Add messy day 6 solution

day6/Makefile

@@ -0,0 +1,15 @@
+CC=g++
+BIN_NAME=day6
+CCFLAGS=-o $(BIN_NAME) -g
+LDFLAGS=-std=c++17
+
+.PHONY: all, clean
+
+all: $(BIN_NAME)
+
+clean:
+	rm -f $(BIN_NAME)
+
+$(BIN_NAME): day6.cpp
+	$(CC) $(CCFLAGS) $(LDFLAGS) day6.cpp
+

day6/day6.cpp

@@ -0,0 +1,90 @@
+#include <folly/String.h>
+
+#include <algorithm>
+#include <execution>
+#include <fstream>
+#include <iostream>
+#include <numeric>
+#include <set>
+#include <string_view>
+#include <vector>
+
+std::vector<std::string> read_input(const std::string &filename) {
+	std::vector<std::string> input;
+	std::string line;
+	std::ifstream file(filename);
+	while (std::getline(file, line)) {
+		input.push_back(line);
+	}
+
+	return input;
+}
+
+int part1(const std::vector<std::string> &input) {
+	int total = 0;
+	std::set<char> groups;
+	for (std::string_view line : input) {
+		if (line.length() == 0) {
+			total += groups.size();
+			groups.clear();
+			continue;
+		}
+
+		for (char item : line) {
+			groups.insert(item);
+		}
+	}
+
+	return total;
+}
+
+int part2(const std::vector<std::string> &input) {
+	int total = 0;
+	std::set<char> commonAnswers;
+	for (char i = 'a'; i <= 'z'; i++) {
+		commonAnswers.insert(i);
+	}
+
+	for (std::string_view line : input) {
+		if (line.length() == 0) {
+			total += commonAnswers.size();
+			commonAnswers.clear();
+			for (char i = 'a'; i <= 'z'; i++) {
+				commonAnswers.insert(i);
+			}
+			continue;
+		}
+
+		std::set<char> personAnswers;
+		for (char item : line) {
+			personAnswers.insert(item);
+		}
+
+		std::set<char> intersection;
+		std::set_intersection(
+			personAnswers.begin(),
+			personAnswers.end(),
+			commonAnswers.begin(),
+			commonAnswers.end(),
+			std::inserter(intersection, intersection.begin()));
+
+		std::string out;
+		folly::join(" ", commonAnswers.begin(), commonAnswers.end(), out);
+		std::cout << out << std::endl;
+		commonAnswers = std::move(intersection);
+	}
+
+	return total;
+}
+
+int main(int argc, char *argv[]) {
+	if (argc != 2) {
+		std::cerr << argv[0] << " <input_file>" << std::endl;
+		return 1;
+	}
+
+	auto input = read_input(argv[1]);
+	input.push_back("");
+	std::cout << part1(input) << std::endl;
+	std::cout << part2(input) << std::endl;
+}