2 files changed, 118 insertions, 1 deletions

diff --git a/makefile b/makefile
index 9d0c4c9..7643188 100644
--- a/makefile
+++ b/makefile
@@ -2,7 +2,8 @@ include config.mk
 
 all: bin/day01 \
          bin/day02 \
-         bin/day03
+         bin/day03 \
+         bin/day04
 
 bin/%: src/%.cpp | bin
         c++ $(CPPFLAGS) $^ -o $@
diff --git a/src/day04.cpp b/src/day04.cpp
new file mode 100644
index 0000000..8afcb2c
--- /dev/null
+++ b/src/day04.cpp
@@ -0,0 +1,116 @@
+#include <cctype>
+#include <fstream>
+#include <iostream>
+#include <set>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <vector>
+
+using namespace std;
+
+vector<string>
+read_file(string_view filename)
+{
+        fstream        input{ filename };
+        vector<string> data;
+
+        for ( string line; getline(input, line); ) {
+                data.emplace_back(line);
+        }
+
+        return data;
+}
+
+vector<string>
+split(const string& line, char sep)
+{
+        vector<string> parts{};
+        stringstream   input{ line };
+
+        for ( string part; getline(input, part, sep); ) {
+                parts.emplace_back(part);
+        }
+
+        return parts;
+}
+
+template<typename Container>
+Container
+read_ints(const string& line)
+{
+        Container    container{};
+        stringstream input{ line };
+
+        for ( typename Container::value_type value; input >> value; ) {
+                container.emplace(value);
+        }
+
+        return container;
+}
+
+vector<unsigned long>
+build_data_set()
+{
+        const auto            lines = read_file("data/day04.txt");
+        vector<unsigned long> values{};
+
+        for ( const auto& line: lines ) {
+                const auto cards   = split(line, ':');
+                const auto numbers = split(cards[1], '|');
+
+                const auto winning_numbers = read_ints<set<int>>(numbers[0]);
+                const auto card_numbers    = read_ints<set<int>>(numbers[1]);
+
+                auto count = 0U;
+                for ( auto number: winning_numbers ) {
+                        if ( card_numbers.contains(number) ) {
+                                ++count;
+                        }
+                }
+                values.emplace_back(count);
+        }
+
+        return values;
+}
+
+void
+pass1()
+{
+        auto sum = 0U;
+        for ( auto value: build_data_set() ) {
+                if ( value != 0 ) {
+                        sum += (1U << (value - 1));
+                }
+        }
+
+        cout << sum << endl;
+}
+
+void
+pass2()
+{
+        const auto values = build_data_set();
+        auto       count  = 0U;
+
+        function<void(size_t, size_t)> process = [&](size_t start, size_t end) -> void {
+                for ( ; start != end; ++start ) {
+                        auto value = values[start];
+
+                        ++count;
+
+                        process(start + 1, start + value + 1);
+                }
+        };
+
+        process(0, values.size());
+
+        cout << count << endl;
+}
+
+int
+main()
+{
+        pass1();
+        pass2();
+}