/*
 * MVCOMP unpacker
 * Copyright (C) 2024 Mateusz Viste
 *
 * MIT License.
 *
 * This is a simple example implementation, not a production-ready tool.
 *
 * https://mvcomp.sourceforge.io
 */


#include <errno.h>
#include <stdio.h>
#include <string.h>


/* unpacks an MVCOMP-compressed token to dst. returns length of uncompressed data. */
static unsigned short mvcomp_depack(unsigned char *dst, unsigned short buff16) {
  static unsigned char rawwords; /* number of uncompressible words */
  unsigned char *dst_start = dst;

  /* token format is LLLL OOOO OOOO OOOO, where:
   * OOOO OOOO OOOO is the back reference offset (number of bytes-1 to rewind)
   * LLLL is the number of bytes (-1) that have to be copied from the offset.
   *
   * However, if LLLL is zero then the token's format is different:
   * 0000 RRRR BBBB BBBB
   *
   * The above form occurs when uncompressible data is encountered:
   * BBBB BBBB is the literal value of a byte to be copied
   * RRRR is the number of RAW (uncompressible) WORDS that follow (possibly 0)
   */

  /* raw word? */
  if (rawwords != 0) {
    unsigned short *dst16 = (void *)dst;
    *dst16 = buff16;
    dst += 2;
    rawwords--;

  /* literal byte? */
  } else if ((buff16 & 0xF000) == 0) {
    *dst = buff16 & 0xff;
    dst++;
    rawwords = buff16 >> 8; /* number of RAW words that are about to follow */

  /* else it's a backreference */
  } else {
    unsigned char *src = dst - (buff16 & 0x0FFF) - 1;
    buff16 >>= 12;
    for (;;) {
      *dst = *src;
      dst++;
      src++;
      if (buff16 == 0) break;
      buff16--;
    }
  }

  return((unsigned short)(dst - dst_start));
}


int main (int argc, char **argv) {
  FILE *in, *out;
  unsigned long bytes_in = 0, bytes_out = 0;
  unsigned char buff[1024 * 8];
  unsigned char *buffptr = buff;

  if (argc != 3) {
    puts("usage: mvucomp file.mvc file.out");
    return(1);
  }

  in = fopen(argv[2], "rb");
  if (in != NULL) {
    fclose(in);
    puts("error: output file already exists");
    return(1);
  }

  in = fopen(argv[1], "rb");
  if (in == NULL) {
    puts("failed to open input file");
    return(1);
  }

  out = fopen(argv[2], "wb");
  if (out == NULL) {
    puts("failed to open output file");
    fclose(in);
    return(1);
  }

  for (;;) {
    unsigned short token;
    unsigned short len;

    if (fread(&token, 1, 2, in) != 2) break;
    bytes_in += 2;

    len = mvcomp_depack(buffptr, token);

    if (fwrite(buffptr, 1, len, out) != len) {
      fprintf(stderr, "ERROR: fwrite() to destination failed - %s\n", strerror(errno));
      break;
    }

    bytes_out += len;
    buffptr += len;

    /* rewind the buffer */
    if ((buffptr - buff) > 6 * 1024) {
      memcpy(buff, buff + 2048, (size_t)((buffptr - buff) - 2048));
      buffptr -= 2048;
    }
  }

  fclose(out);
  fclose(in);

  printf("%s (%lu bytes) -> %s (%lu bytes)\n", argv[1], bytes_in, argv[2], bytes_out);

  return(0);
}