/*
 * http://projecteuler.net
 * 
 * Projecteuler - Problem 11
 * ------------------------
 * 2010-04-27 Henrik Hautakoski
 *
 * Went a little crazy with macros =)
 */

#include <stdio.h>

/* product of 4 integers in direction: x, y, diagonal, reverse diagonal */
#define px(x, y)  (grid[y][x] * grid[y][x+1]   * grid[y][x+2]   * grid[y][x+3])
#define py(x, y)  (grid[y][x] * grid[y+1][x]   * grid[y+2][x]   * grid[y+3][x])
#define pd(x, y)  (grid[y][x] * grid[y+1][x+1] * grid[y+2][x+2] * grid[y+3][x+3])
#define prd(x, y) (grid[y][x] * grid[y-1][x+1] * grid[y-2][x+2] * grid[y-3][x+3])

/* macro for checking all directions of a square x,y */
#define check_square(x, y)      \
    do {                        \
        int p;                  \
        if (x < 17)             \
            setprod(px(x, y));  \
        if (y < 17)             \
            setprod(py(x, y));  \
        if (x < 17 && y < 17)   \
            setprod(pd(x, y));  \
        if (x >= 3 && y >= 3)   \
            setprod(prd(x, y)); \
    } while(0)

#define setprod(x)      \
    do {                \
        p = x;          \
        if (p > result) \
            result = p; \
    } while(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,  1, 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,  2, 62, 12, 20, 95, 63, 94, 39, 63,  8, 40, 91, 66, 49, 94, 21 },
    { 24, 55, 58,  5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72 },
    { 21, 36, 23,  9, 75,  0, 76, 44, 20, 45, 35, 14,  0, 61, 33, 97, 34, 31, 33, 95 },
    { 78, 17, 53, 28, 22, 75, 31, 67, 15, 94,  3, 80,  4, 62, 16, 14,  9, 53, 56, 92 },
    { 16, 39,  5, 42, 96, 35, 31, 47, 55, 58, 88, 24,  0, 17, 54, 24, 36, 29, 85, 57 },
    { 86, 56,  0, 48, 35, 71, 89,  7,  5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58 },
    { 19, 80, 81, 68,  5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77,  4, 89, 55, 40 },
    {  4, 52,  8, 83, 97, 35, 99, 16,  7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66 },
    { 88, 36, 68, 87, 57, 62, 20, 72,  3, 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,  4, 36, 16 },
    { 20, 73, 35, 29, 78, 31, 90,  1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57,  5, 54 },
    {  1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52,  1, 89, 19, 67, 48 }
};

unsigned int result = 0;

int main() {

    int x, y;
    
    for(y=0; y < 20; y++)
        for(x=0; x < 20; x++)
            check_square(x, y);

    printf("%i\n", result);
    
    return 0;
}