AdventOfCode2024/src/DiskFragmenter.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/DiskFragmenter.hpp>

#include <algorithm>

const std::string DiskFragmenter::getPuzzleName() const
{
    return "Disk Fragmenter";
}

const int DiskFragmenter::getPuzzleDay() const
{
    return 9;
}

void DiskFragmenter::processDataLine(const std::string& line)
{
    moveFileBlocks(line);
    moveWholeFiles(line);
}

void DiskFragmenter::moveFileBlocks(const std::string& line)
{
    // Index of the first unprocessed digit in the disk map.
    size_t front{ 1 };
    // ID number of the file 'front' refers to (when 'isFile == true'). Equivalent to 'front / 2', but calculated
    // incrementally.
    size_t frontIdNumber{ 1 };
    // Index of the last unprocessed digit in the disk map.
    size_t back{ line.size() - 1 };
    // Number of remaining fragmented blocks in the file that 'back' refers to.
    unsigned int nRemainingBackBlocks{ getDigit(line, back) };
    // ID number of the file 'back' refers to. Equivalent to 'back / 2', but calculated incrementally.
    size_t backIdNumber{ line.size() / 2 };
    // Current block position.
    size_t position{ getDigit(line, 0) };
    // If 'true', 'front' refers to a file in the digit map, otherwise it refers to an empty space.
    bool isFile{ false };

    while (front < back)
    {
        if (isFile)
        {
            // Adds the checksum for the file at 'front'.
            unsigned int nFileBlocks = getDigit(line, front);
            part1 += calcChecksumPart(frontIdNumber, nFileBlocks, position);
            frontIdNumber++;
        }
        else
        {
            unsigned int nFreeBlocks = getDigit(line, front);
            while (nFreeBlocks > 0)
            {
                if (nFreeBlocks >= nRemainingBackBlocks)
                {
                    // Adds the checksum for all the blocks of the file at 'back'.
                    part1 += calcChecksumPart(backIdNumber, nRemainingBackBlocks, position);
                    backIdNumber--;

                    nFreeBlocks -= nRemainingBackBlocks;
                    back -= 2;
                    nRemainingBackBlocks = getDigit(line, back);
                }
                else
                {
                    // Adds the checksum for the blocks of the file at 'back' that fit into the empty blocks at 'front'.
                    part1 += calcChecksumPart(backIdNumber, nFreeBlocks, position);

                    nRemainingBackBlocks -= nFreeBlocks;
                    nFreeBlocks = 0;
                }
            }
        }
        front++;
        isFile = !isFile;
    }

    // Adds the checksum for the remaining blocks of the file at 'back'.
    part1 += calcChecksumPart(backIdNumber, nRemainingBackBlocks, position);
}

void DiskFragmenter::moveWholeFiles(const std::string& line)
{
    // Index of the first unprocessed empty space in the disk map.
    size_t front{ 1 };
    // Index of the last unprocessed file in the disk map.
    size_t back{ line.size() - 1 };
    // ID number of the file 'back' refers to. Equivalent to 'back / 2', but calculated incrementally.
    size_t backIdNumber{ line.size() / 2 };
    // Current block position.
    size_t position{ getDigit(line, 0) };

    // Contains empty spaces as Interval.
    Intervals emptySpaces{};
    emptySpaces.reserve(backIdNumber / 2);

    // Contains indices of the next empty space with at least as many blocks as their index. 'emptySpaceIndices[0]' is
    // not used, but included for convenience of accessing the other values by index.
    DigitIndexArray emptySpaceIndices;
    emptySpaceIndices.fill(0);

    while (back > 0)
    {
        int nBackBlocks = getDigit(line, back);
        if (emptySpaceIndices[nBackBlocks] < emptySpaces.size() &&
            emptySpaces[emptySpaceIndices[nBackBlocks]].start < position)
        {
            if (front >= back)
            {
                position -= getDigit(line, back) + getDigit(line, back - 1);
            }
            moveBackFileForward(back, backIdNumber, nBackBlocks, emptySpaces[emptySpaceIndices[nBackBlocks]]);
            UpdateEmptySpaceIndices(emptySpaces, emptySpaceIndices);
        }
        else
        {
            AddNewEmptySpaces(emptySpaces, line, front, back, nBackBlocks, position);
            if (front < back)
            {
                moveBackFileForward(back, backIdNumber, nBackBlocks, emptySpaces.back());
            }
            else
            {
                keepBackFile(line, back, backIdNumber, nBackBlocks, position);
            }
            UpdateEmptySpaceIndices(emptySpaces, emptySpaceIndices);
        }
    }
}

void DiskFragmenter::AddNewEmptySpaces(Intervals& emptySpaces, const std::string& line, size_t& front,
    const size_t back, const int nBackBlocks, size_t& position)
{
    int nFrontBlocks{ 0 };
    while (front < back && nFrontBlocks < nBackBlocks)
    {
        // Adds more empty spaces.
        nFrontBlocks = getDigit(line, front);
        emptySpaces.emplace_back(position, nFrontBlocks);
        front++;
        position += static_cast<size_t>(nFrontBlocks) + getDigit(line, front++);
    }
}

void DiskFragmenter::UpdateEmptySpaceIndices(const Intervals& emptySpaces, DigitIndexArray& emptySpaceIndices)
{
    for (size_t i = 1; i < emptySpaceIndices.size(); i++)
    {
        while (emptySpaceIndices[i] < emptySpaces.size() && emptySpaces[emptySpaceIndices[i]].length < i)
        {
            emptySpaceIndices[i]++;
        }
    }
}

// Moves the 'back' file into the empty space.
void DiskFragmenter::moveBackFileForward(size_t& back, size_t& backIdNumber, const int nBackBlocks,
    Interval& emptySpace)
{
    part2 += calcChecksumPart(backIdNumber, nBackBlocks, emptySpace.start);
    emptySpace.length -= nBackBlocks;
    back -= 2;
    backIdNumber--;
}

// Does not move the 'back' file, there is no space for it.
void DiskFragmenter::keepBackFile(const std::string& line, size_t& back, size_t& backIdNumber, const int nBackBlocks,
    size_t& position)
{
    part2 -= calcChecksumPart(backIdNumber, -nBackBlocks, position);
    back--;
    position -= getDigit(line, back--);
    backIdNumber--;
}

void DiskFragmenter::finish()
{
}

unsigned int DiskFragmenter::getDigit(const std::string& line, const size_t index) const
{
    return line[index] - '0';
}

long long int DiskFragmenter::calcChecksumPart(const size_t idNumber, const int nBlocks, size_t& position) const
{
    position += nBlocks;
    // Casting the parameters is required to allow negative block count, resulting in a negative return value.
    return static_cast<long long int>(idNumber) *
        ((nBlocks * (2 * static_cast<long long int>(position) - nBlocks - 1)) / 2);
}