#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#define PRINT_IMG_STATS

/*Program by Robert R. Pavlis. This program takes a binary
ppm file and subtracts a specified value from each pixel in
red green and blue, then normalises to maximum intensity
This version works with 16 bit colour. This code is endian sensitive*/


int filelength (const char *fileName);

int
main (int argc, char *argv[])
{
  unsigned char *ppmimage, *splitit;
  unsigned short putvalue;
  char newname[256], filebase[256];
  int width, height, count, colours, fstrlen,redcor, greencor, bluecor,newvalue;
  long int i, j, k, m, lengthppm, ppmheader;
  double redmult=1.0, greenmult=1.0, bluemult=1.0;  
  FILE *infileppm, *outfileppm;


  if (argc != 2)
    {
      fprintf (stderr, "This program requires a ppm file name\n");
      exit (1);
    }


  infileppm = fopen (argv[1], "rb");

  if (infileppm == NULL)
    {
      printf ("ppm Input file not opened\n");
      exit (1);
    }


  printf("Enter red correction:\n");
  scanf("%d", &redcor);
  printf("Enter green correction:\n");
  scanf("%d", &greencor);
  printf("Enter blue correction:\n");
  scanf("%d", &bluecor);
  printf("Continuing with red %d, blue %d, and green %d\n",redcor, greencor, bluecor);
  lengthppm = filelength (argv[1]);
  ppmimage = (char *) malloc (lengthppm);
  fread (ppmimage, lengthppm, 1, infileppm);
  /*the file must be in the binary ppm format, whose magic number is
     0x50 0x36 */


  if ((ppmimage[0] == 0x50) && (ppmimage[1] == 0x33))	/*magic number for text ppm */
    {
      free (ppmimage);
      fprintf (stderr, "This program must use binary ppm only!\n");
      exit (1);
    }

  if ((ppmimage[0] != 0x50) || (ppmimage[1] != 0x36))
    {
      free (ppmimage);
      fprintf (stderr, "Bad Magic Number!\n");
      fprintf (stderr, "%s is NOT a ppm file!\n", argv[1]);
      exit (1);
    }

  fclose (infileppm);

  /*now we have to parse the start of the headers and see if they contain comments, 
     then we must locate the bytes that have width and length specifications.  We then
     need to search for the 0x0A that ends the preliminary specifications. We must read
     the bit count/colour */

  count = 0;
  ppmheader = 0;
  for (i = 0; i < 256; i++)
    {
      if (count == 1)
	break;
      if (ppmimage[i] == 0x0A)
	if (ppmimage[i + 1] != 0x23)
	  if (count == 0)
	    {
	      width = atoi (ppmimage + i + 1);
	      count = 1;
	      for (j = 1; j < 16; j++)
		{
		  if (ppmimage[i + j] == 0x20)
		    {
		      height = atoi (ppmimage + i + j + 1);
		      i = i + j;
		      break;
		    }
		}
	      for (k = 1; k < 16; k++)
		{
		  if (ppmimage[i + k] == 0x0A)
		    {
		      colours = atoi (ppmimage + i + k + 1);

		      i = i + k;
		      break;
		    }
		}
	    }
    }
  count = 0;
  for (m = i; m < 256; m++)
    {
      if (ppmimage[m] == 0x0A)
	count++;
      if (count == 1)
	{
	  ppmheader = m + 1;
	  break;
	}
    }


  /*at this point the program has parsed the header */
  strcpy (filebase, argv[1]);
  fstrlen = strlen (filebase);
  /*filebase[fstrlen - 2] = 0x67;*/
  strcpy (newname, "cor_");
  strcat (newname, filebase);


  outfileppm = fopen (newname, "wb");
  if (outfileppm == NULL)
    {
      printf ("New output file not opened\n");
      exit (1);
    }
 

  redmult=(double)colours/((double)colours-redcor);
  greenmult=(double)colours/((double)colours-greencor);
  bluemult=(double)colours/((double)colours-bluecor);
  

  if (colours < 0x100)
    for (i = ppmheader; i < lengthppm; i += 3)
      {
        newvalue=(ppmimage[i]-redcor)*redmult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
        ppmimage[i]=newvalue;

        newvalue=(ppmimage[i+1]-greencor)*greenmult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
        ppmimage[i+1]=newvalue;

        newvalue=(ppmimage[i+2]-bluecor)*bluemult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
        ppmimage[i+2]=newvalue;

      }
  else
    { 
 for (i = ppmheader; i < lengthppm; i += 6)
      {
        newvalue=0x100*ppmimage[i]+ ppmimage[i+1];       
        newvalue-=redcor;
        newvalue*=redmult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
	/*newvalue now has the corrected value, needs to be placed bytewise
	  into array*/
        putvalue=newvalue;
        splitit=(unsigned char*)&putvalue;
        ppmimage[i]=splitit[1];
        ppmimage[i+1]=splitit[0];

        newvalue=0x100*ppmimage[i+2]+ ppmimage[i+3];       
        newvalue-=greencor;
        newvalue*=greenmult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
        putvalue=newvalue;
        splitit=(unsigned char*)&putvalue;
        ppmimage[i+2]=splitit[1];
        ppmimage[i+3]=splitit[0];

        newvalue=0x100*ppmimage[i+4]+ ppmimage[i+5];       
        newvalue-=bluecor;
        newvalue*=bluemult;
        if(newvalue<0) newvalue=0;
        if(newvalue>colours) newvalue=colours;
        putvalue=newvalue;
        splitit=(unsigned char*)&putvalue;
        ppmimage[i+4]=splitit[1];
        ppmimage[i+5]=splitit[0];


      }


     
    }
  fwrite (ppmimage,1, lengthppm, outfileppm);
  free (ppmimage);
  fclose (outfileppm);
#ifdef  PRINT_IMG_STATS
  printf ("width=%d, height=%d colours=%d\n", width, height, colours);
#endif

}



int
filelength (const char *fileName)
{
  struct stat *st;

  st = (struct stat *) malloc (sizeof (struct stat));
  if (stat (fileName, st) == -1)
    exit (1);
  return st->st_size;
}