include/minikin/SparseBitSet.h

/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef MINIKIN_SPARSE_BIT_SET_H
#define MINIKIN_SPARSE_BIT_SET_H

#include <minikin/Buffer.h>
#include <sys/types.h>

#include <cstdint>
#include <memory>

// ---------------------------------------------------------------------------

namespace minikin {

// This is an implementation of a set of integers. It is optimized for
// values that are somewhat sparse, in the ballpark of a maximum value
// of thousands to millions. It is particularly efficient when there are
// large gaps. The motivating example is Unicode coverage of a font, but
// the abstraction itself is fully general.
class SparseBitSet {
public:
    // Create an empty bit set.
    SparseBitSet() : mData(nullptr) {}

    // Initialize the set to a new value, represented by ranges. For
    // simplicity, these ranges are arranged as pairs of values,
    // inclusive of start, exclusive of end, laid out in a uint32 array.
    SparseBitSet(const uint32_t* ranges, size_t nRanges) : SparseBitSet() {
        initFromRanges(ranges, nRanges);
    }

    explicit SparseBitSet(BufferReader* reader) : SparseBitSet() { initFromBuffer(reader); }

    SparseBitSet(SparseBitSet&&) = default;
    SparseBitSet& operator=(SparseBitSet&&) = default;

    void writeTo(BufferWriter* writer) const;

    // Determine whether the value is included in the set
    bool get(uint32_t ch) const {
        if (ch >= length()) return false;
        const uint32_t* bitmap = mData->bitmaps() + mData->indices()[ch >> kLogValuesPerPage];
        uint32_t index = ch & kPageMask;
        return (bitmap[index >> kLogBitsPerEl] & (kElFirst >> (index & kElMask))) != 0;
    }

    // One more than the maximum value in the set, or zero if empty
    uint32_t length() const { return mData != nullptr ? mData->mMaxVal : 0; }

    bool empty() const { return mData == nullptr || mData->mMaxVal == 0; }

    // The next set bit starting at fromIndex, inclusive, or kNotFound
    // if none exists.
    uint32_t nextSetBit(uint32_t fromIndex) const;

    static const uint32_t kNotFound = ~0u;

private:
    void initFromRanges(const uint32_t* ranges, size_t nRanges);
    void initFromBuffer(BufferReader* reader);

    static const uint32_t kMaximumCapacity = 0xFFFFFF;
    static const int kLogValuesPerPage = 8;
    static const int kPageMask = (1 << kLogValuesPerPage) - 1;
    static const int kLogBytesPerEl = 2;
    static const int kLogBitsPerEl = kLogBytesPerEl + 3;
    static const int kElMask = (1 << kLogBitsPerEl) - 1;
    // invariant: sizeof(element) == (1 << kLogBytesPerEl)
    typedef uint32_t element;
    static const element kElAllOnes = ~((element)0);
    static const element kElFirst = ((element)1) << kElMask;
    static const uint16_t noZeroPage = 0xFFFF;

    static uint32_t calcNumPages(const uint32_t* ranges, size_t nRanges);
    static int CountLeadingZeros(element x);

    // MappableData represents memory block holding SparseBitSet's fields.
    // 'packed' is used so that the object layout won't change between
    // 32-bit and 64-bit processes.
    // 'aligned(4)' is only for optimization.
    struct __attribute__((packed, aligned(4))) MappableData {
        uint32_t mMaxVal;
        uint32_t mIndicesCount;
        uint32_t mBitmapsCount;
        uint16_t mZeroPageIndex;
        // Whether the memory is mapped (BufferReader::map()) or allocated
        // (malloc()).
        uint16_t mIsMapped;
        // mArray packs two arrays:
        // element mBitmaps[mBitmapsCount];
        // uint16_t mIndices[mIndicesCount];
        __attribute__((aligned(4))) uint32_t mArray[];
        const element* bitmaps() const { return mArray; }
        element* bitmaps() { return mArray; }
        const uint16_t* indices() const {
            return reinterpret_cast<const uint16_t*>(mArray + mBitmapsCount);
        }
        uint16_t* indices() { return reinterpret_cast<uint16_t*>(mArray + mBitmapsCount); }
        size_t size() const { return calcSize(mIndicesCount, mBitmapsCount); }
        static size_t calcSize(uint32_t indicesCount, uint32_t bitmapsCount) {
            static_assert(std::is_same<element, uint32_t>::value);
            static_assert(sizeof(uint32_t) == 4);
            static_assert(sizeof(uint16_t) == 2);
            // Round-up indicesCount / 2
            size_t arrayCount = bitmapsCount + (indicesCount + 1) / 2;
            return offsetof(MappableData, mArray) + sizeof(uint32_t) * arrayCount;
        }
        static MappableData* allocate(uint32_t indicesCount, uint32_t bitmapsCount);
    };

    // MappableDataDeleter does NOT call free() if the data is on a memory map.
    class MappableDataDeleter {
    public:
        void operator()(const MappableData* data) const {
            if (data != nullptr && !data->mIsMapped) free((void*)data);
        }
    };

    std::unique_ptr<const MappableData, MappableDataDeleter> mData;

    // Forbid copy and assign.
    SparseBitSet(const SparseBitSet&) = delete;
    SparseBitSet& operator=(const SparseBitSet&) = delete;
};

}  // namespace minikin

#endif  // MINIKIN_SPARSE_BIT_SET_H