aoc 2025, day 2

1 files changed, 115 insertions, 0 deletions

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);
+}