From a299e1723f0a700a892720badab2e06a202d1eda Mon Sep 17 00:00:00 2001
From: Sebastiano Tronto <sebastiano@tronto.net>
Date: Sun, 8 Dec 2024 07:49:46 +0100
Subject: Day 8 2024

---
 2024/08/day08b.cpp | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 2024/08/day08b.cpp

(limited to '2024/08/day08b.cpp')

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;
+}
-- 
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