forked from mirrors/gecko-dev
265e672179
# ignore-this-changeset --HG-- extra : amend_source : 4d301d3b0b8711c4692392aa76088ba7fd7d1022
129 lines
3.5 KiB
C++
129 lines
3.5 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/**
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* A class to generate unique IDs in the range [ - 2^31, 0 )
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*/
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#ifndef MOZILLA_A11Y_IDSet_h_
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#define MOZILLA_A11Y_IDSet_h_
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#include "mozilla/Attributes.h"
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#include "mozilla/MathAlgorithms.h"
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#include "mozilla/SplayTree.h"
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namespace mozilla {
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namespace a11y {
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/**
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* On windows an accessible's id must be a negative 32 bit integer. It is
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* important to support recycling arbitrary IDs because accessibles can be
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* created and destroyed at any time in the life of a page. IDSet provides 2
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* operations: generate an ID in the range (0, mMaxId], and release an ID so
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* it can be allocated again. Allocated ID are tracked by a sparse bitmap
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* implemented with a splay tree. Nodes in the tree are keyed by the upper N
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* bits of the ID, and the node contains a bitmap tracking the allocation of
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* 2^(ceil(log2(mMaxId)) - N) IDs.
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*
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* Note that negation is handled by MsaaIdGenerator as it performs additional
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* decoration on the ID generated by IDSet.
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* @see mozilla::a11y::MsaaIdGenerator
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*/
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class IDSet {
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public:
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constexpr explicit IDSet(const uint32_t aMaxIdBits)
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: mBitSet(),
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mIdx(0),
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mMaxId((1UL << aMaxIdBits) - 1UL),
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mMaxIdx(mMaxId / bitsPerElt) {}
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/**
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* Return a new unique id.
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*/
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uint32_t GetID() {
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uint32_t idx = mIdx;
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while (true) {
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BitSetElt* elt = mBitSet.findOrInsert(BitSetElt(idx));
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if (elt->mBitvec[0] != UINT64_MAX) {
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uint32_t i = CountTrailingZeroes64(~elt->mBitvec[0]);
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elt->mBitvec[0] |= (1ull << i);
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mIdx = idx;
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return (elt->mIdx * bitsPerElt + i);
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}
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if (elt->mBitvec[1] != UINT64_MAX) {
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uint32_t i = CountTrailingZeroes64(~elt->mBitvec[1]);
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elt->mBitvec[1] |= (1ull << i);
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mIdx = idx;
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return (elt->mIdx * bitsPerElt + bitsPerWord + i);
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}
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idx++;
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if (idx > mMaxIdx) {
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idx = 0;
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}
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if (idx == mIdx) {
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MOZ_CRASH("used up all the available ids");
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}
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}
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}
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/**
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* Free a no longer required id so it may be allocated again.
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*/
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void ReleaseID(uint32_t aID) {
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MOZ_ASSERT(aID < mMaxId);
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uint32_t idx = aID / bitsPerElt;
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mIdx = idx;
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BitSetElt* elt = mBitSet.find(BitSetElt(idx));
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MOZ_ASSERT(elt);
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uint32_t vecIdx = (aID % bitsPerElt) / bitsPerWord;
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elt->mBitvec[vecIdx] &= ~(1ull << (aID % bitsPerWord));
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if (elt->mBitvec[0] == 0 && elt->mBitvec[1] == 0) {
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delete mBitSet.remove(*elt);
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}
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}
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private:
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static const unsigned int wordsPerElt = 2;
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static const unsigned int bitsPerWord = 64;
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static const unsigned int bitsPerElt = wordsPerElt * bitsPerWord;
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struct BitSetElt : mozilla::SplayTreeNode<BitSetElt> {
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explicit BitSetElt(uint32_t aIdx) : mIdx(aIdx) {
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mBitvec[0] = mBitvec[1] = 0;
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}
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uint64_t mBitvec[wordsPerElt];
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uint32_t mIdx;
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static int compare(const BitSetElt& a, const BitSetElt& b) {
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if (a.mIdx == b.mIdx) {
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return 0;
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}
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if (a.mIdx < b.mIdx) {
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return -1;
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}
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return 1;
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}
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};
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SplayTree<BitSetElt, BitSetElt> mBitSet;
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uint32_t mIdx;
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const uint32_t mMaxId;
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const uint32_t mMaxIdx;
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};
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} // namespace a11y
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} // namespace mozilla
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#endif
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