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

2020/1/data

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+1778
+1845
+1813
+1889
+1939
+1635
+1443
+796
+1799
+938
+1488
+1922
+1909
+1258
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+1815
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+1823
+1836
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+1891
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+1637
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+1583
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+1415
+1666
+1155
+1446
+1315
+1345
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+1427
+1242
+1088
+807
+1747
+1514
+1351
+1791
+1612
+1550
+1926
+1455
+85
+1594
+1965
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+1677
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+1631
+1585
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+1566
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+1968
+1927
+1378
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+1710
+1921
+1827
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+1323
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+1951
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+1786
+1111
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+1478
+1453

2020/1/main.cpp

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+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+
+int find2(std::vector<int> &numbers)
+{
+	for(auto i = 0; i < numbers.size(); ++i)
+		for(auto j = i + 1; j < numbers.size(); ++j)
+			if(numbers[i] + numbers[j] == 2020)
+				return numbers[i] * numbers[j];
+
+	return 0;
+}
+int find3(std::vector<int> &numbers)
+{
+	for(auto i = 0; i < numbers.size(); ++i)
+		for(auto j = i + 1; j < numbers.size(); ++j)
+			for(auto k = i + 1; k < numbers.size(); ++k)
+				if(numbers[i] + numbers[j] + numbers[k] == 2020)
+					return numbers[i] * numbers[j] * numbers[k];
+
+	return 0;
+}
+
+int main()
+{
+	std::ifstream ifs("data");
+	if(!ifs.is_open())
+	{
+		std::cerr << "Couldn't open file." << std::endl;
+		return -1;
+	}
+
+	std::vector<int> numbers;
+	for(std::string line; std::getline(ifs, line); )
+		numbers.push_back(std::atoi(line.c_str()));
+
+	std::cout << find2(numbers) << std::endl;
+	std::cout << find3(numbers) << std::endl;
+
+	return 0;
+}

2020/2/Makefile

@@ -0,0 +1,2 @@
+a.out: main.cpp
+	clang++ -std=c++14 main.cpp -g -O0

2020/2/main.cpp

@@ -0,0 +1,72 @@
+#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;
+}