src/base/compression/lzss_compression.cc

   1 /*
   2  * Copyright (c) 2003-2005 The Regents of The University of Michigan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
   7  * met: redistributions of source code must retain the above copyright
   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  *
  28  * Authors: Erik Hallnor
  29  */
  30
  31 /** @file
  32  * LZSSCompression definitions.
  33  */
  34
  35 #include <assert.h>
  36
  37 #include "base/compression/lzss_compression.hh"
  38
  39 #include "base/misc.hh" //for fatal
  40
  41 void
  42 LZSSCompression::findSubString(uint8_t *src, int back, int size, uint16_t &L,
  43                                uint16_t &P)
  44 {
  45     int front = 0;
  46     int max_length = size - back;
  47     L = 0;
  48     P = back - 1;
  49     while (front < back) {
  50         while (src[front] != src[back] && front < back) ++front;
  51         if (front >= back) {
  52             return;
  53         }
  54         int i = 1;
  55         while (src[front+i] == src[back+i] && i < max_length) ++i;
  56         if (i >= L) {
  57             L = i;
  58             P = front;
  59         }
  60         if (src[front+i] != src[back+i-1]) {
  61             // can't find a longer substring until past this point.
  62             front += i;
  63         } else {
  64             ++front;
  65         }
  66     }
  67 }
  68
  69 int
  70 LZSSCompression::emitByte(uint8_t *dest, uint8_t byte)
  71 {
  72     if ((byte >> 5 & 0x7) == 0 || (byte >> 5 & 0x7) == 7) {
  73         // If the top 3 bits are the same, emit 00<6bits>
  74         dest[0] = byte & 0x3f;
  75         return 1;
  76     } else {
  77         // emit 01XXXXXX <8 bits>
  78         dest[0] = 0x40;
  79         dest[1] = byte;
  80         return 2;
  81     }
  82 }
  83
  84 void
  85 LZSSCompression::emitString(uint8_t *dest, uint16_t P, uint16_t L)
  86 {
  87     // Emit 1<7P> <5P><3L> <8L>
  88     dest[0] = 1<<7 | (P >> 5 & 0x7f);
  89     dest[1] = ((P & 0x1f) << 3) | (L>>8 & 0x3);
  90     dest[2] = L & 0xFF;
  91 }
  92
  93 int
  94 LZSSCompression::compress(uint8_t *dest, uint8_t *src, int size)
  95 {
  96     if (size > 4096) {
  97         fatal("Compression can only handle block sizes of 4096 bytes or less");
  98     }
  99
 100     // Encode the first byte.
 101     int dest_index = emitByte(dest, src[0]);
 102     int i = 1;
 103     // A 11 bit field
 104     uint16_t L;
 105     // A 12 bit field
 106     uint16_t P = 0;
 107
 108     while (i < size && dest_index < size) {
 109         L = 0;
 110
 111         if (dest_index+3 >= size) {
 112             dest_index = size;
 113             continue;
 114         }
 115
 116         if (i == size - 1) {
 117             // Output the character
 118             dest_index += emitByte(&dest[dest_index], src[i]);
 119             ++i;
 120             continue;
 121         }
 122         findSubString(src, i, size, L, P);
 123         if (L > 1) {
 124             // Output the string reference
 125             emitString(&dest[dest_index], P, L);
 126             dest_index += 3;
 127             i = i+L;
 128         } else {
 129             // Output the character
 130             dest_index += emitByte(&dest[dest_index], src[i]);
 131             ++i;
 132         }
 133     }
 134
 135     if (dest_index >= size) {
 136         // Have expansion instead of compression, just copy.
 137         memcpy(dest,src,size);
 138         return size;
 139     }
 140     return dest_index;
 141 }
 142
 143 int
 144 LZSSCompression::uncompress(uint8_t *dest, uint8_t *src, int size)
 145 {
 146     int index = 0;
 147     int i = 0;
 148     while (i < size) {
 149         if (src[i] & 1<<7 ) {
 150             // We have a string
 151             // Extract P
 152             int start = (src[i] & 0x3f)<<5 | ((src[i+1] >> 3) & 0x1f);
 153             // Extract L
 154             int len = (src[i+1] & 0x07)<<8 | src[i+2];
 155             i += 3;
 156             for (int j = start; j < start+len; ++j) {
 157                 dest[index++] = dest[j];
 158             }
 159         } else {
 160             // We have a character
 161             if (src[i] & 1<<6) {
 162                 // Value is in the next byte
 163                 dest[index++] = src[i+1];
 164                 i += 2;
 165             } else {
 166                 // just extend the lower 6 bits
 167                 dest[index++] = (src[i] & 0x3f) | ((src[i] & 1<<5) ? 0xC0 : 0);
 168                 ++i;
 169             }
 170         }
 171     }
 172     return index;
 173 }