adventofcode2023/day08.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"unicode"
)

type nextlocation struct {
	left  string
	right string
}

func readsteps(b *bufio.Reader) []byte {
	steps, err := b.ReadBytes('\n')
	must(err)

	return bytes.TrimFunc(steps, unicode.IsSpace)
}

type day08 struct{}

func (d day08) solve1(r io.Reader) string {
	iterations := 0
	current := "AAA"
	buf := bufio.NewReader(r)

	steps := readsteps(buf)
	_, err := buf.ReadBytes('\n')
	must(err)

	m := make(map[string]nextlocation)
	for next := true; next; {
		line, err := buf.ReadBytes('(')
		must(err)
		location := string(line[:3])

		left, err := buf.ReadBytes(',')
		must(err)

		remaining, err := buf.ReadBytes('\n')
		next = !reachedeof(err)

		nextlocation := nextlocation{
			left:  string(bytes.Trim(left, ",")),
			right: string(remaining[1:4]),
		}
		m[location] = nextlocation
	}

	for ; current != "ZZZ"; iterations++ {
		step := steps[iterations%len(steps)]

		if step == 'R' {
			current = m[current].right
		} else {
			current = m[current].left
		}
	}

	return fmt.Sprintf("%d", iterations)
}

func (d day08) solve2(r io.Reader) string {
	return "incomplete"
}