Add day 22 part 2 solution

day22/Makefile

@@ -1,7 +1,7 @@
 CC=g++
 BIN_NAME=day22
-CCFLAGS=-o $(BIN_NAME) -g -std=c++17
-LDFLAGS=-lfolly
+CCFLAGS=-o $(BIN_NAME) -O2 -std=c++17
+LDFLAGS=
 
 .PHONY: all, clean
 

day22/day22.cpp

@@ -1,18 +1,13 @@
-#include <folly/String.h>
-
 #include <fstream>
 #include <functional>
 #include <iostream>
 #include <numeric>
 #include <queue>
-#include <regex>
 #include <set>
 #include <string>
 #include <vector>
 
-// Pair of ingredients and allergens
-using IngredientLineItem = std::pair<std::vector<std::string>, std::vector<std::string>>;
-auto constexpr INGREDIENT_PATTERN = R"((.*) \(contains (.*)\))";
+enum Player { PLAYER1, PLAYER2 };
 
 std::vector<std::string> readInput(const std::string &filename) {
 	std::vector<std::string> input;
@@ -41,27 +36,58 @@ std::pair<std::vector<std::string>, std::vector<std::string>> splitInput(const s
 		std::move(beforeEmptyLine), std::move(afterEmptyLine));
 }
 
-template <typename Iterator, typename IntContainer>
-void containerStoi(Iterator begin, Iterator end, IntContainer &out) {
-	std::transform(begin, end, std::back_inserter(out), [](const std::string &item) { return std::stoi(item); });
+/**
+ * Perform std::stoi on every element in the container, pushing the items to the output iterator.
+ * @tparam Iterator The iterator for the input container
+ * @tparam IntContainer The container to output to
+ * @param begin The start of the range
+ * @param end The end of the range
+ * @param out The output iterator
+ */
+template <typename InputIterator, typename OutputIterator>
+void containerStoi(InputIterator begin, InputIterator end, OutputIterator out) {
+	std::transform(begin, end, out, [](const std::string &item) { return std::stoi(item); });
 }
 
+/**
+ * Parse the decks from the puzzle input
+ * @param input The puzzle input
+ * @return std::pair<std::vector<int>, std::vector<int>> Both decks to play the game (player 1, player 2)
+ */
 std::pair<std::vector<int>, std::vector<int>> parseDecks(const std::vector<std::string> &input) {
 	std::pair<std::vector<std::string>, std::vector<std::string>> inputComponents = splitInput(input);
 	std::pair<std::vector<int>, std::vector<int>> decks;
-	containerStoi(inputComponents.first.cbegin() + 1, inputComponents.first.cend(), decks.first);
-	containerStoi(inputComponents.second.cbegin() + 1, inputComponents.second.cend(), decks.second);
+	containerStoi(inputComponents.first.cbegin() + 1, inputComponents.first.cend(), std::back_inserter(decks.first));
+	containerStoi(inputComponents.second.cbegin() + 1, inputComponents.second.cend(), std::back_inserter(decks.second));
 
 	return decks;
 }
 
-std::vector<int> playBasicGame(const std::pair<std::vector<int>, std::vector<int>> &initialDecks) {
+/**
+ * Calculate the score for a game
+ * @tparam ReverseIterator A reverse iterator for the winning deck. Could technically be any input iterator but the
+ * score calculation requires going backwards
+ * @param begin The start of the deck
+ * @param end The end of the deck
+ * @return int The game score
+ */
+template <typename ReverseIterator>
+int calculateScore(ReverseIterator begin, ReverseIterator end) {
+	int i = 1;
+	int total = 0;
+	for (auto it = begin; it != end; (++it, i++)) {
+		int value = *it;
+		total += i * value;
+	}
+
+	return total;
+}
+
+int part1(const std::pair<std::vector<int>, std::vector<int>> &initialDecks) {
 	std::pair<std::deque<int>, std::deque<int>> decks{
 		std::deque<int>(initialDecks.first.cbegin(), initialDecks.first.cend()),
 		std::deque<int>(initialDecks.second.cbegin(), initialDecks.second.cend())};
-	int i = 0;
 	while (!decks.first.empty() && !decks.second.empty()) {
-		i++;
 		int player1Card = decks.first.front();
 		int player2Card = decks.second.front();
 		decks.first.pop_front();
@@ -77,19 +103,58 @@ std::vector<int> playBasicGame(const std::pair<std::vector<int>, std::vector<int
 
 	std::deque<int> &winnerDeck = decks.first.empty() ? decks.second : decks.first;
 
-	return std::vector<int>(winnerDeck.cbegin(), winnerDeck.cend());
+	return calculateScore(winnerDeck.crbegin(), winnerDeck.crend());
 }
 
-int part1(const std::pair<std::vector<int>, std::vector<int>> &decks) {
-	std::vector<int> winnerDeck = playBasicGame(decks);
-	int i = 1;
-	int total = 0;
-	for (auto it = winnerDeck.crbegin(); it != winnerDeck.crend(); (++it, i++)) {
-		int value = *it;
-		total += i * value;
-	}
+int part2(const std::pair<std::vector<int>, std::vector<int>> &initialDecks) {
+	using DeckPair = std::pair<std::deque<int>, std::deque<int>>;
+	std::function<std::pair<Player, DeckPair>(DeckPair &)> playGame =
+		[&playGame](DeckPair &decks) -> std::pair<Player, std::pair<std::deque<int>, std::deque<int>>> {
+		// Holds game states that have already been used, preventing their resuse within a single sub-game
+		std::set<DeckPair> usedDecks;
+		while (!decks.first.empty() && !decks.second.empty()) {
+			if (usedDecks.find(decks) != usedDecks.end()) {
+				return std::make_pair(PLAYER1, decks);
+			}
 
-	return total;
+			usedDecks.insert(decks);
+			int player1Card = decks.first.front();
+			int player2Card = decks.second.front();
+			Player roundWinner;
+			if (player1Card < decks.first.size() && player2Card < decks.second.size()) {
+				// Must be done after the size check...
+				decks.first.pop_front();
+				decks.second.pop_front();
+				DeckPair truncatedDecks;
+				std::copy_n(decks.first.cbegin(), player1Card, std::back_inserter(truncatedDecks.first));
+				std::copy_n(decks.second.cbegin(), player2Card, std::back_inserter(truncatedDecks.second));
+				roundWinner = playGame(truncatedDecks).first;
+			} else {
+				roundWinner = player1Card > player2Card ? PLAYER1 : PLAYER2;
+				decks.first.pop_front();
+				decks.second.pop_front();
+			}
+
+			if (roundWinner == PLAYER1) {
+				decks.first.push_back(player1Card);
+				decks.first.push_back(player2Card);
+			} else {
+				decks.second.push_back(player2Card);
+				decks.second.push_back(player1Card);
+			}
+		}
+
+		return std::make_pair(decks.first.empty() ? PLAYER2 : PLAYER1, decks);
+	};
+
+	std::pair<std::deque<int>, std::deque<int>> decks{
+		std::deque<int>(initialDecks.first.cbegin(), initialDecks.first.cend()),
+		std::deque<int>(initialDecks.second.cbegin(), initialDecks.second.cend())};
+
+	auto gameResult = playGame(decks);
+	auto winningDeck = gameResult.first == PLAYER1 ? gameResult.second.first : gameResult.second.second;
+
+	return calculateScore(winningDeck.crbegin(), winningDeck.crend());
 }
 
 int main(int argc, char *argv[]) {
@@ -102,4 +167,5 @@ int main(int argc, char *argv[]) {
 	auto decks = parseDecks(input);
 
 	std::cout << part1(decks) << std::endl;
+	std::cout << part2(decks) << std::endl;
 }