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#include <fstream>
#include <iostream>
#include <map>
#include <sstream>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
using program_type = vector<unsigned long>;
using registers_type = map<char, unsigned long>;
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;
}
tuple<registers_type, program_type>
read_file(string_view filename)
{
fstream input{ filename };
map<char, unsigned long> registers;
vector<unsigned long> program;
for ( string line; getline(input, line); ) {
if ( line.empty() ) {
continue;
}
const auto parts = split(line, ':');
if ( parts[0] == "Register A" ) {
registers['A'] = stoul(parts[1]);
}
else if ( parts[0] == "Register B" ) {
registers['B'] = stoul(parts[1]);
}
else if ( parts[0] == "Register C" ) {
registers['C'] = stoul(parts[1]);
}
else if ( parts[0] == "Program" ) {
for ( const auto& value: split(parts[1], ',') ) {
program.emplace_back(stoul(value));
}
}
}
return { registers, program };
}
vector<unsigned long>
run(registers_type registers, const program_type& program)
{
vector<unsigned long> out;
for ( unsigned long ip = 0; ip < program.size(); ) {
const auto opcode = program.at(ip++);
const auto operand = program.at(ip++);
const auto value = (operand < 4) ? operand : registers[char('A' + operand - 4)];
switch ( opcode ) {
case 0: // adv
registers['A'] >>= value;
break;
case 1: // bxl
registers['B'] ^= operand;
break;
case 2: // bst
registers['B'] = value % 8;
break;
case 3: // jnz
if ( registers['A'] != 0 ) {
ip = operand;
}
break;
case 4:
registers['B'] ^= registers['C'];
break;
case 5:
out.push_back(value & 7);
break;
case 6:
registers['B'] = registers['A'] >> value;
break;
case 7:
registers['C'] = registers['A'] >> value;
break;
default:
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);
}
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