Add day 3 implementation

day3/day3.cpp

@@ -0,0 +1,55 @@
+#include <fstream>
+#include <iostream>
+#include <vector>
+
+constexpr char TREE_CHAR = '#';
+
+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;
+}
+
+/**
+ * Find the number of trees that move along a path
+ * @param input The input for the puzzle
+ * @param xDelta The number of tiles to move right each step
+ * @param yDelta The number of tiles to move down each step
+ * @return int The number of trees encountered along this path
+ */
+int findNumTrees(const std::vector<std::string> &input, int xDelta, int yDelta = 1) {
+	int xCursor = 0;
+	int numTrees = 0;
+	for (int i = 0; i < input.size(); i += yDelta) {
+		const std::string &row = input.at(i);
+		numTrees += (row.at(xCursor) == TREE_CHAR);
+		xCursor = (xCursor + xDelta) % row.length();
+	}
+
+	return numTrees;
+}
+
+int part1(const std::vector<std::string> &input) {
+	return findNumTrees(input, 3);
+}
+
+long part2(const std::vector<std::string> &input) {
+	return 1L * findNumTrees(input, 1, 1) * findNumTrees(input, 3, 1) * findNumTrees(input, 5, 1) *
+		   findNumTrees(input, 7, 1) * findNumTrees(input, 1, 2);
+}
+
+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]);
+	std::cout << part1(input) << std::endl;
+	std::cout << part2(input) << std::endl;
+}