1 files changed, 102 insertions, 0 deletions

diff --git a/2016/src/day04.cpp b/2016/src/day04.cpp
new file mode 100644
index 0000000..713b16f
--- /dev/null
+++ b/2016/src/day04.cpp
@@ -0,0 +1,102 @@
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <regex>
+#include <set>
+#include <sstream>
+#include <string>
+#include <tuple>
+#include <vector>
+
+using namespace std;
+
+vector<string>
+split(const string& line, const regex& sep)
+{
+        return { sregex_token_iterator(line.begin(), line.end(), sep, -1), {} };
+}
+
+vector<tuple<vector<string>, long, string>>
+read_file(string_view filename)
+{
+        fstream input{ filename };
+
+        vector<tuple<vector<string>, long, string>> result;
+
+        for ( string line; getline(input, line); ) {
+                static const regex sep{ "[\\[\\]-]" };
+
+                auto parts = split(line, sep);
+
+                auto checksum = parts.at(parts.size() - 1);
+                parts.pop_back();
+
+                auto selector = stol(parts.at(parts.size() - 1));
+                parts.pop_back();
+
+                result.emplace_back(parts, selector, checksum);
+        }
+
+        return result;
+}
+
+void
+part1(const vector<tuple<vector<string>, long, string>>& data)
+{
+        long sum = 0;
+        for ( const auto& [ids, selector, checksum]: data ) {
+                map<char, long> counts;
+                for ( const auto& id: ids ) {
+                        for ( const auto chr: id ) {
+                                ++counts[chr];
+                        }
+                }
+
+                set<tuple<long, char>> tops;
+                for ( const auto& [chr, count]: counts ) {
+                        tops.emplace(-count, chr);
+                }
+
+                string result;
+                for ( const auto& [count, chr]: tops ) {
+                        result += chr;
+                }
+
+                if ( result.starts_with(checksum) ) {
+                        sum += selector;
+                }
+        }
+        cout << sum << endl;
+}
+
+void
+part2(const vector<tuple<vector<string>, long, string>>& lines)
+{
+        auto rot = [](string str, int n) {
+                for ( auto& chr: str ) {
+                        chr = ((chr - 'a' + n) % 26) + 'a';
+                }
+                return str;
+        };
+
+        auto is_northpole = [&](const string& str, int n) {
+                return rot(str, n) == "northpole";
+        };
+
+        for ( const auto& line: lines ) {
+                const auto& ids      = get<0>(line);
+                const auto  selector = get<1>(line);
+                if ( ranges::any_of(ids, [&](auto& str) { return is_northpole(str, (int) selector); }) ) {
+                        cout << selector << endl;
+                        return;
+                }
+        }
+}
+
+int
+main()
+{
+        auto data = read_file("data/day04.txt");
+        part1(data);
+        part2(data);
+}