/* eql_hst.c */
#include <stdio.h>
#include <stdlib.h>
#include "mypgm.h"
#define FINAL_LEVEL 64  /* No. of final gray levels */

void equal_histogram( )
     /* Input: image1 -------- Output: image2 */
{
  int hist1[GRAYLEVEL], hist2[GRAYLEVEL];
  int trans_table[GRAYLEVEL]; /* Table of gray level transformation */
  int i, x, y; /* Loop variable */
  int target_value; /* Target occurrences after transformation */
  int gray;
  double gray_step; /* Descriptive gray level interval */
  int min, max;
  
  printf("Histogram equalization is now processing.\n");
  
  /* Generation of gray level histogram of input image */
  for (i = 0; i < GRAYLEVEL; i++) hist1[i] = 0;
  for (y = 0; y < y_size1; y++) {
    for (x = 0; x < x_size1; x++) {
      hist1[image1[y][x]]++;
    }
  }
  
  /* Generation of transformation table */
  target_value = (int)((double)(x_size1 * y_size1) / FINAL_LEVEL);
  for (i = 0; i < FINAL_LEVEL; i++)
    hist2[i] = 0;

  gray = 0;
  for (i = 0; i < GRAYLEVEL; i++) {
    if (abs(target_value - hist2[gray]) <
	abs(target_value - (hist2[gray] + hist1[i]))) {
      gray++;
      if (gray >= FINAL_LEVEL) gray = FINAL_LEVEL - 1;
    }
    trans_table[i] = gray;
    hist2[gray] = hist2[gray] + hist1[i];
  }

  /* Output of image2 subject to histogram equalization */
  x_size2 = x_size1;
  y_size2 = y_size1;
  gray_step = (double)GRAYLEVEL / FINAL_LEVEL;
  for (y = 0; y < y_size2; y++) {
    for (x = 0; x < x_size2; x++) {
      image2[y][x] = (unsigned char)
	(trans_table[image1[y][x]] * gray_step);
    }
  }
  
  /* linear transformation of gray level histogram of output image */
  max = min = image2[0][0];
  for (y = 0; y < y_size2; y++) {
    for (x = 0; x < x_size2; x++) {
      if (min > image2[y][x]) min = image2[y][x];
      if (max < image2[y][x]) max = image2[y][x];
    }
  }
  
  if ((max - min) == 0) {
    printf("Nothing exists!!!\n");
    exit(1);
  }
  
  for (y = 0; y < y_size2; y++) {
    for (x = 0; x < x_size2; x++) {
      image2[y][x] = (unsigned char)((double)(image2[y][x] - min)
				     / (max - min) * MAX_BRIGHTNESS);
    }
  }
}

main( )
{
  load_image_data( );  /* Input of image1 */
  equal_histogram( );  /* Histogram equalization is applied to image1 */
  save_image_data( );  /* Output of image2*/
  return 0;
}