aoc 2024, day 17, part 2

1 files changed, 52 insertions, 6 deletions

diff --git a/2024/src/day17.cpp b/2024/src/day17.cpp
index f0d1010..a4af373 100644
--- a/2024/src/day17.cpp
+++ b/2024/src/day17.cpp
@@ -57,10 +57,10 @@ read_file(string_view filename)
         return { registers, program };
 }
 
-void
-part1(const tuple<registers_type, program_type>& data)
+vector<unsigned long>
+run(registers_type registers, const program_type& program)
 {
-        auto [registers, program] = data;
+        vector<unsigned long> out;
 
         for ( unsigned long ip = 0; ip < program.size(); ) {
                 const auto opcode  = program.at(ip++);
@@ -86,7 +86,7 @@ part1(const tuple<registers_type, program_type>& data)
                         registers['B'] ^= registers['C'];
                         break;
                 case 5:
-                        cout << (value & 7) << ',';
+                        out.push_back(value & 7);
                         break;
                 case 6:
                         registers['B'] = registers['A'] >> value;
@@ -95,16 +95,62 @@ part1(const tuple<registers_type, program_type>& data)
                         registers['C'] = registers['A'] >> value;
                         break;
                 default:
-                        cout << "Error" << endl;
-                        return;
+                        throw runtime_error("Error");
                 }
         }
+        return out;
+}
+
+void
+part1(const tuple<registers_type, program_type>& data)
+{
+        auto [registers, program] = data;
+
+        auto result = run(registers, program);
+
+        copy(result.begin(), result.end(), ostream_iterator<unsigned long>(cout, ","));
         cout << endl;
 }
 
+optional<unsigned long>
+find_register_a(const registers_type& registers, const program_type& program, size_t pos)
+{
+        if ( pos > program.size() ) {
+                return registers.at('A');
+        }
+
+        for ( unsigned long i = 0; i != 8; ++i ) {
+                auto registers_copy = registers;
+
+                registers_copy['A'] <<= 3UL;
+                registers_copy['A'] |= i;
+
+                const auto try_run = run(registers_copy, program);
+
+                if ( try_run.front() == program[program.size() - pos] ) {
+                        if ( auto result = find_register_a(registers_copy, program, pos + 1) ) {
+                                return *result;
+                        }
+                }
+        }
+        return nullopt;
+}
+
+void
+part2(const tuple<registers_type, program_type>& data)
+{
+        auto [registers, program] = data;
+
+        registers['A'] = 0;
+        if ( auto result = find_register_a(registers, program, 1) ) {
+                cout << *result << endl;
+        }
+}
+
 int
 main()
 {
         auto data = read_file("data/day17.txt");
         part1(data);
+        part2(data);
 }