From a6c923706a0447e65c3b0b2644727367494d3947 Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Tue, 2 Dec 2025 19:58:55 +0100
Subject: aoc 2025, day 2

---
 2025/src/day02.cpp | 115 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 2025/src/day02.cpp

diff --git a/2025/src/day02.cpp b/2025/src/day02.cpp
new file mode 100644
index 0000000..1d17cd9
--- /dev/null
+++ b/2025/src/day02.cpp
@@ -0,0 +1,115 @@
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <ranges>
+#include <sstream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+namespace {
+
+vector<string>
+split(const string& line, char sep)
+{
+	istringstream  strm{ line };
+	vector<string> parts;
+
+	for ( string part; getline(strm, part, sep); ) {
+		parts.emplace_back(part);
+	}
+	return parts;
+}
+
+vector<tuple<long, long>>
+read_file(const filesystem::path& filename)
+{
+	ifstream file{ filename };
+	string   line;
+	getline(file, line);
+
+	vector<tuple<long, long>> ranges;
+	for ( const auto& part: split(line, ',') ) {
+		const auto range = split(part, '-');
+		ranges.emplace_back(stol(range.at(0)), stol(range.at(1)));
+	}
+	return ranges;
+}
+
+long
+accumulate_invalids(const vector<tuple<long, long>>& ranges, const function<bool(long)>& is_invalid)
+{
+	long sum = 0;
+	for ( const auto [from, to]: ranges ) {
+		for ( auto i = from; i <= to; ++i ) {
+			if ( is_invalid(i) ) {
+				sum += i;
+			}
+		}
+	}
+	return sum;
+}
+
+void
+part1(const vector<tuple<long, long>>& ranges)
+{
+	auto is_invalid = [](long num) {
+		const auto str = to_string(num);
+		const auto len = str.length();
+
+		if ( len % 2 == 1 ) {
+			return false;
+		}
+
+		const auto half = len / 2;
+
+		for ( size_t i = 0; i != half; ++i ) {
+			if ( str[i] != str[i + half] ) {
+				return false;
+			}
+		}
+
+		return true;
+	};
+
+	cout << "Part 1: " << accumulate_invalids(ranges, is_invalid) << '\n';
+}
+
+void
+part2(const vector<tuple<long, long>>& ranges)
+{
+	auto is_invalid = [](long num) {
+		const auto str  = to_string(num);
+		const auto len  = str.length();
+		const auto half = len / 2;
+
+		for ( size_t i = 1; i <= half; ++i ) {
+			if ( len % i != 0 ) {
+				continue;
+			}
+
+			string pattern;
+			while ( pattern.length() != len ) {
+				pattern += str.substr(0, i);
+			}
+			if ( pattern == str ) {
+				return true;
+			}
+		}
+
+		return false;
+	};
+
+	cout << "Part 2: " << accumulate_invalids(ranges, is_invalid) << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+	auto ranges = read_file("data/day02.txt");
+	part1(ranges);
+	part2(ranges);
+}
-- 
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