p0008_s0.c

problem 8 · 54 lines

/* Solution to Euler Problem 8: Largest Product in a Series. */
#include "runner.h"

static const char *number =
    "73167176531330624919225119674426574742355349194934"
    "96983520312774506326239578318016984801869478851843"
    "85861560789112949495459501737958331952853208805511"
    "12540698747158523863050715693290963295227443043557"
    "66896648950445244523161731856403098711121722383113"
    "62229893423380308135336276614282806444486645238749"
    "30358907296290491560440772390713810515859307960866"
    "70172427121883998797908792274921901699720888093776"
    "65727333001053367881220235421809751254540594752243"
    "52584907711670556013604839586446706324415722155397"
    "53697817977846174064955149290862569321978468622482"
    "83972241375657056057490261407972968652414535100474"
    "82166370484403199890008895243450658541227588666881"
    "16427171479924442928230863465674813919123162824586"
    "17866458359124566529476545682848912883142607690042"
    "24219022671055626321111109370544217506941658960408"
    "07198403850962455444362981230987879927244284909188"
    "84580156166097919133875499200524063689912560717606"
    "05886116467109405077541002256983155200055935729725"
    "71636269561882670428252483600823257530420752963450";

/* Sliding-window brute force: largest digit-product over every length-wide window, skipping
   windows that contain a zero; O(N*length) for N digits. */
const char *solve(int argc, char *argv[]) {
    static char _answer[32];
    int length = (argc >= 2) ? parse_int(argv[1]) : 13;
    int n = (int)strlen(number);
    long long best = 0;

    for (int i = 0; i <= n - length; i++) {
        /* Skip windows containing '0' */
        int has_zero = 0;
        for (int j = i; j < i + length; j++) {
            if (number[j] == '0') {
                has_zero = 1;
                break;
            }
        }
        if (has_zero) continue;

        long long product = 1;
        for (int j = i; j < i + length; j++) {
            product *= (number[j] - '0');
        }
        if (product > best) best = product;
    }

    { snprintf(_answer, sizeof _answer, "%lld", (long long)(best)); return _answer; }
}