aoc2023/day8/day8.go

package day8

import (
	"math"
	"regexp"

	"gitea.paas.celticinfo.fr/oabrivard/aoc2023/utils"
)

type Step struct {
	src   string
	left  string
	right string
}

func CountSteps(lines []string) int {
	result := 0
	steps := map[string]Step{}
	directions := lines[0]

	//matches occurrences like RFK = (DBD, FDM)
	var re = regexp.MustCompile(`([A-Z]{3}?)\s*=\s*\(([A-Z]{3}?)\s*,\s*([A-Z]{3}?)\)`)

	for i := 2; i < len(lines); i++ {
		matches := re.FindStringSubmatch(lines[i])

		src := matches[1]
		left := matches[2]
		right := matches[3]

		steps[src] = Step{src, left, right}
	}

	loops := math.MaxInt32
	step := steps["AAA"]

	for {
		for _, d := range directions {
			result++

			if d == 'L' {
				step = steps[step.left]
			} else {
				step = steps[step.right]
			}

			if step.src == "ZZZ" {
				return result
			}
		}

		loops--
		if loops == 0 {
			break
		}
	}

	return 0
}

func CountParallelSteps(lines []string) int {
	steps := map[string]Step{}
	threads := []Step{}
	directions := lines[0]

	//matches occurrences like RFK = (DBD, FDM)
	var re = regexp.MustCompile(`([A-Z0-9]{3}?)\s*=\s*\(([A-Z0-9]{3}?)\s*,\s*([A-Z0-9]{3}?)\)`)

	for i := 2; i < len(lines); i++ {
		matches := re.FindStringSubmatch(lines[i])

		src := matches[1]
		left := matches[2]
		right := matches[3]

		step := Step{src, left, right}
		steps[src] = step
		if src[2] == 'A' {
			threads = append(threads, step)
		}
	}

	loops := math.MaxInt32
	reachedZ := 0
	countZ := make([]int, len(threads))
	results := make([]int, len(threads))

	for {
		for _, d := range directions {

			for i := range threads {
				countZ[i]++

				if d == 'L' {
					threads[i] = steps[threads[i].left]
				} else {
					threads[i] = steps[threads[i].right]
				}

				if threads[i].src[2] == 'Z' && results[i] == 0 {
					reachedZ++
					results[i] = countZ[i]
				}
			}

			if reachedZ == len(threads) {
				return utils.LCM(results[0], results[1], results...)
			}
		}

		loops--
		if loops == 0 {
			break
		}
	}

	return 0
}