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