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

#define kRows 16
#define kCols 16
#define kOffset 127U
#define kFactor 31U
#define kMax 255U

// Value grid (e.g. image) to find matches for.  bestMatch is index of closest
// grid match thus far, and bestDifference is how different that match is.
typedef struct {
   long long vals[kRows][kCols];
   int bestMatch;
   int bestDifference;
} Grid;

// Create |len| made-up grids, putting them into array |grids|
int MockupGrids(Grid *grids, int len) {
   int val, g, r, c;

   for (g = 0; g < len; g++) {
      val = g;
      for (r = 0; r < kRows; r++) {
         for (c = 0; c < kCols; c++) {
            val = (val + kOffset) * kFactor % INT_MAX;
            grids[g].vals[r][c] = val % kMax;
         }
      }
      grids[g].bestMatch = -1;
      grids[g].bestDifference = INT_MAX;
   }
}

inline int sqr(int x) {return x*x;}

// Given an other Grid's id as |other|, and the |bestDifference| between
// Grid |*g| and |other|, update |*g|s info accordingly.
inline void UpdateGrid(Grid *g, int other, int difference) {
   if (difference < g->bestDifference) {
      g->bestMatch = other;
      g->bestDifference = difference;
   }
}

int main(int argc, char **argv) {
   unsigned checkSum = 0;
   int difference, numGrids, g1, g2, row, col;
   Grid *grids, *grid1, *grid2;

   if (argc != 2) {
      printf("Usage: GridMatcher <numGrids>\n");
      exit(1);
   }

   numGrids = atoi(argv[1]);
   grids = (Grid *)calloc(sizeof(Grid), numGrids);
   MockupGrids(grids, numGrids);

   for (g1 = 0; g1 < numGrids; g1++) {
      grid1 = grids + g1;
      for (g2 = g1+1; g2 < numGrids; g2++) {
         grid2 = grids + g2;
         difference = 0;
         for (row = 0; row < kRows; row++) {
            for (col = 0; col < kCols; col += 4) 
               difference += sqr(grid1->vals[row][col] - grid2->vals[row][col]);
         }
         UpdateGrid(grid1, g2, difference);
         UpdateGrid(grid2, g1, difference);
      }
      checkSum += grid1->bestDifference;
   }
   printf("Last grid matches %d with %d\n", grid1->bestMatch, grid1->bestDifference);
   printf("Checksum is %0u\n", checkSum);
}