Problem statement

The four adjacent digits in the 1000-digit number that have the greatest product are $9 \times 9 \times 8 \times 9 = 5832$.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

Thoughts

This can be solved using a "shifting window", 13 characters in size, moving along the string one character at a time.

Solution

Move over the list, multiplying each consecutive 13 digit string. Store it if it's larger than the current largest.

#include <stdio.h>
#include <string.h>
#define SEARCH_LENGTH 13

char chars[] = "73167176531330624919225119674426574742355349194934"
	       "96983520312774506326239578318016984801869478851843"
	       "85861560789112949495459501737958331952853208805511"
	       "12540698747158523863050715693290963295227443043557"
	       "66896648950445244523161731856403098711121722383113"
	       "62229893423380308135336276614282806444486645238749"
	       "30358907296290491560440772390713810515859307960866"
	       "70172427121883998797908792274921901699720888093776"
	       "65727333001053367881220235421809751254540594752243"
	       "52584907711670556013604839586446706324415722155397"
	       "53697817977846174064955149290862569321978468622482"
	       "83972241375657056057490261407972968652414535100474"
	       "82166370484403199890008895243450658541227588666881"
	       "16427171479924442928230863465674813919123162824586"
	       "17866458359124566529476545682848912883142607690042"
	       "24219022671055626321111109370544217506941658960408"
	       "07198403850962455444362981230987879927244284909188"
	       "84580156166097919133875499200524063689912560717606"
	       "05886116467109405077541002256983155200055935729725"
	       "71636269561882670428252483600823257530420752963450";

int main()
{
	const int len = strlen(chars);

	unsigned long largest_val = 0;
	char *start = &chars[0];

	for (int i = 0; i < len - SEARCH_LENGTH; i++) {
		unsigned long val = 1;
		for (int j = 0; j < SEARCH_LENGTH; j++) {
			val *= *(start + j) - '0';
		}
		if (val > largest_val) {
			largest_val = val;
		}
		start++;
	}
	printf("%lu", largest_val);
}