euler011.c

// Find the greatest product of any four adjacent numbers in
// a 20x20 grid.

#include <stdio.h>

int main(void)
{
  int i, j;
  int max=0;
  int current=0;
  int grid[20][20] =
    {
      {8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8},
      {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0},
      {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65},
      {52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 01, 32, 56, 71, 37, 2, 36, 91},
      {22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80},
      {24, 47, 32, 60, 99, 3, 45, 2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50},
      {32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70},
      {67, 26, 20, 68, 02, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21},
      {24, 55, 58, 05, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
      {21, 36, 23, 9, 75, 00, 76, 44, 20, 45, 35, 14, 00, 61, 33, 97, 34, 31, 33, 95},
      {78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 03, 80, 04, 62, 16, 14, 9, 53, 56, 92},
      {16, 39, 05, 42, 96, 35, 31, 47, 55, 58, 88, 24, 00, 17, 54, 24, 36, 29, 85, 57},
      {86, 56, 00, 48, 35, 71, 89, 07, 05, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
      {19, 80, 81, 68, 05, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 04, 89, 55, 40},
      {04, 52, 8, 83, 97, 35, 99, 16, 07, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
      {88, 36, 68, 87, 57, 62, 20, 72, 03, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
      {4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 8, 46, 29, 32, 40, 62, 76, 36},
      {20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 04, 36, 16},
      {20, 73, 35, 29, 78, 31, 90, 01, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 05, 54},
      {01, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 01, 89, 19, 67, 48}
    };

  for(i = 0; i < 20; i++) {
    for(j = 0; j < 20; j++) {
      // go right
      if(j < 17) {
        current=grid[i][j]*grid[i][j+1]*grid[i][j+2]*grid[i][j+3];
        if(current > max) {
          max = current;
          printf("right max! %d = %d, %d, %d, %d\n",
            current, grid[i][j], grid[i][j+1], grid[i][j+2], grid[i][j+3]);
        }
      }
      // go down
      if(i < 17) {
        current=grid[i][j]*grid[i+1][j]*grid[i+2][j]*grid[i+3][j];
        if(current > max) {
          max = current;
          printf("down max! %d = %d, %d, %d, %d\n",
            current, grid[i][j], grid[i+1][j], grid[i+2][j], grid[i+3][j]);
        }
      }
      // go down-right
      if(i < 17 && j < 17) {
        current=grid[i][j]*grid[i+1][j+1]*grid[i+2][j+2]*grid[i+3][j+3];
        if(current > max) {
          max = current;
          printf("down-right max! %d = %d, %d, %d, %d\n",
            current, grid[i][j], grid[i+1][j+1], grid[i+2][j+2], grid[i+3][j+3]);
        }
      }
      // go down-left
      if(i < 17 && j > 2) {
        current=grid[i][j]*grid[i+1][j-1]*grid[i+2][j-2]*grid[i+3][j-3];
        if(current > max) {
          max = current;
          printf("left-right max! %d = %d, %d, %d, %d\n",
            current, grid[i][j], grid[i+1][j-1], grid[i+2][j-2], grid[i+3][j-3]);
        }
      }
    }
  }

  printf("%d\n", max);
}