Day 8 2024

1 files changed, 68 insertions, 0 deletions

diff --git a/2024/08/day08b.cpp b/2024/08/day08b.cpp
new file mode 100644
index 0000000..1d1381f
--- /dev/null
+++ b/2024/08/day08b.cpp
@@ -0,0 +1,68 @@
+#include <algorithm>
+#include <iostream>
+#include <map>
+#include <set>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+using namespace std;
+
+#define N 50
+#define M 50
+
+bool in_range(int x, int y) {
+        return x >= 0 && x < N && y >= 0 && y < M;
+}
+
+vector<pair<int, int>> antinode_all(pair<int, int> p, pair<int, int> q) {
+        auto [xp, yp] = p;
+        auto [xq, yq] = q;
+        /* In theory these two coefficients must be divided by the gcd of
+         * of the coordinates, which leads to possibly more points,
+         * as far as I understood the problem statement. Apparently, this
+         * is not the case. */
+        int gx = (xp - xq);
+        int gy = (yp - yq);
+        vector<pair<int, int>> v;
+        for (int i = 0; in_range(xp+i*gx, yp+i*gy); i++)
+                v.push_back(make_pair(xp+i*gx, yp+i*gy));
+        for (int i = 0; in_range(xq-i*gx, yq-i*gy); i++)
+                v.push_back(make_pair(xq-i*gx, yq-i*gy));
+        return v;
+}
+
+void add_antinodes(set<pair<int, int>>& points,
+    pair<int, int> p, set<pair<int, int>>& antinodes) {
+        for (auto q : points) {
+                auto a = antinode_all(p, q);
+                for (auto r : a)
+                        antinodes.insert(r);
+        }
+}
+
+int main() {
+        string line;
+        map<char, set<pair<int, int>>> d;
+        set<pair<int, int>> antinodes;
+        int i, j;
+        for (i = 0; getline(cin, line); i++) {
+                stringstream s(line);
+                char c;
+                for (j = 0; s >> c; j++) {
+                        if (c != '.') {
+                                pair<int, int> p(i, j);
+                                add_antinodes(d[c], p, antinodes);
+                                d[c].insert(p);
+                        }
+                }
+        }
+        
+        int tot = 0;
+        for (auto [x, y] : antinodes)
+                if (in_range(x, y))
+                        tot++;
+
+        cout << tot << endl;
+        return 0;
+}