#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

void usage(const char *app, const char *msg)
{
  if(msg != NULL) {
    fprintf(stderr, msg);
    fprintf(stderr, " ");
  }
  fprintf(stderr, "Usage:\n");
  fprintf(stderr, "\t%s [e|d] old.bin new.bin\n\n", app);
  fprintf(stderr, "First argument should be `e' for encrypt, `d' for decrypt.\n");
}

/* Actual codec action happens here */
int ww_crypt_core(FILE *ifp, FILE *ofp, int dir)
{
  int ret = 0;
  uint8_t last, buf;
  uint8_t byte_cnt;
  uint8_t c;

  byte_cnt = 0;
  for(;;) {
    if(fread(&c, 1, 1, ifp) != 1) {
      if(feof(ifp)) /* End of file reached */
        break;
      /* Otherwise it was an error! */
      ret = -1;
      break;
    }

    if(byte_cnt == 0) /* Reset last to 0xFF for each XMODEM sector */
      last = 0xFF;

    if(dir) { /* Encrypt */
      c ^= last;
      buf = c;
    }else{ /* Decrypt */
      buf = c;
      c ^= last;
    }

    last = buf;

    if(fwrite(&c, 1, 1, ofp) != 1) {
      /* Had to be an error! */
      ret = -1;
      break;
    }

    byte_cnt++;
    byte_cnt &= 0x7F; /* XMODEM sectors are every 0x80 bytes */
  }

  return ret;
}

int ww_crypt(const char *infn, const char *outfn, int dir)
{
  int ret = 0;
  FILE *ifp, *ofp;

  ifp = fopen(infn, "rb");
  if(ifp == NULL) {
    ret = -1;
    goto fail_noclose;
  }
  ofp = fopen(outfn, "wb");
  if(ofp == NULL) {
    ret = -1;
    goto fail_closeifp;
  }

  if(ww_crypt_core(ifp, ofp, dir)) {
    ret = -1;
    goto fail_closeall;
  }

fail_closeall:
  fclose(ofp);
fail_closeifp:
  fclose(ifp);
fail_noclose:
  return ret;
}

int ww_decrypt(const char *infn, const char *outfn)
{
  return ww_crypt(infn, outfn, 0);
}
int ww_encrypt(const char *infn, const char *outfn)
{
  return ww_crypt(infn, outfn, 1);
}

int main(int argc, const char *argv[])
{
  int ret = -1;
  fprintf(stderr, "WW_CODEC (c)2014 Alex \"trap15\" Marshall\n");

  if(argc < 3) {
    usage(argv[0], "Too few arguments.");
    return EXIT_FAILURE;
  }

  switch(argv[1][0]) {
    case 'e': case 'E':
      ret = ww_encrypt(argv[2], argv[3]);
      break;
    case 'd': case 'D':
      ret = ww_decrypt(argv[2], argv[3]);
      break;
    default:
      usage(argv[0], "Invalid operation type.");
      return EXIT_FAILURE;
  }

  if(ret) {
    fprintf(stderr, "Error in codec!\n");
    return EXIT_FAILURE;
  }

  return EXIT_SUCCESS;
}