LZSS算法简介

LZSS算法是LZ77算法的改良版，由James A. Storer和Thomas Szymacski于1982年发表在ACM杂志上。LZSS算法充分地利用了位的标记，从而使得压缩效率相比较于LZ77而言，有了显著提高。

参考维基百科lzss

算法逻辑

在看本篇博文之前，需要知道LZ77算法的大体流程，如果不清楚可以点击这里浏览。

LZSS算法和LZ77算法的编码/解码流程逻辑是一致的，区别在于数据编码标记上，这里详细叙述一下LZSS的数据编码标记，可以看到它这种方式是如何显著提高压缩效率的。

在LZSS算法的数据编码里面有一个重要概念，就是：数据分组。

一个字节由8位组成，因此可以用一个字节来标记后面的至少8字节是否存在压缩编码，而不用\0x00\0x00的前缀来区分是否被压缩，从而节省了大量的\0x00\0x00前缀空间开销。

数据分组标记方式：

由此看见，这种数据分组的方式，最坏的情况下，只会每8个字节增加一个字节的额外空间开销，比起每个字节另外需要2个字节的空间开销而言，效率大幅提升。

源码实现

辅助函数：

unsigned int search_maxlen_p( unsigned char**  pt, \
                              unsigned char* head, \
                              unsigned char* tail, \
                              unsigned char* end)
{
  unsigned char* tmp = *pt;
  unsigned int leng = 0, i;

  for(tmp;tmp >= head;tmp--){
    if(tmp[0] == tail[0]){
      for(i = 0;(i < ((1 << LENGTHBITS) + 1)) && (&tmp[i] > head) && (&tmp[i] < tail) && (&tail[i] < end) && (tmp[i] == tail[i]);i++);
      if(i > leng){
        leng = i;
        *pt = tmp;
      }
    }
  }
  return leng;
}

编码实现：

int lzss_encode(FILE* in,FILE* out)
{
  unsigned char* buf = 0,* head = 0,* tail = 0;
  long size = 0;

  if(! in || ! out)return 1;

  get_filesize(in,size);
  if(size <= 0)return 1;

  buf = (unsigned char*)malloc(size* sizeof(unsigned char));
  if(! buf)return 1;

  if(fread(buf,sizeof(unsigned char),size,in) == size){
    unsigned int  np      =  1 ,control_count = 0;
    unsigned char suf[17] = {0}; // 2*8 + 1

    fputc((size >> 24) & 0xff,out);
    fputc((size >> 16) & 0xff,out);
    fputc((size >>  8) & 0xff,out);
    fputc((size >>  0) & 0xff,out);

    head = tail = buf;

    for(;;){
      head = tail - WINDOWSIZE;
      if(head < buf)head = buf;

      if(control_count >= 8){
        put_bytes(suf,np,out);
        memset(suf,0,sizeof(suf));
        np = 1;
        control_count = 1;
      } else {
        control_count++;
      }

      if(tail > buf + size - 1)break;

      if(head < tail){
        unsigned int bytes = 0;
        unsigned char* pt = tail - 1;
        bytes = search_maxlen_p(&pt,head,tail,buf + size);
        if(bytes > 2){ // if find a pattern more than 2 bytes, save its offset-length.
          suf[0]  <<= 1;
          suf[0]   |= OFFLENFLAG;
          suf[np++] = (unsigned char)(( (tail - pt) >> LENGTHBITS) & 0xff);
          suf[np++] = (unsigned char)((((tail - pt) << LENGTHBITS) + bytes - 2) & 0xff);
          tail += bytes;
        } else {
          suf[0]  <<= 1;
          suf[np++] = tail[0];
          tail++;
        }
      } else {
        suf[0]  <<= 1;
        suf[np++] = tail[0];
        tail++;
      }

    }

    for(control_count;control_count <= 8;control_count++)suf[0] <<= 1; // must keep highest bit is aligned to first byte.
    put_bytes(suf,np,out);
  }

  free(buf);

  return 0;
}

解码实现：

int lzss_decode(FILE* in,FILE* out)
{
  unsigned char* in_buffer = 0,* out_buffer = 0,* head = 0,* tail = 0;
  long in_size = 0,out_size = 0;

  if(! in || ! out)return 1;

  get_filesize(in,in_size);
  if(in_size <= 0)return 1;

  out_size  = (fgetc(in) & 0xff) << 24;
  out_size += (fgetc(in) & 0xff) << 16;
  out_size += (fgetc(in) & 0xff) <<  8;
  out_size += (fgetc(in) & 0xff) <<  0;
  if(out_size <= 0)return 1;

  in_buffer = (unsigned char*)malloc(sizeof(unsigned char)* in_size);
  if(! in_buffer)return 1;

  out_buffer = (unsigned char*)malloc(sizeof(unsigned char)* out_size);
  if(out_buffer){
    if(fread(in_buffer,sizeof(unsigned char),in_size,in) == in_size - 4){
      unsigned int  control_count = 0;
      unsigned char control_flags = 0;
      unsigned char*           pt = (unsigned char*)in_buffer;

      head  = tail  = out_buffer;
      control_flags = pt[0];
      pt++;

      for(;;){
        if(pt >= (in_buffer + in_size - 4))break;

        if(control_count >= 8){
          control_flags = pt[0];
          pt++;
          control_count = 1;
        } else {
          control_count++;
        }

        if(control_flags >> 7){ // offset-length.
          unsigned int offset = 0,length = 0,i;
          offset  =  pt[0] << LENGTHBITS;
          offset += (pt[1] >> LENGTHBITS) & 0x0f;
          length  = (pt[1] & 0x0f) + 2;
          for(i = 0;i < length;i++){
            tail[i] = ((unsigned char*)(tail - offset))[i];
          }
          tail += length;
          pt += 2;
          control_flags <<= 1;
        } else { // literal.
          tail[0] = pt[0];
          tail++;
          pt++;
          control_flags <<= 1;
        }
      }
      fwrite(out_buffer,sizeof(unsigned char),out_size,out);

    }
    free(out_buffer);
  }

  free(in_buffer);

  return 0;
}