/*	equal_probability.c
**	this program is used to normalize
**      pixel value of input images for textural
**      feature test, so as to avoid the unequal
**      brightness and contrast
**      from:IEEE Trans. on systems, man, and
**      cybernetics. vol. SMC-3, no.6, nov. 1973
**      pp. 611-621.
*/

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define gray_level 256


void main(void)
{
  FILE 		*fin, *fout;
  unsigned int num_pix,gray[gray_level];
  unsigned int num_lev,gray_value,start,sum;
  unsigned int count1, count2,temp_num;
  double probability;
  unsigned char store[2],*look_up,in_file[50];


      printf("Please input the total number of image pixels:");
	  scanf("%d",&num_pix);
	  printf("Please input the gray levels of input image:");
	  scanf("%d",&num_lev);


      look_up = (unsigned char *)calloc(num_lev+1,sizeof(unsigned char));

      printf("Please input image file name:");
	  scanf("%s",in_file);
      if((fin = fopen(in_file,"rb")) == NULL)
	{
	  fprintf(stderr,"Failed to open %s\n",in_file);
	  exit(EXIT_FAILURE);
	}
      printf("Ouput image file name:");
	  scanf("%s",in_file);
      if((fout = fopen(in_file,"wb")) == NULL)
       {
         fprintf(stderr,"Failed to open %s\n",in_file);
         exit(EXIT_FAILURE);
       }

      for(count1 = 0;count1 < gray_level;count1++)
          gray[count1] = 0;

      for(count1 = 0;count1 < num_pix; count1++)
		{
         fread(store,1,1,fin);
         gray_value = store[0];
         gray[gray_value]++;
		}

      fseek(fin,0,0);
      start = 0;
      sum = 0;
      count1 = 1;
      count2 = 0;
      temp_num = num_pix;
      while(temp_num != 0)
		{
        probability = (double) temp_num/(num_lev-count1+1);
        sum = 0;
     	while(sum < probability)
			{
			sum = sum + gray[count2];
			count2++;
			}
        look_up[count1] = count2 - 1;
        count1++;
        temp_num = temp_num - sum;
		}

      for(count1 = 0;count1 < num_pix; count1++)
		{
        count2 = 1;
        fread(store,1,1,fin);
        while(look_up[count2] < store[0])
			{
           count2++;
			}
        store[0] = count2-1;
        fwrite(store,1,1,fout);
		}

      fclose(fin);
      fclose(fout);
      free(look_up);

}