From 6bf92f2a2517f8eebddbdb139a877dcffd381edc Mon Sep 17 00:00:00 2001
From: Sebastiano Tronto <sebastiano@tronto.net>
Date: Wed, 18 Dec 2024 06:38:01 +0100
Subject: Day 18 2024

---
 2024/18/day18b.cpp | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 2024/18/day18b.cpp

(limited to '2024/18/day18b.cpp')

diff --git a/2024/18/day18b.cpp b/2024/18/day18b.cpp
new file mode 100644
index 0000000..dadd259
--- /dev/null
+++ b/2024/18/day18b.cpp
@@ -0,0 +1,74 @@
+/*
+I just look the shortest path after every byte drop and stop when there
+is none.  This code is not very fast (~25 seconds on my laptop), but it
+took just a couple of minutes to modify part 1 to get this.
+*/
+
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <map>
+#include <queue>
+#include <ranges>
+#include <set>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+#define N 71
+
+bool b[N][N];
+
+class Qelem {
+public:
+	int d, x, y;
+	Qelem(int dis, int xx, int yy) : d{dis}, x{xx}, y{yy} {}
+	bool operator<(const Qelem& other) const { return d > other.d; }
+};
+
+
+int coord(int x, int y) {
+	return N*x+y;
+}
+
+bool inrange(int x, int y) {
+	return x >= 0 && x < N && y >= 0 && y < N;
+}
+
+int shortest_path() {
+	vector<bool> vis(N*N);
+
+	priority_queue<Qelem> q;
+	q.push(Qelem(0, 0, 0));
+
+	while (!q.empty()) {
+		auto v = q.top();
+		q.pop();
+		if (!inrange(v.x, v.y) || b[v.x][v.y] || vis[coord(v.x, v.y)])
+			continue;
+		if (v.x == N-1 && v.y == N-1) return v.d;
+		vis[coord(v.x, v.y)] = true;
+		q.push(Qelem(v.d+1, v.x+1, v.y));
+		q.push(Qelem(v.d+1, v.x-1, v.y));
+		q.push(Qelem(v.d+1, v.x, v.y+1));
+		q.push(Qelem(v.d+1, v.x, v.y-1));
+	}
+	return -1;
+}
+
+int main() {
+	int x, y;
+	while (cin >> x >> y) {
+		b[x][y] = true;
+		if (shortest_path() == -1) {
+			cout << x << "," << y << endl;
+			exit(0);
+		}
+	}
+
+	cout << "Not found" << endl;
+
+	return 0;
+}
-- 
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