011.cpp

#include <algorithm>
#include <iostream>

static const unsigned int input[][20] = {{ 8, 02, 22, 97, 38, 15, 00, 40, 00, 75, 04, 05, 07, 78, 52, 12, 50, 77, 91,  8},
                                         {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 04, 56, 62, 00},
                                         {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 03, 49, 13, 36, 65},
                                         {52, 70, 95, 23, 04, 60, 11, 42, 69, 24, 68, 56, 01, 32, 56, 71, 37, 02, 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, 03, 45, 02, 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},
                                         {04, 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}};

int main(int argc, char* argv[])
{
    unsigned int largest = 0u;

    // vertical
    for(std::size_t i = 0u; i < 17u; ++i)
    {
        for(std::size_t j = 0u; j < 20u; ++j)
        {
            auto prod = input[i][j] * input[i + 1][j] * input[i + 2][j] * input[i + 3][j];
            largest = std::max(largest, prod);
        }
    }

    // horizontal
    for(std::size_t i = 0u; i < 20u; ++i)
    {
        for(std::size_t j = 0u; j < 17u; ++j)
        {
            auto prod = input[i][j] * input[i][j + 1] * input[i][j + 2] * input[i][j + 3];
            largest = std::max(largest, prod);
        }
    }

    // diagonal: top left to bottom right
    for(std::size_t i = 0u; i < 17u; ++i)
    {
        for(std::size_t j = 0u; j < 17u; ++j)
        {
            auto prod = input[i][j] * input[i + 1][j + 1] * input[i + 2][j + 2] * input[i + 3][j + 3];
            largest = std::max(largest, prod);
        }
    }

    // diagonal: top right to bottom left
    for(std::size_t i = 0u; i < 17u; ++i)
    {
        for(std::size_t j = 3u; j < 20u; ++j)
        {
            auto prod = input[i][j] * input[i + 1][j - 1] * input[i + 2][j - 2] * input[i + 3][j - 3];
            largest = std::max(largest, prod);
        }
    }

    std::cout << largest << std::endl;

    return 0;
}