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    */
buildHuffmanTable(int[] rootTable, int tableOffset, int rootBits, int[] codeLengths, int codeLengthsSize)62   static void buildHuffmanTable(int[] rootTable, int tableOffset, int rootBits, int[] codeLengths,
63       int codeLengthsSize) {
64     int key; // Reversed prefix code.
65     int[] sorted = new int[codeLengthsSize]; // Symbols sorted by code length.
66     // TODO: fill with zeroes?
67     int[] count = new int[MAX_LENGTH + 1]; // Number of codes of each length.
68     int[] offset = new int[MAX_LENGTH + 1]; // Offsets in sorted table for each length.
69     int symbol;
70 
71     // Build histogram of code lengths.
72     for (symbol = 0; symbol < codeLengthsSize; symbol++) {
73       count[codeLengths[symbol]]++;
74     }
75 
76     // Generate offsets into sorted symbol table by code length.
77     offset[1] = 0;
78     for (int len = 1; len < MAX_LENGTH; len++) {
79       offset[len + 1] = offset[len] + count[len];
80     }
81 
82     // Sort symbols by length, by symbol order within each length.
83     for (symbol = 0; symbol < codeLengthsSize; symbol++) {
84       if (codeLengths[symbol] != 0) {
85         sorted[offset[codeLengths[symbol]]++] = symbol;
86       }
87     }
88 
89     int tableBits = rootBits;
90     int tableSize = 1 << tableBits;
91     int totalSize = tableSize;
92 
93     // Special case code with only one value.
94     if (offset[MAX_LENGTH] == 1) {
95       for (key = 0; key < totalSize; key++) {
96         rootTable[tableOffset + key] = sorted[0];
97       }
98       return;
99     }
100 
101     // Fill in root table.
102     key = 0;
103     symbol = 0;
104     for (int len = 1, step = 2; len <= rootBits; len++, step <<= 1) {
105       for (; count[len] > 0; count[len]--) {
106         replicateValue(rootTable, tableOffset + key, step, tableSize, len << 16 | sorted[symbol++]);
107         key = getNextKey(key, len);
108       }
109     }
110 
111     // Fill in 2nd level tables and add pointers to root table.
112     int mask = totalSize - 1;
113     int low = -1;
114     int currentOffset = tableOffset;
115     for (int len = rootBits + 1, step = 2; len <= MAX_LENGTH; len++, step <<= 1) {
116       for (; count[len] > 0; count[len]--) {
117         if ((key & mask) != low) {
118           currentOffset += tableSize;
119           tableBits = nextTableBitSize(count, len, rootBits);
120           tableSize = 1 << tableBits;
121           totalSize += tableSize;
122           low = key & mask;
123           rootTable[tableOffset + low] =
124               (tableBits + rootBits) << 16 | (currentOffset - tableOffset - low);
125         }
126         replicateValue(rootTable, currentOffset + (key >> rootBits), step, tableSize,
127             (len - rootBits) << 16 | sorted[symbol++]);
128         key = getNextKey(key, len);
129       }
130     }
131   }
132 }
133