55 lines
1.5 KiB
C
55 lines
1.5 KiB
C
/* Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:
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*
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* 21 22 23 24 25
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* 20 7 8 9 10
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* 19 6 1 2 11
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* 18 5 4 3 12
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* 17 16 15 14 13
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*
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* It can be verified that the sum of the numbers on the diagonals is 101.
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*
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* What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?*/
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#define _POSIX_C_SOURCE 199309L
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#include <stdio.h>
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#include <stdlib.h>
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#include <time.h>
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#define N 1001
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int main(int argc, char **argv)
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{
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int i, j, step = 0, limit = N * N, sum = 1;
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double elapsed;
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struct timespec start, end;
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clock_gettime(CLOCK_MONOTONIC, &start);
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/* Starting with the central 1, it's easy to see that the next four numbers in the diagonal
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* are 1+2, 1+2+2, 1+2+2+2 and 1+2+2+2+2, then for the next four number the step is increased
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* by two, so from 9 to 9+4, 9+4+4 etc, for the next four number the step is again increased
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* by two, and so on. We go on until the value is equal to N*N, with N=1001 for this problem.*/
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for(i = 0, j = 1; j < limit; i = (i + 1) % 4)
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{
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if(i == 0)
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{
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step += 2;
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}
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j += step;
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sum += j;
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}
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clock_gettime(CLOCK_MONOTONIC, &end);
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elapsed = (end.tv_sec - start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / 1000000000;
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printf("Project Euler, Problem 28\n");
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printf("Answer: %d\n", sum);
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printf("Elapsed time: %.9lf seconds\n", elapsed);
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return 0;
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}
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