main.go

package main

import (
	input "aoc2020/inpututils"

	"errors"
	"fmt"
)

func main() {
	fmt.Println("--- Part One ---")
	fmt.Println(Part1("input.txt"))

	fmt.Println("--- Part Two ---")
	fmt.Println(Part2("input.txt"))
}

// create set type as Go doesn't have one
type set map[int]struct{}

func (s set) add(n int) {
	s[n] = struct{}{}
}

func (s set) has(n int) bool {
	_, ok := s[n]
	return ok
}

func Part1(filename string) int {
	nums := input.ReadNumbers(filename)

	product, err := twoSum(nums, 2020)
	if err != nil {
		return -1
	}
	return product
}

func Part2(filename string) int {
	nums := input.ReadNumbers(filename)

	for i := 0; i < len(nums)-2; i += 1 {
		num := nums[i]
		product, err := twoSum(nums[i+1:], 2020-num)
		if err == nil {
			return product * num
		}
	}
	return -1
}

func twoSum(ints []int, target int) (int, error) {
	seen := set{}
	for _, i := range ints {
		if seen.has(target - i) {
			return (target - i) * i, nil
		}
		seen.add(i)
	}
	return -1, errors.New("No solution")
}