adventofcode2024/day04/sol.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <stdbool.h>

struct grid {
	char** A;
	int x, y;
};

void* xrealloc(void* ptr, size_t size) {
	if (size == 0) {
		size = 1;
	}
	ptr = realloc(ptr, size);
	if (ptr == NULL) {
		fprintf(stderr, "fatal: alloc(%zd) failed", size);
		exit(1);
	}
	return ptr;
}

int readfile(const char* filename, struct grid* out) {
	FILE *fp = fopen(filename, "r");
	if (fp == NULL) {
		perror("fopen");
		exit(1);
	}

	int len = 0;
	int cap = 100;
	char* data = xrealloc(NULL, cap);
	for (;;) {
		if (len >= cap) {
			cap *= 2;
			data = xrealloc(data, cap);
		}
		int c = fgetc(fp);
		if (c == -1) {
			break;
		}
		data[len] = c;
		len++;
	}
	int i, nl=0;
	for (i = 0; i < len; i++) {
		if (data[i] == '\n') {
			nl++;
		}
	}
	// split on newlines
	char** lines = xrealloc(NULL, nl * sizeof(char*));
	char* p = data;
	int mincol = INT_MAX;
	i = 0;
	while (p != NULL && p < data+len) {
		lines[i++] = p;
		char* q = memchr(p, '\n', (data+len)-p);
		if (!q) {
			break; // TODO: final line?
		}
		int col = q - p;
		if (mincol > col && col > 0) {
			mincol = col;
		}
		p = q+1; // skip newline
	}

	out->A = lines;
	out->x = mincol;
	out->y = nl;

	return 0;
}

bool search(struct grid *g, int x, int y, int dx, int dy, const char* word) {
	int i;
	for (i = 0; word[i] != '\0'; i++) {
		if (!(0 <= x && x < g->x && 0 <= y && y < g->y)) {
			return false;
		}
		if (g->A[y][x] != word[i]) {
			return false;
		}
		x += dx;
		y += dy;
	}
	return true;
}

int solve(struct grid *g) {
	int x, y;
	int count = 0;
	const char *word = "XMAS";
	for (y = 0; y < g->y; y++)
	for (x = 0; x < g->x; x++) {
		if (g->A[y][x] != 'X') continue;
		if (search(g, x, y,  0, +1, word)) { count++; }
		if (search(g, x, y, +1,  0, word)) { count++; }
		if (search(g, x, y, -1,  0, word)) { count++; }
		if (search(g, x, y,  0, -1, word)) { count++; }
		if (search(g, x, y, +1, +1, word)) { count++; }
		if (search(g, x, y, -1, -1, word)) { count++; }
		if (search(g, x, y, +1, -1, word)) { count++; }
		if (search(g, x, y, -1, +1, word)) { count++; }
	}
	printf("%d\n", count);
	return 0;
}

int solve2(struct grid *g) {
	int x, y;
	int count = 0;
	const char *word = "MAS";
	for (y = 0; y < g->y; y++)
	for (x = 0; x < g->x; x++) {
		if (g->A[y][x] != 'A') continue;
		int mas = 0;
		if (search(g, x-1, y-1,  +1, +1, word)) { mas++; }
		if (search(g, x+1, y+1,  -1, -1, word)) { mas++; }
		if (search(g, x-1, y+1,  +1, -1, word)) { mas++; }
		if (search(g, x+1, y-1,  -1, +1, word)) { mas++; }
		if (mas == 2) { count++; }
	}
	printf("%d\n", count);
	return 0;

}

int main() {
	struct grid g;
	//readfile("sample2.in", &g);
	readfile("input", &g);
	printf("%d %d\n", g.x, g.y);
	solve(&g);
	solve2(&g);
}