AdventOfCode2024/src/GuardGallivant.cpp

// Solutions to the Advent Of Code 2024.
// Copyright (C) 2024-2025  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/GuardGallivant.hpp>

const std::string GuardGallivant::getPuzzleName() const
{
    return "Guard Gallivant";
}

const int GuardGallivant::getPuzzleDay() const
{
    return 6;
}

void GuardGallivant::processDataLine(const std::string& line)
{
    auto pos = line.find(getStartChar());
    if (pos != std::string::npos)
    {
        start_ = Point2{ static_cast<int>(pos), static_cast<int>(lines.size()) };
    }

    LinesSolver::processDataLine(line);
}

void GuardGallivant::finish()
{
    tracePath();
}

constexpr size_t GuardGallivant::getStartDirectionIndex()
{
    return 2;
}

constexpr char GuardGallivant::getStartChar()
{
    return '^';
}

constexpr char GuardGallivant::getObstructionChar()
{
    return '#';
}

constexpr char GuardGallivant::getEmptyChar()
{
    return '.';
}

void GuardGallivant::tracePath()
{
    VisitGrid visitGrid{ lines.size(), lines[0].size() };
    visitGrid.fill(false);
    PathGrid pathGrid{ lines.size(), lines[0].size() };
    pathGrid.fill(0);

    // PathGrid for cycle tests, reused to avoid repeated memory allocation.
    PathGrid cyclePathGrid{ lines.size(), lines[0].size() };

    size_t directionIndex{ getStartDirectionIndex() };
    Point2 current{ start_ };
    visitPosition(current, directionIndex, visitGrid);
    tracePosition(current, directionIndex, pathGrid);
    backtracePath(current, directionIndex, pathGrid);
    auto next = current + Point2::cardinalDirections[directionIndex];
    while (isInBounds(next))
    {
        if (!checkTurn(current, next, directionIndex, pathGrid))
        {
            if (!visitGrid.cell(next))
            {
                if (pathGrid.cell(current)[turnDirectionRight(directionIndex)])
                {
                    part2++;
                }
                else
                {
                    setCharAt(next, getObstructionChar());
                    if (tracePathCycleTest(current, turnDirectionRight(directionIndex), pathGrid, cyclePathGrid))
                    {
                        part2++;
                    }
                    setCharAt(next, getEmptyChar());
                }
            }
            current = next;
            visitPosition(current, directionIndex, visitGrid);
            tracePosition(current, directionIndex, pathGrid);
        }
        next = current + Point2::cardinalDirections[directionIndex];
    }
}

bool GuardGallivant::tracePathCycleTest(const Point2& startPosition, const size_t startDirectionIndex,
    PathGrid& mainPathGrid, PathGrid& cyclePathGrid)
{
    cyclePathGrid.fill(0);

    size_t directionIndex{ startDirectionIndex };
    Point2 current{ startPosition };
    tracePosition(current, directionIndex, cyclePathGrid);
    backtracePath(current, directionIndex, cyclePathGrid);
    auto next = current + Point2::cardinalDirections[directionIndex];
    while (isInBounds(next))
    {
        if (!checkTurn(current, next, directionIndex, cyclePathGrid))
        {
            if (mainPathGrid.cell(next)[directionIndex] || cyclePathGrid.cell(next)[directionIndex])
            {
                return true;
            }
            current = next;
            tracePosition(current, directionIndex, cyclePathGrid);
        }
        next = current + Point2::cardinalDirections[directionIndex];
    }
    return false;
}

bool GuardGallivant::checkTurn(const Point2& current, const Point2& next, size_t& directionIndex, PathGrid& pathGrid)
{
    if (getCharAt(next) == getObstructionChar())
    {
        directionIndex = turnDirectionRight(directionIndex);
        backtracePath(current, directionIndex, pathGrid);
        return true;
    }
    return false;
}

void GuardGallivant::visitPosition(const Point2& current, const size_t directionIndex, VisitGrid& visitGrid)
{
    if (!visitGrid.cell(current))
    {
        visitGrid.cell(current) = true;
        part1++;
    }
}

void GuardGallivant::tracePosition(const Point2& current, const size_t directionIndex, PathGrid& pathGrid)
{
    pathGrid.cell(current)[directionIndex] = true;
    backtraceTurn(current, turnDirectionLeft(directionIndex), pathGrid);
}

void GuardGallivant::backtracePath(const Point2& current, const size_t directionIndex, PathGrid& pathGrid)
{
    auto oppositeDirectionIndex = revertDirection(directionIndex);
    auto next = current + Point2::cardinalDirections[oppositeDirectionIndex];
    while (isObstruction(next) && !pathGrid.cell(next)[directionIndex])
    {
        tracePosition(next, oppositeDirectionIndex, pathGrid);
        next = next + Point2::cardinalDirections[oppositeDirectionIndex];
    }
}

void GuardGallivant::backtraceTurn(const Point2& current, const size_t reverseTurnDirectionIndex, PathGrid& pathGrid)
{
    auto left = current + Point2::cardinalDirections[reverseTurnDirectionIndex];
    if (isObstruction(left))
    {
        backtracePath(current, reverseTurnDirectionIndex, pathGrid);
    }
}

size_t GuardGallivant::turnDirectionRight(const size_t currentDirectionIndex) const
{
    return currentDirectionIndex == 0 ? 3 : currentDirectionIndex - 1;
}

size_t GuardGallivant::turnDirectionLeft(const size_t currentDirectionIndex) const
{
    return currentDirectionIndex == 3 ? 0 : currentDirectionIndex + 1;
}

size_t GuardGallivant::revertDirection(const size_t currentDirectionIndex) const
{
    return currentDirectionIndex > 1 ? currentDirectionIndex - 2 : currentDirectionIndex + 2;
}

bool GuardGallivant::isObstruction(const Point2& position) const
{
    return isInBounds(position) && getCharAt(position) == getObstructionChar();
}