1 files changed, 90 insertions, 0 deletions
diff --git a/2020/1a.cpp b/2020/1a.cpp
new file mode 100644
index 0000000..179598b
--- /dev/null
+++ b/2020/1a.cpp
@@ -0,0 +1,90 @@
+#include <iostream>
+#include <fstream>
+#include <string>
+#include <vector>
+#include <bitset>
+#include <functional>
+#include <cstdlib>
+#include <climits>
+#include <ctime>
+#include <cassert>
+
+
+static double time_func(std::function<void()> func) {
+	const double minmeasure = 2.0;
+	const double factor = 1.5;
+
+	for (int reps = 1;
+			reps < INT_MAX / factor;
+			reps = std::max<int>(reps + 1, reps * factor)) {
+		const clock_t t_start = clock();
+		for (int k = 0; k < reps; k++) func();
+		const clock_t t_end = clock();
+		const double time_taken = (double)(t_end - t_start) / CLOCKS_PER_SEC;
+		std::cerr << reps << "x -> " << time_taken << "s" << std::endl;
+		if (time_taken >= minmeasure) return time_taken / reps;
+	}
+
+	assert(false);
+}
+
+static std::string read_input() {
+	std::ios::sync_with_stdio(false);
+
+	std::ifstream file{"1.in"};
+	file.seekg(0, std::ios::end);
+	size_t filesize = file.tellg();
+	file.seekg(0);
+
+	std::string input(filesize, '\0');
+	size_t cursor = 0;
+	while (cursor < filesize) {
+		file.read(&input[cursor], filesize - cursor);
+		cursor += file.gcount();
+	}
+
+	return input;
+}
+
+static std::pair<int, int> solve(const std::string &input) {
+	std::vector<int> numbers;
+	numbers.reserve(input.size() / 2);
+
+	std::bitset<2021> have;
+
+	char *endp = nullptr;
+	const char *cursor = input.data();
+	while (true) {
+		int val = strtol(cursor, &endp, 10);
+		if (endp == cursor) break;
+		numbers.push_back(val);
+		have[val] = true;
+		cursor = endp;
+	}
+
+	int ans1 = -1;
+	for (int n : numbers) {
+		if (2020 - n >= 0 && have[2020 - n]) {
+			ans1 = n * (2020 - n);
+			break;
+		}
+	}
+
+	for (int n : numbers) {
+		for (int m : numbers) {
+			const int k = 2020 - (n + m);
+			if (k >= 0 && have[k]) {
+				return std::make_pair(ans1, n * m * k);
+			}
+		}
+	}
+
+	assert(false);
+}
+
+int main() {
+	const std::string input = read_input();
+	auto out = solve(input);
+	std::cout << out.first << '\n' << out.second << std::endl;
+	std::cout << time_func([&input](){ solve(input); }) << std::endl;
+}