PackBits RLE compressor

packbits.c

// PackBits RLE compressor
// Usage: packbits c|d input output
// Originally by Matt Mahoney, adapted for PackBits. Public domain.

#include <stdio.h>

int main(int argc, char** argv) {

  // Check args
  if (argc!=4)
    return printf("To compress|decompress: packbits c|d input output\n"), 1;

  // Open files
  FILE* in=fopen(argv[2], "rb");
  if (!in) return perror(argv[2]), 1;
  FILE* out=fopen(argv[3], "wb");
  if (!out) return perror(argv[3]), 1;

  int c, c1=-1;  // current and last char
  char t[128];  // buffer for literals
  int run=0;  // current run length
  int lrun=0;  // current literal run length

  // Compress
  if (argv[1][0]=='c') {

    // RLE encode
    do {
      c=getc(in);
      if (c==c1) {
        if (lrun>1) { putc(lrun-2, out); fwrite(t, 1, lrun-1, out); }
        lrun=0;
        if (++run==128) { putc(-127, out); putc(c1, out); run=0; t[lrun++]=c; }
      }
      else {
        if (run>0) { putc(-run, out); putc(c1, out); run=0; }
        if (lrun==128) { putc(127, out); fwrite(t, 1, 128, out); lrun=0; }
        t[lrun++]=c;
      }
      c1=c;
    } while (c!=EOF);
    if (lrun>1) { putc(lrun-2, out); fwrite(t, 1, lrun-1, out); }
  }

  // Decompress
  else if (argv[1][0]=='d') {

    // Decode
    while ((c=getc(in))!=EOF) {
      if (c>127) {
        if (c==128) continue;
        run=257-c;
        if ((c=getc(in))==EOF) return 1;
        for (; run>0; --run) putc(c, out);
      }
      else {
        lrun=c+1;
        for (; lrun>0; --lrun) {
          if ((c=getc(in))==EOF) return 1;
          putc(c, out);
        }
      }
    }
  }

  // Print result
  printf("PRLE %lu -> %lu\n", ftell(in), ftell(out));
  fclose(in);
  fclose(out);
  return 0;
}