aoc24d11.c

#include <stdio.h>

#define N 0x2000
typedef struct {
	long long k, v;
} tbl[N];

static long long *
cell(tbl t, long long k) {
	unsigned i;
	for (i = k; i %= N, t[i].v && t[i].k != k; i++);
	t[i].k = k;
	return &t[i].v;
}

static long long
sum(const tbl t) {
	long long x;
	int i;
	x = 0;
	for (i = 0; i < N; i++)
		x += t[i].v;
	return x;
}

static void
blink(const tbl a, tbl b) {
	int i;
	long long d;
	memset(b, 0, sizeof (tbl));
	for (i = 0; i < N; i++)
		if (a[i].k == 0) {
			*cell(b, 1) += a[i].v;
		}
		else {
			for (d = 10; d <= a[i].k/d; d *= 10);
			if (a[i].k/d >= d/10) {
				*cell(b, a[i].k/d) += a[i].v;
				*cell(b, a[i].k%d) += a[i].v;
			}
			else {
				*cell(b, a[i].k*2024) += a[i].v;
			}
		}
}

int
main(void) {
	tbl a[2];
	int x, i;
	memset(a, 0, sizeof a);
	do {
		scanf("%d", &x);
		*cell(a[0], x) = 1;
	}
	while (getchar() != '\n');
	for (i = 0; i < 75; i++) {
		if (i == 25)
			printf("%lld\n", sum(a[1]));
		blink(a[i%2 != 0], a[i%2 == 0]);
	}
	printf("%lld\n", sum(a[1]));
}