AdventOfCode2020/05/main.go

package main

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

type boardingPass struct {
	passID string
	row    int
	seat   int
	id     int
}

func parsePassID(s string) (int, int) {
	bottom := 0
	top := 127
	left := 0
	right := 7

	for _, r := range s {
		switch {
		case r == 'F':
			top = ((top - bottom - 1) / 2) + bottom
		case r == 'B':
			bottom = (top + bottom + 1) / 2
		case r == 'L':
			right = ((right - left - 1) / 2) + left
		case r == 'R':
			left = (left + right + 1) / 2
		}
	}
	return top, left
}

// Instead of zones or groups, this airline uses binary space partitioning to seat people.
// A seat might be specified like FBFBBFFRLR, where F means "front", B means "back", L means "left", and R means "right".
// Every seat also has a unique seat ID: multiply the row by 8, then add the column.
// Here are some other boarding passes:
// BFFFBBFRRR: row 70, column 7, seat ID 567.
// FFFBBBFRRR: row 14, column 7, seat ID 119.
// BBFFBBFRLL: row 102, column 4, seat ID 820.
// As a sanity check, look through your list of boarding passes. What is the highest seat ID on a boarding pass?

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

	maxID := 0
	for scanner.Scan() {
		line := scanner.Text()
		row, column := parsePassID(line)
		if (row*8)+column > maxID {
			maxID = (row * 8) + column
		}
	}

	fmt.Println(maxID)
}

// It's a completely full flight, so your seat should be the only missing boarding pass in your list.
// However, there's a catch: some of the seats at the very front and back of the plane don't exist on this aircraft, so they'll be missing from your list as well.
// Your seat wasn't at the very front or back, though; the seats with IDs +1 and -1 from yours will be in your list.

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

	passes := []boardingPass{}

	for scanner.Scan() {
		line := scanner.Text()
		row, column := parsePassID(line)
		passes = append(passes, boardingPass{
			passID: line,
			row:    row,
			seat:   column,
			id:     (row * 8) + column,
		})
	}

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

	for i, s := range passes {
		if i != 0 && passes[i-1].id != s.id-1 {
			fmt.Println(s.id - 1)
		}
	}
}