Added 2025 day 1.

2025/1/Makefile

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+a.out: main.cpp
+	clang++ -std=c++20 main.cpp

2025/1/data_test.txt

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+L68
+L30
+R48
+L5
+R60
+L55
+L1
+L99
+R14
+L82

2025/1/main.cpp

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+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+
+int main()
+{
+	std::vector<std::string> input;
+
+	std::ifstream ifs("data.txt");
+	if(!ifs.is_open())
+	{
+		std::cerr << "Missing data.txt." << std::endl;
+		return -1;
+	}
+
+	std::string line;
+	while(!ifs.eof())
+	{
+		ifs >> line;
+		if(ifs.eof())
+			break;
+
+		input.push_back(line);
+	}
+
+	int answer = 0;
+	int num = 50;
+	for(const std::string &line : input)
+	{
+		bool to_right = ('R' == line[0]);
+		int amount = std::atoi(line.substr(1).c_str());
+
+		if(!to_right)
+			amount *= -1;
+
+		num += amount;
+		num %= 100;
+		if(num == 0)
+			++answer;
+	}
+
+	std::cout << "Part 1: " << answer << std::endl;
+
+	answer = 0;
+	num = 50;
+	for(const std::string &line : input)
+	{
+		bool to_right = ('R' == line[0]);
+		int amount = std::atoi(line.substr(1).c_str());
+
+		// slow, but accurate. :)
+		for(auto i = 0; i < amount; ++i)
+		{
+			num += (to_right ? 1 : -1);
+			num %= 100;
+			if(num < 0)
+				num += 100;
+			if(num == 0)
+				++answer;
+		}
+	}
+
+	std::cout << "Part 2: " << answer << std::endl;
+
+	return 0;
+}