118 lines
3.8 KiB
C++
118 lines
3.8 KiB
C++
// Solutions to the Advent Of Code 2024.
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// Copyright (C) 2025 Stefan Müller
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//
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// This program is free software: you can redistribute it and/or modify it under
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// the terms of the GNU General Public License as published by the Free Software
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// Foundation, either version 3 of the License, or (at your option) any later
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// version.
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//
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// This program is distributed in the hope that it will be useful, but WITHOUT
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// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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// FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License along with
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// this program. If not, see <http://www.gnu.org/licenses/>.
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#include <aoc/RaceCondition.hpp>
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RaceCondition::RaceCondition(const int threshold)
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: threshold_{ threshold }
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{
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}
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const std::string RaceCondition::getPuzzleName() const
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{
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return "Race Condition";
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}
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const int RaceCondition::getPuzzleDay() const
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{
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return 20;
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}
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void RaceCondition::finish()
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{
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// Vector of positions that form the path. The index of an element is also the time at which the position is passed.
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std::vector<Point2> path{};
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path.reserve(static_cast<size_t>(threshold_) * 2);
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path.push_back(findChar(getStartChar()));
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bool isMoving{ true };
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while (isMoving)
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{
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// Checks if there is a cheat leading to the current position.
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checkCheat(path);
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// Progresses the race path.
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isMoving = tryFindNextPathPosition(path);
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}
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}
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constexpr char RaceCondition::getStartChar()
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{
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return 'S';
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}
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constexpr char RaceCondition::getWallChar()
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{
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return '#';
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}
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constexpr int RaceCondition::getPart1CheatLength()
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{
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return 2;
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}
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constexpr int RaceCondition::getPart2CheatLength()
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{
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return 20;
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}
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bool RaceCondition::tryFindNextPathPosition(std::vector<Point2>& path)
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{
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auto previous = path.size() <= 1 ? Point2{ -1, -1 } : path[path.size() - 2];
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for (const auto& direction : Point2::cardinalDirections)
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{
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auto next = path.back() + direction;
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if (next != previous && getCharAt(next) != getWallChar())
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{
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path.push_back(next);
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return true;
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}
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}
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return false;
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}
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void RaceCondition::checkCheat(const std::vector<Point2>& path)
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{
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// Checks previously encountered path positions that are at least the threshold away from the current position in
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// reverse order for valid cheat opportunities.
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int64_t i{ static_cast<int64_t>(path.size()) - threshold_ - 2 };
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while (i >= 0)
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{
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int64_t distance{ path.back().calcManhattanDistance(path[i]) };
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// Checks if the time saved by the cheat reaches at least the threshold, and if the cheat is not longer than
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// permitted. The permitted cheat time is longer for part 2 than for part 1.
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int64_t thresholdMinusTimeSaved{ threshold_ - static_cast<int64_t>(path.size()) + i + distance };
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int64_t cheatLengthDiff{ distance - getPart2CheatLength() };
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if (thresholdMinusTimeSaved < 0 && cheatLengthDiff <= 0)
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{
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part2++;
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if (distance <= getPart1CheatLength())
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{
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part1++;
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}
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i--;
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}
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else
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{
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// Backtracks the path as much as possible for the next potential position. This is possible because we know
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// that the 'distance' between 'path.back()' and 'path[i]' cannot change by more than the change of 'i',
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// since the positions in 'path' are contiguous. In other words, from this iteration to the next, both
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// 'cheatLengthDiff' and 'thresholdMinusTimeSaved / 2' cannot change more than 'i'. We use this to skip the
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// positions that cannot fulfill the condition above.
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i -= std::max<int64_t>(std::max(thresholdMinusTimeSaved >> 1, cheatLengthDiff), 1);
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}
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}
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}
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