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      1 /*
      2  * Copyright (C) 2012 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #ifndef MINIKIN_SPARSE_BIT_SET_H
     18 #define MINIKIN_SPARSE_BIT_SET_H
     19 
     20 #include <stdint.h>
     21 #include <sys/types.h>
     22 #include <UniquePtr.h>
     23 
     24 // ---------------------------------------------------------------------------
     25 
     26 namespace android {
     27 
     28 // This is an implementation of a set of integers. It is optimized for
     29 // values that are somewhat sparse, in the ballpark of a maximum value
     30 // of thousands to millions. It is particularly efficient when there are
     31 // large gaps. The motivating example is Unicode coverage of a font, but
     32 // the abstraction itself is fully general.
     33 
     34 class SparseBitSet {
     35 public:
     36     SparseBitSet(): mMaxVal(0) {
     37     }
     38 
     39     // Clear the set
     40     void clear();
     41 
     42     // Initialize the set to a new value, represented by ranges. For
     43     // simplicity, these ranges are arranged as pairs of values,
     44     // inclusive of start, exclusive of end, laid out in a uint32 array.
     45     void initFromRanges(const uint32_t* ranges, size_t nRanges);
     46 
     47     // Determine whether the value is included in the set
     48     bool get(uint32_t ch) const {
     49         if (ch >= mMaxVal) return false;
     50         uint32_t *bitmap = &mBitmaps[mIndices[ch >> kLogValuesPerPage]];
     51         uint32_t index = ch & kPageMask;
     52         return (bitmap[index >> kLogBitsPerEl] & (kElFirst >> (index & kElMask))) != 0;
     53     }
     54 
     55     // One more than the maximum value in the set, or zero if empty
     56     uint32_t length() const {
     57         return mMaxVal;
     58     }
     59 
     60     // The next set bit starting at fromIndex, inclusive, or kNotFound
     61     // if none exists.
     62     uint32_t nextSetBit(uint32_t fromIndex) const;
     63 
     64     static const uint32_t kNotFound = ~0u;
     65 
     66 private:
     67     static const int kLogValuesPerPage = 8;
     68     static const int kPageMask = (1 << kLogValuesPerPage) - 1;
     69     static const int kLogBytesPerEl = 2;
     70     static const int kLogBitsPerEl = kLogBytesPerEl + 3;
     71     static const int kElMask = (1 << kLogBitsPerEl) - 1;
     72     // invariant: sizeof(element) == (1 << kLogBytesPerEl)
     73     typedef uint32_t element;
     74     static const element kElAllOnes = ~((element)0);
     75     static const element kElFirst = ((element)1) << kElMask;
     76     static const uint32_t noZeroPage = ~0u;
     77 
     78     static uint32_t calcNumPages(const uint32_t* ranges, size_t nRanges);
     79     static int CountLeadingZeros(element x);
     80 
     81     uint32_t mMaxVal;
     82     UniquePtr<uint32_t[]> mIndices;
     83     UniquePtr<element[]> mBitmaps;
     84     uint32_t mZeroPageIndex;
     85 };
     86 
     87 // Note: this thing cannot be used in vectors yet. If that were important, we'd need to
     88 // make the copy constructor work, and probably set up move traits as well.
     89 
     90 }; // namespace android
     91 
     92 #endif // MINIKIN_SPARSE_BIT_SET_H
     93