#include <stdio.h>

int main(int argc, char **argv) {
  FILE *inFile1 = 0;
  FILE *inFile2 = 0;
  FILE *outFile;
  char *outName = 0;
  char ppmString[256];
  int t1, t2, x1, x2, y1, y2, i, max1, max2;
  
  if(arg > 4) {
    printf("too much arguments\n");
    return 5;
  }
  
  for (i = 1; i < argc; i++) {
    if (!strcmp(argv[i], "-o")) {
      outName = argv[++i];
      if(!(outFile = fopen(outName, "w"))) {
        printf("failed to open outputfile %s\n", outName);
        return 10;
      }
    }
    else if(!inFile1) {
      if(!(inFile1 = fopen(argv[i], "r"))) {
        printf("failed to open first inputfile %s\n", argv[i]);
        return 10;
      }
    }
    else if(!inFile2) {
      if(!(inFile2 = fopen(argv[i], "r"))) {
        printf("failed to open second inputfile %s\n", argv[i]);
        return 10;
      }
    }
  }
  
  if(!inFile1 || !inFile2) {
    printf("you must specify two sources\n");
    return 5;
  }
  if(!outFile)
    outFile = stdout;
  
  fscanf(inFile1, "P%1d\n%d %d\n%d\n", &t1, &x1, &y1, &max1);
  fscanf(inFile2, "P%1d\n%d %d\n%d\n", &t2, &x2, &y2, &max2);
  
  if((t1 != t2) || (x1 != x2) || (y1 != y2) || (max1 != max2)) {
    printf("cannot compare pictures with different sizes or formats\n");
    return 5;
  }
  if(!((t1 == 5) || (t1 == 6))) {
    printf("cannot handle this format P%1d\n", t1);
    return 5;
  }
  
  fprintf(outFile, "P5\n%d %d\n255\n", x1, y1);
  
  for(i = 0; i < (x1 * y1); i++) {
    int r, g, b, p;
    
    r  = fgetc(inFile2) - fgetc(inFile1);
    r >>= 1;
    r += 127;
    if(t1 == 6) {
      g  = fgetc(inFile2) - fgetc(inFile1);
      g >>= 1;
      g += 127;
      b  = fgetc(inFile2) - fgetc(inFile1);
      b >>= 1;
      b += 127;
      
      p = (r + g + b) / 3;
      fputc(p, outFile);
    }
    else
      fputc(r, outFile);
  }
  
  fclose(inFile1);
  fclose(inFile2);
  fclose(outFile);
}