base/compression/lzss_compression.cc: base/compression/lzss_compression.hh: Rework for better performance --HG-- extra : convert_revision : b13d706e0e23cbe7122b611258354c66cf5f3c70
171 lines
5 KiB
C++
171 lines
5 KiB
C++
/*
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* Copyright (c) 2003 The Regents of The University of Michigan
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* met: redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer;
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* redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution;
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* neither the name of the copyright holders nor the names of its
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* contributors may be used to endorse or promote products derived from
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* this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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/** @file
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* LZSSCompression definitions.
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*/
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#include <assert.h>
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#include "base/compression/lzss_compression.hh"
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#include "base/misc.hh" //for fatal
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void
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LZSSCompression::findSubString(uint8_t *src, int back, int size, uint16_t &L,
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uint16_t &P)
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{
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int front = 0;
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int max_length = size - back;
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L = 0;
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P = back - 1;
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while (front < back) {
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while (src[front] != src[back] && front < back) ++front;
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if (front >= back) {
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return;
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}
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int i = 1;
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while (src[front+i] == src[back+i] && i < max_length) ++i;
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if (i >= L) {
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L = i;
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P = front;
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}
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if (src[front+i] != src[back+i-1]) {
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// can't find a longer substring until past this point.
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front += i;
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} else {
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++front;
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}
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}
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}
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int
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LZSSCompression::emitByte(uint8_t *dest, uint8_t byte)
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{
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if ((byte >> 5 & 0x7) == 0 || (byte >> 5 & 0x7) == 7) {
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// If the top 3 bits are the same, emit 00<6bits>
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dest[0] = byte & 0x3f;
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return 1;
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} else {
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// emit 01XXXXXX <8 bits>
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dest[0] = 0x40;
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dest[1] = byte;
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return 2;
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}
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}
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void
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LZSSCompression::emitString(uint8_t *dest, uint16_t P, uint16_t L)
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{
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// Emit 1<7P> <5P><3L> <8L>
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dest[0] = 1<<7 | (P >> 5 & 0x7f);
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dest[1] = ((P & 0x1f) << 3) | (L>>8 & 0x3);
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dest[2] = L & 0xFF;
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}
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int
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LZSSCompression::compress(uint8_t *dest, uint8_t *src, int size)
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{
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if (size > 4096) {
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fatal("Compression can only handle block sizes of 4096 bytes or less");
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}
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// Encode the first byte.
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int dest_index = emitByte(dest, src[0]);
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int i = 1;
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// A 11 bit field
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uint16_t L;
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// A 12 bit field
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uint16_t P = 0;
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while (i < size && dest_index < size) {
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L = 0;
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if (dest_index+3 >= size) {
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dest_index = size;
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continue;
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}
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if (i == size - 1) {
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// Output the character
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dest_index += emitByte(&dest[dest_index], src[i]);
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++i;
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continue;
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}
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findSubString(src, i, size, L, P);
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if (L > 1) {
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// Output the string reference
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emitString(&dest[dest_index], P, L);
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dest_index += 3;
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i = i+L;
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} else {
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// Output the character
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dest_index += emitByte(&dest[dest_index], src[i]);
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++i;
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}
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}
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if (dest_index >= size) {
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// Have expansion instead of compression, just copy.
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memcpy(dest,src,size);
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return size;
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}
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return dest_index;
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}
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int
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LZSSCompression::uncompress(uint8_t *dest, uint8_t *src, int size)
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{
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int index = 0;
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int i = 0;
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while (i < size) {
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if (src[i] & 1<<7 ) {
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// We have a string
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// Extract P
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int start = (src[i] & 0x3f)<<5 | ((src[i+1] >> 3) & 0x1f);
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// Extract L
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int len = (src[i+1] & 0x07)<<8 | src[i+2];
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i += 3;
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for (int j = start; j < start+len; ++j) {
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dest[index++] = dest[j];
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}
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} else {
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// We have a character
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if (src[i] & 1<<6) {
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// Value is in the next byte
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dest[index++] = src[i+1];
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i += 2;
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} else {
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// just extend the lower 6 bits
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dest[index++] = (src[i] & 0x3f) | ((src[i] & 1<<5) ? 0xC0 : 0);
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++i;
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}
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}
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}
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return index;
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}
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