Day 18 2024

3 files changed, 161 insertions, 0 deletions

diff --git a/2024/18/Makefile b/2024/18/Makefile
new file mode 100644
index 0000000..1372bff
--- /dev/null
+++ b/2024/18/Makefile
@@ -0,0 +1,24 @@
+CC=g++ -std=c++20 -g -Wall
+
+a:
+        ${CC} -o a.out day18a.cpp
+
+b:
+        ${CC} -o b.out day18b.cpp
+
+clean:
+        rm -f a b
+
+atest: a
+        ./a.out
+
+btest: b
+        ./b.out
+
+arun: a
+        ./a.out < input
+
+brun: b
+        ./b.out < input
+
+.PHONY: a b clean atest btest arun brun
diff --git a/2024/18/day18a.cpp b/2024/18/day18a.cpp
new file mode 100644
index 0000000..7c795a9
--- /dev/null
+++ b/2024/18/day18a.cpp
@@ -0,0 +1,63 @@
+#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
+#define S 1024
+
+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(bool b[N][N]) {
+        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;
+        for (int i = 0; i < S && (cin >> x >> y); i++)
+                b[x][y] = true;
+
+        cout << shortest_path(b) << endl;
+        return 0;
+}
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;
+}