1 /* Copyright 2015 Google Inc. All Rights Reserved.
2 
3    Distributed under MIT license.
4    See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
5 */
6 
7 package org.brotli.dec;
8 
9 /**
10  * Utilities for building Huffman decoding tables.
11  */
12 final class Huffman {
13 
14   private static final int MAX_LENGTH = 15;
15 
16   /**
17    * Returns reverse(reverse(key, len) + 1, len).
18    *
19    * <p> reverse(key, len) is the bit-wise reversal of the len least significant bits of key.
20    */
getNextKey(int key, int len)21   private static int getNextKey(int key, int len) {
22     int step = 1 << (len - 1);
23     while ((key & step) != 0) {
24       step >>= 1;
25     }
26     return (key & (step - 1)) + step;
27   }
28 
29   /**
30    * Stores {@code item} in {@code table[0], table[step], table[2 * step] .., table[end]}.
31    *
32    * <p> Assumes that end is an integer multiple of step.
33    */
replicateValue(int[] table, int offset, int step, int end, int item)34   private static void replicateValue(int[] table, int offset, int step, int end, int item) {
35     do {
36       end -= step;
37       table[offset + end] = item;
38     } while (end > 0);
39   }
40 
41   /**
42    * @param count histogram of bit lengths for the remaining symbols,
43    * @param len code length of the next processed symbol.
44    * @return table width of the next 2nd level table.
45    */
nextTableBitSize(int[] count, int len, int rootBits)46   private static int nextTableBitSize(int[] count, int len, int rootBits) {
47     int left = 1 << (len - rootBits);
48     while (len < MAX_LENGTH) {
49       left -= count[len];
50       if (left <= 0) {
51         break;
52       }
53       len++;
54       left <<= 1;
55     }
56     return len - rootBits;
57   }
58 
59   /**
60    * Builds Huffman lookup table assuming code lengths are in symbol order.
61    *
62    * @return number of slots used by resulting Huffman table
63    */
buildHuffmanTable(int[] tableGroup, int tableIdx, int rootBits, int[] codeLengths, int codeLengthsSize)64   static int buildHuffmanTable(int[] tableGroup, int tableIdx, int rootBits, int[] codeLengths,
65       int codeLengthsSize) {
66     int tableOffset = tableGroup[tableIdx];
67     int key; // Reversed prefix code.
68     int[] sorted = new int[codeLengthsSize]; // Symbols sorted by code length.
69     // TODO: fill with zeroes?
70     int[] count = new int[MAX_LENGTH + 1]; // Number of codes of each length.
71     int[] offset = new int[MAX_LENGTH + 1]; // Offsets in sorted table for each length.
72     int symbol;
73 
74     // Build histogram of code lengths.
75     for (symbol = 0; symbol < codeLengthsSize; symbol++) {
76       count[codeLengths[symbol]]++;
77     }
78 
79     // Generate offsets into sorted symbol table by code length.
80     offset[1] = 0;
81     for (int len = 1; len < MAX_LENGTH; len++) {
82       offset[len + 1] = offset[len] + count[len];
83     }
84 
85     // Sort symbols by length, by symbol order within each length.
86     for (symbol = 0; symbol < codeLengthsSize; symbol++) {
87       if (codeLengths[symbol] != 0) {
88         sorted[offset[codeLengths[symbol]]++] = symbol;
89       }
90     }
91 
92     int tableBits = rootBits;
93     int tableSize = 1 << tableBits;
94     int totalSize = tableSize;
95 
96     // Special case code with only one value.
97     if (offset[MAX_LENGTH] == 1) {
98       for (key = 0; key < totalSize; key++) {
99         tableGroup[tableOffset + key] = sorted[0];
100       }
101       return totalSize;
102     }
103 
104     // Fill in root table.
105     key = 0;
106     symbol = 0;
107     for (int len = 1, step = 2; len <= rootBits; len++, step <<= 1) {
108       for (; count[len] > 0; count[len]--) {
109         replicateValue(tableGroup, tableOffset + key, step, tableSize,
110             len << 16 | sorted[symbol++]);
111         key = getNextKey(key, len);
112       }
113     }
114 
115     // Fill in 2nd level tables and add pointers to root table.
116     int mask = totalSize - 1;
117     int low = -1;
118     int currentOffset = tableOffset;
119     for (int len = rootBits + 1, step = 2; len <= MAX_LENGTH; len++, step <<= 1) {
120       for (; count[len] > 0; count[len]--) {
121         if ((key & mask) != low) {
122           currentOffset += tableSize;
123           tableBits = nextTableBitSize(count, len, rootBits);
124           tableSize = 1 << tableBits;
125           totalSize += tableSize;
126           low = key & mask;
127           tableGroup[tableOffset + low] =
128               (tableBits + rootBits) << 16 | (currentOffset - tableOffset - low);
129         }
130         replicateValue(tableGroup, currentOffset + (key >> rootBits), step, tableSize,
131             (len - rootBits) << 16 | sorted[symbol++]);
132         key = getNextKey(key, len);
133       }
134     }
135     return totalSize;
136   }
137 }
138