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

   12/22/11
                                                                           */

#include<stdio.h>
#define BRIGHTNESS_INCREMENT 64

/* Find pixel indices: max,mid,min */
static void sort_pixels(int *color, int *i)
{
   if( color[0] > color[1] )
   {
      if( color[1] > color[2] )      { i[0] = 0; i[1] = 1; i[2] = 2; }
      else if( color[0] > color[2] ) { i[0] = 0; i[1] = 2; i[2] = 1; }
      else                           { i[0] = 2; i[1] = 0; i[2] = 1; }
   }
   else
   {
      if( color[0] > color[2] )      { i[0] = 1; i[1] = 0; i[2] = 2; }
      else if( color[1] > color[2] ) { i[0] = 1; i[1] = 2; i[2] = 0; }
      else                           { i[0] = 2; i[1] = 1; i[2] = 0; }
   }
}

int main(int argc, char **argv)
{
   FILE *infile = stdin;
   FILE *outfile = stdout;
   char buffer[1024];
   int width, height, range, size;
   int i, c, color[3], index[3];
   float chroma, scale;

   /* Open files */
   if( argc > 1 && (argv[1][0] != '-' || argv[1][1] != '\0') )
   {
      if( (infile = fopen(argv[1], "rb")) == NULL )
         return printf("Can not open %s for reading\n", argv[1]);
   }
   if( argc > 2 && (argv[2][0] != '-' || argv[2][1] != '\0') )
   {
      if( (outfile = fopen(argv[2], "wb+")) == NULL )
         return printf("Can not open %s for writing\n", argv[2]);
   }

   /* Read header */
   size = fread(buffer, 1, 1024, infile);
   if( size < 2 ||
       buffer[0] != 'P' || buffer[1] != '6' ||
       sscanf(buffer, "P6 %d %d %d", &width, &height, &range) != 3 ||
       range != 255 )
   {
      printf("Error parsing header from %s\n", argv[1]);
      fclose(infile);
      fclose(outfile);
      return 1;
   }

   /* Write header to output */
   fprintf(outfile, "P6\n%d %d\n%d\n", width, height, range);

   /* Skip header */
   c = 0;
   for(i = 0; i < size; i++)
   {
      if( buffer[i] == '\n' )
      {
         if( ++c == 3 )
            break;
      }
   }
   if( c < 3 )
   {
      printf("Error parsing header from %s\n", argv[1]);
      fclose(infile);
      fclose(outfile);
      return 1;
   }

   /* Read bytes */
   for(i++;;)
   {
      /* Read 3 pixels at once */
      for(c = 0; c < 3; c++)
      {
         if( i == size )
         {
            if( (size = fread(buffer, 1, 1024, infile)) == 0 )
               break;
            i = 0;
         }
         color[c] = (int)(buffer[i++]) & 255;
      }
      if( size == 0 )
         break;

      /* Compute chroma */
      sort_pixels(color, index);

      /* Raise brightness */
      color[0] += BRIGHTNESS_INCREMENT;
      color[1] += BRIGHTNESS_INCREMENT;
      color[2] += BRIGHTNESS_INCREMENT;

      if( color[index[0]] > 255 )
      {
         if( color[index[2]] < 255 )
         {
            /* Some (but not all) pixels exceeded valid range,
               adjust mid pixel to preserve chroma.            */
            chroma = (float)(color[index[1]] - color[index[2]]) /
                     (float)(color[index[0]] - color[index[2]]);
            scale = (float)(255 - color[index[2]]) /
                    (float)(color[index[0]] - color[index[2]]);
            color[index[1]] = (int)(color[index[2]] + chroma * scale);
         }
         if( color[0] > 255 ) color[0] = 255;
         if( color[1] > 255 ) color[1] = 255;
         if( color[2] > 255 ) color[2] = 255;
      }

      /* Output pixels */
      fputc(color[0], outfile);
      fputc(color[1], outfile);
      fputc(color[2], outfile);
   }

   /* Cleanup */
   fclose(infile);
   fclose(outfile);
   return 0;
}