AdventOfCode2020/10/main.go

package main

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

// Find a chain that uses all of your adapters to connect the charging outlet to your device's built-in adapter
// and count the joltage differences between the charging outlet, the adapters, and your device.
// What is the number of 1-jolt differences multiplied by the number of 3-jolt differences?

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

	// wall
	adapters := []int{0}

	// load
	for scanner.Scan() {
		line := scanner.Text()
		i, _ := strconv.Atoi(line)
		adapters = append(adapters, i)
	}

	// sort
	sort.Slice(adapters, func(i, j int) bool {
		return adapters[i] < adapters[j]
	})

	// device
	adapters = append(adapters, adapters[len(adapters)-1]+3)

	one := 0
	three := 0

	// count gaps
	for i, j := range adapters {
		if i < len(adapters)-1 {
			if adapters[i+1]-j == 3 {
				three = three + 1
			} else if adapters[i+1]-j == 1 {
				one = one + 1
			}
		}
	}

	fmt.Println(three * one)
}

// You glance back down at your bag and try to remember why you brought so many adapters; there must be more than a trillion valid ways to arrange them!
// Surely, there must be an efficient way to count the arrangements.
// What is the total number of distinct ways you can arrange the adapters to connect the charging outlet to your device?

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

	// wall
	adapters := []int{0}

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

	// sort
	sort.Slice(adapters, func(i, j int) bool {
		return adapters[i] < adapters[j]
	})

	// device
	adapters = append(adapters, adapters[len(adapters)-1]+3)

	combinations := 1
	ones := 0
	three := 0

	// count gaps
	for i, j := range adapters {
		if i < len(adapters)-1 {
			if adapters[i+1]-j == 1 {
				// small steps...
				ones = ones + 1
			} else if adapters[i+1]-j == 3 {
				combinations = combinations + 1
				if ones == 2 {
					combinations = combinations * 1
				}
				if ones == 3 {
					combinations = combinations * 2
				}
				if ones == 4 {
					combinations = combinations * 4
				}
				if ones == 5 {
					combinations = combinations * 7
				}
				ones = 0
				three = three + 1
			}
		}
	}

	fmt.Println(adapters, combinations)
}