day 6 part 2

day06/Makefile

@@ -1,2 +1,2 @@
-CFLAGS=-O2 -std=c99 -Wall -Wextra
+CFLAGS=-O2 -std=c99 -ggdb -Wall -Wextra
 all: sol

day06/sol.c

@@ -1,4 +1,5 @@
 #include <stdlib.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <string.h>
 #include <limits.h>
@@ -80,7 +81,6 @@ inline bool inbounds(struct grid *g, int x, int y) {
 
 struct step_r { int x, y, dx, dy; bool ok; };
 struct step_r step(struct grid *g, int x, int y, int dx, int dy) {
-	int i;
 	struct step_r r = {0};
 	if (!inbounds(g, x, y)) {
 		return r;
@@ -104,45 +104,111 @@ struct step_r step(struct grid *g, int x, int y, int dx, int dy) {
 	}
 }
 
-
-int solve(struct grid *g) {
+struct point { int x, y; };
+struct point startingPoint(struct grid *g) {
 	int x, y;
-	int cx = -1, cy = -1;
-	int count = 0;
 	for (y = 0; y < g->y; y++)
 	for (x = 0; x < g->x; x++) {
 		if (g->A[y][x] == '^') {
-			cx = x;
-			cy = y;
-			break;
+			struct point r = {x, y};
+			return r;
 		}
 	}
+	struct point r = {-1,-1};
+	return r;
+}
+
+int solve(struct grid *g) {
+	struct point start = startingPoint(g);
+	int x = start.x, y = start.y;
+	int count = 0;
 	int dx = 0, dy = -1; // up
-	while(inbounds(g, cx, cy)) {
-		if (g->A[cy][cx] != 'x') {
+	while(inbounds(g, x, y)) {
+		if (g->A[y][x] != 'x') {
 			count++;
 		}
-		g->A[cy][cx] = 'x';
+		g->A[y][x] = 'x';
 
-		struct step_r r = step(g, cx, cy, dx, dy);
+		struct step_r r = step(g, x, y, dx, dy);
 		if (r.ok != true) {
 			break;
 		}
-		cx = r.x;
-		cy = r.y;
+		x = r.x;
+		y = r.y;
 		dx = r.dx;
 		dy = r.dy;
 	}
+	g->A[start.y][start.x] = '^';
 	printf("%d\n", count);
 	return 0;
 }
 
+// returns whether the chosen grid will cause the guard to get stuck in a loop
+bool loops(struct grid *g, struct point start, uint16_t* state) {
+	int x = start.x;
+	int y = start.y;
+	int dx = 0, dy = -1; // up
+	int stride = g->x;
+	//printf("loops(%d,%d)\n", x,y);
+	while(inbounds(g, x, y)) {
+		uint16_t dir = 15;
+		if (dx == 1) { dir = 1; }
+		else if (dx == -1) { dir = 2; }
+		else if (dy == 1) { dir = 4; }
+		else if (dy == -1) { dir = 8; }
+
+		uint16_t *s = &state[y*stride + x];
+		if ((*s & dir) != 0) {
+			// revisited a state, loop!
+			return true;
+		}
+		*s |= dir;
+
+		struct step_r r = step(g, x, y, dx, dy);
+		if (r.ok != true) {
+			// step failed, no loop
+			return false;
+		}
+		x = r.x;
+		y = r.y;
+		dx = r.dx;
+		dy = r.dy;
+	}
+	// left the map, no loop
+	return false;
+}
+
+
+int solve2(struct grid *g) {
+	int count = 0;
+	int ox, oy; // obstacle coords
+	struct point p = startingPoint(g);
+	size_t state_size = g->x * g->y * sizeof(uint16_t);
+	uint16_t *state = xrealloc(NULL, state_size);
+	for (oy = 0; oy < g->y; oy++)
+	for (ox = 0; ox < g->x; ox++) {
+		int c = g->A[oy][ox];
+		if (c != '#' && c != '^') {
+			g->A[oy][ox] = '#';
+			memset(state, 0, state_size);
+			if (loops(g, p, state)) {
+				count++;
+			}
+			g->A[oy][ox] = c;
+		}
+	}
+	free(state);
+	printf("%d\n", count);
+	return 0;
+}
+
+
 int main() {
 	struct grid g;
-	readfile("sample1.in", &g);
+	//readfile("sample1.in", &g);
 	readfile("input", &g);
 	printf("%d %d\n", g.x, g.y);
 	solve(&g);
-	//solve2(&g);
-	printf("%s", g.A[0]);
+	//printf("%s", g.A[0]);
+	solve2(&g);
 }