AdventOfCode2020/21/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
	"time"
)

func main() {
	start := time.Now()
	partOne()
	duration := time.Since(start)
	partTwo()
	duration2 := time.Since(start)
	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
}

type food struct {
	allergens   []string
	ingredients []string
}

func intersection(a, b []string) []string {
	// special-case starting out
	if len(a) == 0 {
		return b
	} else if len(b) == 0 {
		return a
	}
	result := []string{}
	for _, s := range b {
		for _, t := range a {
			if s == t {
				result = append(result, s)
			}
		}
	}
	return result
}

func countSafeIngredients(f food, allergens map[string][]string) int {
	for c, i := range f.ingredients {
		for _, l := range allergens {
			for _, a := range l {
				if i == a {
					f.ingredients[c] = ""
				}
			}
		}
	}
	result := 0
	for _, i := range f.ingredients {
		if i != "" {
			result = result + 1
		}
	}
	return result
}

func remove(ingredients []string, ingredient string) []string {
	for i, j := range ingredients {
		if j == ingredient {
			ingredients[i] = ingredients[len(ingredients)-1]
			ingredients[len(ingredients)-1] = ""
			return ingredients[:len(ingredients)-1]
		}
	}
	return ingredients
}

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

	foods := []food{}
	allergens := map[string][]string{}

	for scanner.Scan() {
		line := strings.Split(scanner.Text(), " (contains ")
		f := food{
			ingredients: strings.Split(line[0], " "),
			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
		}
		foods = append(foods, f)
		for _, a := range f.allergens {
			allergens[a] = intersection(f.ingredients, allergens[a])
		}
	}

	sum := 0
	for _, f := range foods {
		sum = sum + countSafeIngredients(f, allergens)
	}
	fmt.Println(sum)
}

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

	foods := []food{}
	allergens := map[string][]string{}

	for scanner.Scan() {
		line := strings.Split(scanner.Text(), " (contains ")
		f := food{
			ingredients: strings.Split(line[0], " "),
			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
		}
		foods = append(foods, f)
		for _, a := range f.allergens {
			allergens[a] = intersection(f.ingredients, allergens[a])
		}
	}

	// hey, this is the same reduction logic as day 16!
	trueAllergens := map[string]string{}
	var lastAllergenSolved string
	for i := 0; i < len(allergens); i = i + 1 {
		for i, a := range allergens {
			if len(a) == 1 {
				trueAllergens[i] = a[0]
				lastAllergenSolved = a[0]
			}
		}
		for i := range allergens {
			allergens[i] = remove(allergens[i], lastAllergenSolved)
		}
	}

	// formatting the result correctly is left as an exercise for the reader
	// because I don't want to deal with go's map order randomization
	fmt.Println(trueAllergens)
}