main.go

package main

import (
	"encoding/csv"
	"fmt"
	"io"
	"os"
	"path"
)

func solve(r io.Reader) {
	csvReader := csv.NewReader(r)

	rows, err := csvReader.ReadAll()
	if err != nil {
		panic(err)
	}

	max := 0
	for _, row := range rows {
		if getId(row[0]) > max {
			max = getId(row[0])
		}
	}

	fmt.Println(max)
}

func getId(pass string) int {
	min, max := 0, 128

	for i := 0; i < 7; i++ {
		if pass[i] == 'F' {
			max -= (max - min) / 2
		} else {
			min += (max - min) / 2
		}
	}

	row := min

	colMin, colMax := 0, 8
	for i := 0; i < 3; i++ {
		if pass[7+i] == 'L' {
			colMax -= (colMax - colMin) / 2
		} else {
			colMin += (colMax - colMin) / 2
		}
	}

	col := colMin

	return row*8 + col
}

func main() {
	input, err := os.Open(path.Join("2020", "5", "input.txt"))
	if err != nil {
		panic(err)
	}

	solve(input)

	fmt.Println(getId("BFFFBBFRRR"))
	fmt.Println(getId("FFFBBBFRRR"))
	fmt.Println(getId("BBFFBBFRLL"))
}