bitsPerWord;
+    elt->mBitvec[vecIdx] &= ~(1ull << (aID % bitsPerWord));
+    if (elt->mBitvec[0] == 0 && elt->mBitvec[1] == 0) {
+      delete mBitSet.remove(*elt);
+    }
+  }
+
+private:
+  static const unsigned int wordsPerElt = 2;
+  static const unsigned int bitsPerWord = 64;
+  static const unsigned int bitsPerElt = wordsPerElt * bitsPerWord;
+  static const uint32_t sMaxIdx = INT32_MAX / bitsPerElt;
+
+  struct BitSetElt : mozilla::SplayTreeNode<BitSetElt>
+  {
+    explicit BitSetElt(uint32_t aIdx) :
+      mIdx(aIdx)
+    { mBitvec[0] = mBitvec[1] = 0; }
+
+    uint64_t mBitvec[wordsPerElt];
+    uint32_t mIdx;
+
+    static int compare(const BitSetElt& a, const BitSetElt& b)
+    {
+      if (a.mIdx == b.mIdx) {
+        return 0;
+      }
+
+      if (a.mIdx < b.mIdx) {
+        return -1;
+      }
+      return 1;
+    }
+  };
+
+  SplayTree<BitSetElt, BitSetElt> mBitSet;
+  uint32_t mIdx;
+};
+
+}
+}
+
+#endif

accessible/windows/msaa/moz.build

@@ -12,6 +12,7 @@ EXPORTS.mozilla.a11y += [
     'AccessibleWrap.h',
     'Compatibility.h',
     'HyperTextAccessibleWrap.h',
+    'IDSet.h',
 ]
 
 UNIFIED_SOURCES += [