adventoc/2024/day05.go

package adventoc2024

import (
	"bufio"
	"io"
	"slices"
	"sort"
	"strings"
)

type Day5 struct{}

func (d Day5) Part1(r io.Reader) string {
	s := bufio.NewScanner(r)
	before := make(map[int][]int, 100)
	var sum int

	var updates bool
Scan:
	for s.Scan() {
		updates = updates || s.Text() == ""

		if !updates {
			s1, s2, _ := strings.Cut(s.Text(), "|")
			i, j := atoi(s1), atoi(s2)
			before[j] = append(before[j], i)
		} else if s.Text() != "" {
			pages := strings.Split(s.Text(), ",")
			var v1, v2 int
			for i := range pages {
				v1 = atoi(pages[i])

				for j := range pages[i+1:] {
					v2 = atoi(pages[i+j+1])
					for _, mustbefore := range before[v1] {
						if mustbefore == v2 {
							continue Scan
						}
					}
				}
			}
			sum += atoi(pages[(len(pages)-1)/2])
		}
	}

	return printi(sum)
}

func (d Day5) Part2(r io.Reader) string {
	s := bufio.NewScanner(r)
	before := make(map[int][]int, 100)
	var sum int

	var updates bool
	for s.Scan() {
		updates = updates || s.Text() == ""

		if !updates {
			s1, s2, _ := strings.Cut(s.Text(), "|")
			i, j := atoi(s1), atoi(s2)
			before[j] = append(before[j], i)
		} else if s.Text() != "" {
			pages := strings.Split(s.Text(), ",")
			pageNums := make([]int, len(pages))
			pageCopy := make([]int, len(pages))

			for i := range pages {
				pageNums[i], pageCopy[i] = atoi(pages[i]), atoi(pages[i])
			}

			sortPages(pageNums, before)
			if !slices.Equal(pageNums, pageCopy) {
				sum += middlePage(pageNums)
			}
		}
	}

	return printi(sum)
}

func sortPages(update []int, before map[int][]int) {
	sort.Slice(update, func(i, j int) bool {
		v1 := update[i]
		v2 := update[j]

		search := before[v1]
		for k := range search {
			if search[k] == v2 {
				return false
			}
		}

		return true
	})
}

func middlePage(pages []int) int {
	return pages[(len(pages)-1)/2]
}