/* decode.c - Don Yang (uguu.org)

   Test encoded data (from encode.c).

   11/28/01
                                                                           */
/*@ -shiftsigned -type @*/

#include<ctype.h>
#include<stdio.h>

#define BITS_PER_GROUP  2
#define BASE64_OFFSET   '0'


static int GetBit(FILE *infile, int *byte, int *bit, int *size);
static int GetCode(FILE *infile, int *byte, int *bit, int *size);

/******************************************************************** main */
int main(int argc, char **argv)
{
   FILE *infile, *outfile;
   int i, bit, byte, size;
   int cstate, csize, code;

   infile = stdin;
   outfile = stdout;
   if( argc > 1 )
   {
      if( (infile = fopen(argv[1], "rb")) == NULL )
         return printf("Can not open %s\n", argv[1]);
      if( argc > 2 )
      {
         if( (outfile = fopen(argv[2], "wt+")) == NULL )
         {
            (void)fclose(infile);
            return printf("Can not create %s\n", argv[2]);
         }
      }
   }


   (void)fseek(infile, 1, SEEK_SET);
   while( (i = fgetc(infile)) != '\"' );
   while( !isdigit(i = fgetc(infile)) );
   for(size = i - '0'; isdigit(i = fgetc(infile)); size = size * 10 + i - '0');

   (void)fseek(infile, 1, SEEK_SET);
   bit = byte = 6;
   for(cstate = csize = 0; csize < size;)
   {
      code = GetCode(infile, &byte, &bit, &csize);
      if( code == 0 )
      {
         (void)fputc('\n', outfile);
         cstate = 0;
      }
      else
      {
         for(i = 0; i < code; i++)
            (void)fputc(cstate == 0 ? ' ' : 'X', outfile);
         cstate ^= 1;
      }
   }


   (void)fclose(infile);
   (void)fclose(outfile);
   return 0;

} /* main() */

/****************************************************************** GetBit */
static int GetBit(FILE *infile, int *byte, int *bit, int *size)
{
   int i;

   if( *bit > 5 )
   {
      if( (i = fgetc(infile)) == '\\' )
         i = fgetc(infile);
      *byte = i - BASE64_OFFSET;
      *bit = 0;
   }
   (*size)++;
   return (*byte & (1 << (*bit)++)) == 0 ? 0 : 1;

} /* GetBit() */

/***************************************************************** GetByte */
static int GetCode(FILE *infile, int *byte, int *bit, int *size)
{
   int i, j, code;

   for(i = code = 0;; i += BITS_PER_GROUP)
   {
      for(j = 0; j < BITS_PER_GROUP; j++)
         code |= GetBit(infile, byte, bit, size) << (i + j);
      if( GetBit(infile, byte, bit, size) == 0 )
         break;
   }
   return code;

} /* GetByte() */