advent_of_code/2020/days/day9.cpp

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 * Copyright (c) 2020 Gnarwhal
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#include <vector>
#include <string>
#include <iostream>
#include <fstream>

#include "../misc/types.hpp"
#include "../misc/print.hpp"

auto find_sum(usize index, std::vector<u64> xmas) {
	for (auto i = index - 25; i < index - 1; ++i) {
		for (auto j = i + 1; j < index; ++j) {
			if (xmas[i] + xmas[j] == xmas[index]) {
				return true;
			}
		}
	}
	return false;
}

auto day9() -> void {
	auto xmas = std::vector<u64>();
	auto line = std::string();
	auto file = std::ifstream("inputs/day9.input");
	while (getline(file, line)) {
		xmas.push_back(std::stoll(line));
	}

	auto invalid = u64(0);
	for (auto i = usize(25); i < xmas.size(); ++i) {
		if (!find_sum(i, xmas)) {
			print((invalid = xmas[i]));
			break;
		}
	}
	
	auto head_index = 0;
	auto tail_index = 1;
	auto sum        = xmas[0] + xmas[1];
	while (sum != invalid) {
		if (sum < invalid) {
			sum += xmas[++tail_index];
		} else if (head_index + 1 < tail_index) {
			sum -= xmas[head_index];
			++head_index;
		} else {
			head_index = tail_index + 1;
			tail_index = head_index + 1;
			sum        = xmas[head_index] + xmas[tail_index];
		}
	}
	auto min = xmas[tail_index];
	auto max = xmas[tail_index];
	for (auto i = head_index; i < tail_index; ++i) {
		if (xmas[i] < min) {
			min = xmas[i];
		} else if (xmas[i] > max) {
			max = xmas[i];
		}
	}
	print((min + max));
}