/* This program changes the byte order in a data file of 
   the form 

   HEADERLENGTH=N\n  ... (N Bytes) (4 byte word) (4 byte word) ...

   The order of the bytes in each of the 4 byte words is reversed.

   usage:  byte_swap input_file output_file [block_size]

*/


#include <strings.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void critical(char *message);

void main (int argc, char *argv[]) {

   
   char *input_file, *output_file, *buffer_in, *buffer_out; 
   long word, hsize, bsize, bytes_read, count;
   int hflag;
   FILE *input, *output;

   if ((argc < 3) || (argc > 5))  {
      printf ("usage: %s  [-nh] input_file output_file [block_size]\n", 
	 &argv[0][0]);
      _exit(1);
   }
   if (strcmp(argv[1],"-nh") != 0) hflag = 0; else hflag = 1 ;
   input_file = argv[1+hflag];
   output_file = argv[2+hflag];
   if (argc == 4+hflag) {
     bsize = atoi(argv[3+hflag]);
   } else {
     bsize = 1024;
   }
   /* open the relevant files */

   if ((input = fopen(input_file, "r+b")) == NULL)
      critical("ERROR: Cannot open input file.\n");
   if ((output = fopen(output_file, "r+b")) == NULL) {
      if ((output = fopen(output_file, "wb")) == NULL)
	 critical("ERROR: Cannot create output file.\n");
   }
   hsize = 0;
   if (hflag == 0) {
     if (fscanf (input, "HEADERLENGTH=%li", &hsize) != 1)
       {
	 critical ("ERROR:  Could not find header length, use -nh if no header.\n");
       }
     buffer_in = malloc (hsize);
     if ( fseek(input, 0, SEEK_SET) != 0)
       critical("ERROR: Seek error in input.\n");
     if ( ( fread(buffer_in ,1 , hsize, input) != hsize) 
	  ||  ferror(input) ) 
       critical ("ERROR: Error reading header.\n");
     if ( ( fwrite(buffer_in ,1 , hsize, output) != hsize) 
	  ||  ferror(output) ) 
       critical ("ERROR: Error writing header.\n");
     free (buffer_in);
   }
   buffer_in = malloc (bsize);
   buffer_out = malloc (bsize);

   bytes_read = bsize;
   for (count = 0; bytes_read == bsize; count++) {

      if ( fseek(input, hsize + count*bsize, SEEK_SET) != 0)
	 critical("ERROR: Seek error in input.\n");
      bytes_read = fread(buffer_in, 1, bsize, input); 
      if ( ferror(input) ) critical ("ERROR: File read error.\n");


      for (word = 0; word < (bytes_read/4); word++) {
         buffer_out [word*4 + 0] = buffer_in [word*4 + 3];
         buffer_out [word*4 + 1] = buffer_in [word*4 + 2];
         buffer_out [word*4 + 2] = buffer_in [word*4 + 1];
         buffer_out [word*4 + 3] = buffer_in [word*4 + 0];
      }



      if (fseek(output, hsize + count*bsize, SEEK_SET) != 0) 
         critical("ERROR: Seek error in output.\n");
      if ( (bytes_read != fwrite(buffer_out, 1, bytes_read, output)) 
         || ferror(output) ) 
	 critical ("ERROR:File write error.\n");

   }

     


   if (fclose(output)) critical("ERROR: Cannot close output file.\n");
   if (fclose(input)) critical("ERROR: Cannot input output file.\n");

   free (buffer_in);
   free (buffer_out);

   printf ("%d header bytes copied\n", hsize);
   printf ("%d bytes reordered\n", count*bsize + bytes_read);

}

void critical (char *message) {
   printf("%s",message);
   _exit(1);
}