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:
SparseBitSet()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
get(uint32_t ch)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
length()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