Day 9: Mirage Maintenance

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
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  • @sjmulder
    1•11 months ago

    C

    Thought there must be some clever way to infer the formula without storing those derived rows but implemented it like the example to get started. Turns out that was fine for part 2 as well so yay!

    GitHub link

    (Slightly abridged) code 
    int main()
{
	char line[128], *tok,*rest;
	size_t n,d,i;
	int a[24][24], p1=0,p2=0, nz, acc1,acc2;

	while ((rest = fgets(line, sizeof(line), stdin))) {
		for (n=0; (tok = strsep(&rest, " ")); n++)
			a[0][n] = atoi(tok);

		/* generate rows until all 0, 'd' is depth */
		for (d=1, nz=1; nz && d<n; d++)
		for (i=0, nz=0; i<n-d; i++)
			nz |= (a[d][i] = a[d-1][i+1] - a[d-1][i]);

		/* extrapolate forward and backwards */
		for (i=0, acc1=acc2=0; i<d; i++) {
			acc1 += a[d-i-1][n-d+i];
			acc2  = a[d-i-1][0] - acc2;
		}

		p1 += acc1;
		p2 += acc2;
	}

	printf("09: %d %d\n", p1, p2);
	return 0;
}