From e01295c03d5a0c8affeb9cc9acda01223e5f8a89 Mon Sep 17 00:00:00 2001
From: Curtis McEnroe <june@causal.agency>
Date: Sun, 23 Dec 2018 22:47:52 -0500
Subject: Solve day 22 part 2

---
 2018/day22.c | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)

diff --git a/2018/day22.c b/2018/day22.c
index 459f8cb..88f7040 100644
--- a/2018/day22.c
+++ b/2018/day22.c
@@ -35,6 +35,62 @@ static enum Type type(uint x, uint y) {
 	return erosionLevel(x, y) % 3;
 }
 
+enum Tool {
+	Neither,
+	Torch,
+	Gear,
+};
+
+static enum Tool validTool(enum Tool tool, enum Type a, enum Type b) {
+	if (a == Rocky && b == Wet) return Gear;
+	if (a == Wet && b == Rocky) return Gear;
+	if (a == Rocky && b == Narrow) return Torch;
+	if (a == Narrow && b == Rocky) return Torch;
+	if (a == Wet && b == Narrow) return Neither;
+	if (a == Narrow && b == Wet) return Neither;
+	return tool;
+}
+
+static uint costs[1000][1000][3];
+
+static struct Point {
+	uint x, y;
+	enum Tool tool;
+} point(uint x, uint y, enum Tool tool) {
+	return (struct Point) { x, y, tool };
+}
+
+static struct Q {
+	struct Point point;
+	uint cost;
+	struct Q *next;
+} *qHead;
+
+static void qPush(struct Point point, uint cost) {
+	struct Q *q = malloc(sizeof(*q));
+	q->point = point;
+	q->cost = cost;
+	if (!qHead || q->cost < qHead->cost) {
+		q->next = qHead;
+		qHead = q;
+		return;
+	}
+	for (struct Q *u = qHead; u; u = u->next) {
+		if (u->next && q->cost > u->cost) continue;
+		q->next = u->next;
+		u->next = q;
+		break;
+	}
+}
+
+static struct Point qPop(void) {
+	struct Q *q = qHead;
+	struct Point p = q->point;
+	qHead = q->next;
+	free(q);
+	return p;
+}
+
 int main(void) {
 	scanf("depth: %u\n", &depth);
 	scanf("target: %u,%u\n", &targetX, &targetY);
@@ -46,4 +102,42 @@ int main(void) {
 		}
 	}
 	printf("%u\n", riskLevel);
+
+	qPush(point(0, 0, Torch), 0);
+	while (qHead) {
+		struct Point p = qPop();
+		if (p.x == targetX && p.y == targetY) {
+			if (p.tool == Torch) break;
+			struct Point next = point(p.x, p.y, Torch);
+			uint cost = costs[p.x][p.y][p.tool] + 7;
+			costs[next.x][next.y][next.tool] = cost;
+			qPush(next, cost);
+		}
+
+		uint len = 0;
+		struct Point nexts[4];
+		nexts[len++] = point(p.x + 1, p.y, p.tool);
+		nexts[len++] = point(p.x, p.y + 1, p.tool);
+		if (p.x) nexts[len++] = point(p.x - 1, p.y, p.tool);
+		if (p.y) nexts[len++] = point(p.x, p.y - 1, p.tool);
+
+		for (uint i = 0; i < len; ++i) {
+			struct Point next = nexts[i];
+			uint cost = costs[p.x][p.y][p.tool] + 1;
+			enum Type prevType = type(p.x, p.y);
+			enum Type nextType = type(next.x, next.y);
+			enum Tool tool = validTool(next.tool, prevType, nextType);
+			if (next.tool != tool) {
+				next.tool = tool;
+				cost += 7;
+			}
+
+			uint oldCost = costs[next.x][next.y][next.tool];
+			if (next.x == 0 && next.y == 0) continue;
+			if (oldCost && cost >= oldCost) continue;
+			costs[next.x][next.y][next.tool] = cost;
+			qPush(next, cost);
+		}
+	}
+	printf("%u\n", costs[targetX][targetY][Torch]);
 }
-- 
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