AdventOfCode2020/01/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func main() {
	partOne()
	partTwo()
}

// [...] they need you to find the two entries that sum to 2020 and then multiply those two numbers together.
// For example, suppose your expense report contained the following:
// 1721
// 979
// 366
// 299
// 675
// 1456
// In this list, the two entries that sum to 2020 are 1721 and 299.
// Multiplying them together produces 1721 * 299 = 514579, so the correct answer is 514579.

func partOne() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	numbers := []int{}

	for scanner.Scan() {
		i, _ := strconv.Atoi(scanner.Text())
		numbers = append(numbers, i)
	}

	solution := make(chan int, 1)
	for i := range numbers {
		go func(a int) {
			if r := checkTwoSums(numbers[a], numbers[a+1:]); r != 0 {
				solution <- r
			}
		}(i)
	}
	fmt.Println(<-solution)
}

func checkTwoSums(i int, rest []int) int {
	for _, j := range rest {
		if i+j == 2020 {
			return i * j
		}
	}
	return 0
}

// Can find three numbers in your expense report that meet the same criteria?
// Using the above example again, the three entries that sum to 2020 are 979, 366, and 675.
// Multiplying them together produces the answer, 241861950.

func partTwo() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	numbers := []int{}

	for scanner.Scan() {
		i, _ := strconv.Atoi(scanner.Text())
		numbers = append(numbers, i)
	}

	solution := make(chan int, 1)

	for i := range numbers {
		go func(a int) {
			for j := range numbers[a+1:] {
				if r := checkThreeSums(numbers[a], numbers[j], numbers[j+1:]); r != 0 {
					solution <- r
				}
			}
		}(i)
	}
	fmt.Println(<-solution)
}

func checkThreeSums(i int, j int, rest []int) int {
	for _, k := range rest {
		if i+j+k == 2020 {
			return i * j * k
		}
	}
	return 0
}