/*----------------------------------------------------*/
/* b64decode.c --   Base64-Decoding Filter            */
/*----------------------------------------------------*/
/*  Usage:                                            */
/*    b64decode [infile] > <file>                     */
/*        infile: filename of the file to be decoded  */
/*                (if omitted, input from stdin)      */
/*     result(stdout) should be redirected to <file>  */
/*----------------------------------------------------*/
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{ int c, m, c1, c2, k=0;
  FILE *fp;
# ifdef O_BINARY
  setmode(1,O_BINARY);
# endif
  if (argc == 1) {
    fp = stdin;
# ifdef O_BINARY
    setmode(0,O_BINARY);
# endif
  } else if ((fp=fopen(argv[1],"rb")) == NULL) {
    fprintf(stderr, "file %s not found\n", argv[1]); exit(1);
  }
  while ((c=getc(fp)) != EOF) {
    if      ((c=='\n') || (c=='\r')) continue;
    else if ((c>=0x41) && (c<=0x5A)) m = c-0x41;       /* 'A'-'Z':  0-25 */
    else if ((c>=0x61) && (c<=0x7A)) m = c-0x61 + 26;  /* 'a'-'z': 26-51 */
    else if ((c>=0x30) && (c<=0x39)) m = c-0x30 + 52;  /* '0'-'9': 52-61 */
    else if (c == '+') m = 62;
    else if (c == '/') m = 63;
    else if (c == '=') {
      if (k == 0)
        { fprintf(stderr, "\n*** unexpected '=' char neglected\n"); continue; }
      if (c1 != 0)
        { fprintf(stderr, "\n*** illegal padding('='), partial data lost\n"); }
      c1 = 0; if (++k > 3) k = 0; continue;
    }
    else {
      fprintf(stderr, "\n*** illegal Base64 char '%c' neglected\n", c);
      continue;
    }
    if      (k == 0) { c1 = (m & 0x3F) << 2; k++; }
    else if (k == 1) { c2 = (m & 0x30) >> 4; putchar(c1|c2);
                       c1 = (m & 0x0F) << 4; k++; }
    else if (k == 2) { c2 = (m & 0x3C) >> 2; putchar(c1|c2);
                       c1 = (m & 0x03) << 6; k++; }
    else if (k == 3) { c2 = (m & 0x3F); putchar(c1|c2); c1 = 0; k = 0; }
  }
  if (k != 0) fprintf(stderr, "\n*** incomplete trailing char\n");
  fclose(fp); fclose(stdout);
  return(0);
}