1 files changed, 231 insertions, 3 deletions

diff --git a/2024/src/day24.cpp b/2024/src/day24.cpp
index bf439c7..3f252a1 100644
--- a/2024/src/day24.cpp
+++ b/2024/src/day24.cpp
@@ -1,3 +1,4 @@
+#include <array>
 #include <fstream>
 #include <iostream>
 #include <map>
@@ -7,6 +8,8 @@
 #include <vector>
 using namespace std;
 
+static const auto MAX_BITS = 45UL;
+
 vector<string>
 split(string_view line, string_view delimiter)
 {
@@ -26,6 +29,23 @@ split(string_view line, string_view delimiter)
         return res;
 }
 
+template<typename Container>
+string
+join(const Container& container, const string& delimiter)
+{
+        ostringstream oss;
+        auto          iter = container.begin();
+        if ( iter != container.end() ) {
+                oss << *iter;
+                ++iter;
+        }
+        while ( iter != container.end() ) {
+                oss << delimiter << *iter;
+                ++iter;
+        }
+        return oss.str();
+}
+
 tuple<map<string, unsigned long>, map<string, tuple<string, string, string>>>
 read_file(string_view filename)
 {
@@ -76,9 +96,6 @@ part1(const tuple<map<string, unsigned long>, map<string, tuple<string, string,
                 else if ( op == "OR" ) {
                         value = eval(lhs) | eval(rhs);
                 }
-                else {
-                        cerr << "Das darf nicht passieren! Input: " << input << endl;
-                }
 
                 inputs[input] = value;
 
@@ -97,9 +114,220 @@ part1(const tuple<map<string, unsigned long>, map<string, tuple<string, string,
         cout << value << endl;
 }
 
+string
+make_wire(char chr, unsigned long num)
+{
+        static const size_t  MAX_LEN = 10;
+        array<char, MAX_LEN> tmp{};
+        snprintf(tmp.data(), tmp.size(), "%c%02lu", chr, num); // NOLINT
+        return tmp.data();
+}
+
+class RippleCarryAdder {
+public:
+        explicit RippleCarryAdder(const tuple<map<string, unsigned long>, map<string, tuple<string, string, string>>>& data)
+            : inputs_(get<0>(data))
+            , deps_(get<1>(data))
+        {
+        }
+
+        RippleCarryAdder(const map<string, tuple<string, string, string>>& deps, unsigned long x_value, unsigned long y_value)
+            : deps_(deps)
+        {
+                build_inputs(x_value, y_value);
+        }
+
+        void swap_wire(const string& lhs, const string& rhs)
+        {
+                swap(deps_.at(lhs), deps_.at(rhs));
+        }
+
+        bool eval_z(unsigned long x_value, unsigned long y_value)
+        {
+                build_inputs(x_value, y_value);
+
+                bool overflow = false;
+                for ( auto bit = 0UL; bit != MAX_BITS + 1; ++bit ) {
+                        static const int MAX_RECURSION_DEPTH = 99;
+                        eval(make_wire('z', bit), MAX_RECURSION_DEPTH, overflow);
+                }
+                return !overflow;
+        }
+
+        [[nodiscard]] vector<string> test(unsigned long x_value, unsigned long y_value) const
+        {
+                const auto z_value = x_value + y_value;
+
+                vector<string> failed_bits;
+                auto           bit = 0UL;
+                for ( ; bit != MAX_BITS + 1; ++bit ) {
+                        const auto z_bit = (z_value & (1UL << bit)) != 0 ? 1UL : 0UL;
+                        const auto wire  = make_wire('z', bit);
+                        if ( inputs_.at(wire) != z_bit ) {
+                                failed_bits.push_back(wire);
+                        }
+                }
+                return failed_bits;
+        };
+
+        void print(unsigned long x_value, unsigned long y_value, ostream& out) const
+        {
+                const auto z_value = x_value + y_value;
+
+                out << "  x:  " << bitset<MAX_BITS>{ x_value } << '\n'
+                    << "+ y:  " << bitset<MAX_BITS>{ y_value } << '\n'
+                    << "= z: " << bitset<MAX_BITS + 1>{ z_value } << '\n';
+
+                out << "===: ";
+                for ( auto bit = MAX_BITS + 1; bit-- > 0; ) {
+                        const auto wire = make_wire('z', bit);
+                        if ( inputs_.contains(wire) ) {
+                                out << inputs_.at(wire);
+                        }
+                        else {
+                                out << '_';
+                        }
+                }
+                out << endl;
+        };
+
+        void collect(const string& input, set<string>& possible_fails) const
+        {
+                collect(input, 2, possible_fails);
+        }
+
+private:
+        void build_inputs(unsigned long x_value, unsigned long y_value)
+        {
+                inputs_.clear();
+
+                for ( auto bit = 0UL; bit != MAX_BITS; ++bit ) {
+                        inputs_.emplace(make_wire('x', bit), (x_value & (1UL << bit)) != 0 ? 1 : 0);
+                        inputs_.emplace(make_wire('y', bit), (y_value & (1UL << bit)) != 0 ? 1 : 0);
+                }
+        }
+
+        unsigned long eval(const string& input, size_t depth, bool& overflow)
+        {
+                if ( depth == 0 ) {
+                        overflow = true;
+                        return 0;
+                }
+
+                if ( inputs_.contains(input) ) {
+                        return inputs_.at(input);
+                }
+
+                const auto& [lhs, op, rhs] = deps_.at(input);
+
+                unsigned long value = 0;
+
+                if ( op == "XOR" ) {
+                        value = eval(lhs, depth - 1, overflow) ^ eval(rhs, depth - 1, overflow);
+                }
+                else if ( op == "AND" ) {
+                        value = eval(lhs, depth - 1, overflow) & eval(rhs, depth - 1, overflow);
+                }
+                else if ( op == "OR" ) {
+                        value = eval(lhs, depth - 1, overflow) | eval(rhs, depth - 1, overflow);
+                }
+
+                inputs_[input] = value;
+
+                return value;
+        }
+
+        void collect(const string& input, size_t depth, set<string>& possible_fails) const
+        {
+                if ( !deps_.contains(input) ) {
+                        return;
+                }
+
+                possible_fails.insert(input);
+
+                if ( depth == 0 ) {
+                        return;
+                }
+
+                const auto& [lhs, op, rhs] = deps_.at(input);
+
+                collect(lhs, depth - 1, possible_fails);
+                collect(rhs, depth - 1, possible_fails);
+        };
+
+        map<string, unsigned long> inputs_;
+
+        // register => { reg1 operator reg2 }
+        map<string, tuple<string, string, string>> deps_;
+};
+
+void
+part2(const tuple<map<string, unsigned long>, map<string, tuple<string, string, string>>>& data)
+{
+        RippleCarryAdder rca(data);
+
+        set<string> solution;
+
+        for ( auto bit = 0UL; bit != MAX_BITS; ++bit ) {
+                const auto x_value = 1UL << bit; // (1UL << (bit+1)) - 1;
+                const auto y_value = 0UL << bit;
+
+                if ( !rca.eval_z(x_value, y_value) ) {
+                        return;
+                }
+
+                const auto failed_bits = rca.test(x_value, y_value);
+                if ( !failed_bits.empty() ) {
+                        set<string> candidates;
+                        for ( const auto& wire: failed_bits ) {
+                                rca.collect(wire, candidates);
+                        }
+
+                        const vector<string> foo{ candidates.begin(), candidates.end() };
+
+                        for ( auto i = foo.begin(); i != foo.end(); ++i ) {
+                                for ( auto j = i + 1; j != foo.end(); ++j ) {
+                                        auto do_test = [&](unsigned long lhs, unsigned long rhs) -> bool {
+                                                RippleCarryAdder rca_tester{ data };
+
+                                                rca_tester.swap_wire(*i, *j);
+
+                                                if ( !rca_tester.eval_z(lhs, rhs) ) {
+                                                        return false;
+                                                }
+                                                return rca_tester.test(lhs, rhs).empty();
+                                        };
+
+                                        if ( do_test(x_value, y_value) ) {
+                                                const auto x_value2 = 7UL << (bit - 1);
+                                                const auto y_value2 = 3UL << (bit - 1);
+
+                                                if ( do_test(x_value2, y_value2) ) {
+                                                        const auto x_value3 = 1UL << bit;
+                                                        const auto y_value3 = 1UL << bit;
+
+                                                        if ( do_test(x_value3, y_value3) ) {
+                                                                const auto x_value4 = 5UL << (bit - 1);
+                                                                const auto y_value4 = 3UL << (bit - 1);
+
+                                                                if ( do_test(x_value4, y_value4) ) {
+                                                                        solution.insert(*i);
+                                                                        solution.insert(*j);
+                                                                }
+                                                        }
+                                                }
+                                        }
+                                }
+                        }
+                }
+        }
+        cout << join(solution, ",") << endl;
+}
+
 int
 main()
 {
         const auto data = read_file("data/day24.txt");
         part1(data);
+        part2(data);
 }