Added part one of day four, plus a few 2020's from my laptop.

2020/2/Makefile

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+a.out: main.cpp
+	clang++ -std=c++14 main.cpp -g -O0

2020/2/main.cpp

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+#include <algorithm>
+#include <fstream>
+#include <iostream>
+#include <string>
+
+// 6-7 w: wwhmzwtwwk
+bool Valid1(const std::string &str)
+{
+	int min, max;
+	char letter;
+	std::string password;
+
+	auto dash = str.find('-');
+	auto space = str.find(' ');
+	auto space_two = str.find(' ', space + 1);
+
+	min = std::atoi(str.substr(0, dash).c_str());
+	max = std::atoi(str.substr(dash + 1, space).c_str());
+	letter = str.at(space + 1);
+	password = str.substr(space_two + 1);
+
+	// std::cout << "min:    " << min << std::endl
+	//           << "max:    " << max << std::endl
+	//           << "letter: " << letter << std::endl
+	//           << "pass:   " << password << std::endl;
+
+	int total = std::count(password.begin(), password.end(), letter);
+
+	return (total >= min && total <= max);
+}
+
+bool Valid2(const std::string &str)
+{
+	int min, max;
+	char letter;
+	std::string password;
+
+	auto dash = str.find('-');
+	auto space = str.find(' ');
+	auto space_two = str.find(' ', space + 1);
+
+	min = std::atoi(str.substr(0, dash).c_str());
+	max = std::atoi(str.substr(dash + 1, space).c_str());
+	letter = str.at(space + 1);
+	password = str.substr(space_two + 1);
+
+	bool a = password[min - 1] == letter;
+	bool b = password[max - 1] == letter;
+
+	return a != b;
+}
+
+int main()
+{
+	std::ifstream ifs("data");
+	if(!ifs.is_open())
+		return -1;
+
+	int valid1 = 0;
+	int valid2 = 0;
+	for(std::string line; std::getline(ifs, line); )
+	{
+		if(Valid1(line))
+			++valid1;
+		if(Valid2(line))
+			++valid2;
+	}
+
+	std::cout << valid1 << " valid1 passwords." << std::endl;
+	std::cout << valid2 << " valid2 passwords." << std::endl;
+	return 0;
+}