AdventOfCode2024/src/ChronospatialComputer.cpp

// Solutions to the Advent Of Code 2024.
// Copyright (C) 2025  Stefan Müller
//
// This program is free software: you can redistribute it and/or modify it under
// the terms of the GNU General Public License as published by the Free Software
// Foundation, either version 3 of the License, or (at your option) any later
// version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
// FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License along with
// this program.  If not, see <http://www.gnu.org/licenses/>.

#include <aoc/ChronospatialComputer.hpp>

#include <sstream>
#include <string>

ChronospatialComputer::ChronospatialComputer()
    : state_{}, program_{}, instructions_{
          std::make_unique<ChronospatialComputerDivisionInstruction>(0),   // adv
          std::make_unique<ChronospatialComputerXorLiteralInstruction>(),  // bxl
          std::make_unique<ChronospatialComputerModuloInstruction>(),      // bst
          std::make_unique<ChronospatialComputerJumpInstruction>(),        // jnz
          std::make_unique<ChronospatialComputerXorRegisterInstruction>(), // bxc
          std::make_unique<ChronospatialComputerOutInstruction>(),         // out
          std::make_unique<ChronospatialComputerDivisionInstruction>(1),   // bdv
          std::make_unique<ChronospatialComputerDivisionInstruction>(2)    // cdv
      }
{
}

const std::string ChronospatialComputer::getPuzzleName() const
{
    return "Chronospatial Computer";
}

const int ChronospatialComputer::getPuzzleDay() const
{
    return 17;
}

void ChronospatialComputer::processDataLine(const std::string& line)
{
    std::istringstream stream{ line };
    std::string token{};
    char c;
    int value;
    if (stream >> token)
    {
        if (token == "Register")
        {
            stream >> c >> token >> value;
            // c must be 'A', 'B', or 'C'.
            size_t regIndex{ static_cast<size_t>(c - 65) };
            state_.registers[regIndex] = value;
        }
        else
        {
            while (stream >> value)
            {
                program_.push_back(value);
                // Streams a comma from between values.
                stream >> c;
            }
        }
    }
}

void ChronospatialComputer::finish()
{
    runProgram(program_, state_);
    part1 = state_.output.str();
    part2 = findQuineState();
}

void ChronospatialComputer::runProgram(const std::vector<int>& program, ChronospatialComputerState& state) const
{
    state.instructionPointer = 0;
    while (state.instructionPointer < program.size())
    {
        instructions_[program[state.instructionPointer]]->run(state, program[state.instructionPointer + 1]);
    }
}

int64_t ChronospatialComputer::findQuineState()
{
    // The following masks and output patterns are specific to a single program and have been precalculated manually.
    // The given Chronospatial Computer program essentially does the following calculations.
    //
    //     A' = A >> 3
    //     OUT = (((A mod 8) xor 4) xor (A shr ((A mod 8) xor 1))) mod 8,
    //
    // where A is the initial value of register A, A' is the value of register A when program flow reaches the jump
    // instruction at the end of the program, and OUT is the single value written to the output when program flow reaches
    // that same jump instruction.
    //
    // So given a desired output OUT, the number to be appended to the accumulated result can be found by matching the
    // output patterns below against the masked trailing bits of the accumulated result, see the calculations below.

    // Array index corresponds to number to append.
    std::array<int, 8> masks{ 0b1, 0b0, 0b111, 0b11, 0b11100, 0b1110, 0b1110000, 0b111000 };

    // Array index is the output number.
    // First pair element is the number appended to register A.
    // Second pair element is the pattern to match against register A.
    std::array<std::vector<std::pair<int, int>>, 8> outputPatterns
    { {
        // output 0
        { { 0, 0b1 }, { 2, 0b110 }, { 4, 0b00000 }, { 5, 0b0010 }, { 6, 0b0100000 }, { 7, 0b0110000 } },
        // output 1
        { { 2, 0b111 }, { 3, 0b11 }, { 4, 0b00100 }, { 5, 0b0000 }, { 6, 0b0110000 }, { 7, 0b010000 } },
        // output 2
        { { 2, 0b100 }, { 4, 0b01000 }, { 5, 0b0110 }, { 6, 0b0000000 }, { 7, 0b001000 } },
        // output 3
        { { 2, 0b101 }, { 3, 0b10 }, { 4, 0b01100 }, { 5, 0b0100 }, { 6, 0b0010000 }, { 7, 0b000000 } },
        // output 4
        { { 0, 0b0 }, { 1, 0b0 }, { 2, 0b010 }, { 4, 0b10000 }, { 5, 0b1010 }, { 6, 0b1100000 }, { 7, 0b111000 } },
        // output 5
        { { 2, 0b011 }, { 3, 0b01 }, { 4, 0b10100 }, { 5, 0b1000 }, { 6, 0b1110000 }, { 7, 0b110000 } },
        // output 6
        { { 2, 0b000 }, { 4, 0b11000 }, { 5, 0b1110 }, { 6, 0b1000000 }, { 7, 0b101000 } },
        // output 7
        { { 2, 0b001 }, { 3, 0b00 }, { 4, 0b11100 }, { 5, 0b1100 }, { 6, 0b1010000 }, { 7, 0b100000 } }
    } };

    int64_t result{ 0 };
    size_t i{ program_.size() };
    int lastAppend{ -1 };

    // Appends 3-digit binary numbers to the end of 'result' that will cause the program to output the next desired
    // number (from 'program_'). The target output numbers are considered in reverse order.
    while (i > 0)
    {
        auto& patterns = outputPatterns[program_[i - 1]];
        auto it = std::find_if(patterns.begin(), patterns.end(),
            [&lastAppend, &result, &masks](auto& x) {
                return lastAppend < x.first && (result & masks[x.first]) == x.second; });
        if (it != patterns.end())
        {
            // Appends the next number 'it->first' that will result in the desired output 'program_[i - 1]'.
            lastAppend = -1;
            result = (result << 3) + it->first;
            i--;
        }
        else if (i < program_.size())
        {
            // Backtrack because no 3-digit number can be appended to yield the desired output.
            lastAppend = result & 0b111;
            result >>= 3;
            i++;
        }
        else
        {
            // This should not execute, it's a safeguard against an infinite loop in case of unexpected input.
            return result;
        }
    }

    return result;
}