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SparseBitSet.h
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4  * Licensed under the Apache License, Version 2.0 (the "License");
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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 
23 #include <memory>
24 
25 // ---------------------------------------------------------------------------
26 
27 namespace minikin {
28 
29 // This is an implementation of a set of integers. It is optimized for
30 // values that are somewhat sparse, in the ballpark of a maximum value
31 // of thousands to millions. It is particularly efficient when there are
32 // large gaps. The motivating example is Unicode coverage of a font, but
33 // the abstraction itself is fully general.
34 class SparseBitSet {
35  public:
36  // Create an empty bit set.
37  SparseBitSet() : mMaxVal(0) {}
38 
39  // Initialize the set to a new value, represented by ranges. For
40  // simplicity, these ranges are arranged as pairs of values,
41  // inclusive of start, exclusive of end, laid out in a uint32 array.
42  SparseBitSet(const uint32_t* ranges, size_t nRanges) : SparseBitSet() {
43  initFromRanges(ranges, nRanges);
44  }
45 
46  SparseBitSet(SparseBitSet&&) = default;
47  SparseBitSet& operator=(SparseBitSet&&) = default;
48 
49  // Determine whether the value is included in the set
50  bool get(uint32_t ch) const {
51  if (ch >= mMaxVal)
52  return false;
53  const uint32_t* bitmap = &mBitmaps[mIndices[ch >> kLogValuesPerPage]];
54  uint32_t index = ch & kPageMask;
55  return (bitmap[index >> kLogBitsPerEl] & (kElFirst >> (index & kElMask))) !=
56  0;
57  }
58 
59  // One more than the maximum value in the set, or zero if empty
60  uint32_t length() const { return mMaxVal; }
61 
62  // The next set bit starting at fromIndex, inclusive, or kNotFound
63  // if none exists.
64  uint32_t nextSetBit(uint32_t fromIndex) const;
65 
66  static const uint32_t kNotFound = ~0u;
67 
68  private:
69  void initFromRanges(const uint32_t* ranges, size_t nRanges);
70 
71  static const uint32_t kMaximumCapacity = 0xFFFFFF;
72  static const int kLogValuesPerPage = 8;
73  static const int kPageMask = (1 << kLogValuesPerPage) - 1;
74  static const int kLogBytesPerEl = 2;
75  static const int kLogBitsPerEl = kLogBytesPerEl + 3;
76  static const int kElMask = (1 << kLogBitsPerEl) - 1;
77  // invariant: sizeof(element) == (1 << kLogBytesPerEl)
78  typedef uint32_t element;
79  static const element kElAllOnes = ~((element)0);
80  static const element kElFirst = ((element)1) << kElMask;
81  static const uint16_t noZeroPage = 0xFFFF;
82 
83  static uint32_t calcNumPages(const uint32_t* ranges, size_t nRanges);
84  static int CountLeadingZeros(element x);
85 
86  uint32_t mMaxVal;
87 
88  std::unique_ptr<uint16_t[]> mIndices;
89  std::unique_ptr<element[]> mBitmaps;
90  uint16_t mZeroPageIndex;
91 
92  // Forbid copy and assign.
93  SparseBitSet(const SparseBitSet&) = delete;
94  void operator=(const SparseBitSet&) = delete;
95 };
96 
97 } // namespace minikin
98 
99 #endif // MINIKIN_SPARSE_BIT_SET_H
SparseBitSet(const uint32_t *ranges, size_t nRanges)
Definition: SparseBitSet.h:42
uint32_t length() const
Definition: SparseBitSet.h:60
static const uint32_t kNotFound
Definition: SparseBitSet.h:66
SparseBitSet & operator=(SparseBitSet &&)=default
uint32_t nextSetBit(uint32_t fromIndex) const