Binary Boarding

Advent of Code 2020 Day 5

Try the simulator!

Task: Solve for X where...

Part 1:

X = the highest boarding pass ID

Part 2:

X = my boarding pass ID

Example input

FBFBBFFRLR
BFFFBBFRRR
FFFBBBFRRR
BBFFBBFRLL

It represents:

• A list of encrypted boarding pass IDs

Part 1

1. Using the title as a hint
2. Writing a working algorithm
3. Building the simulator

Using the title as a hint

• The title is Binary Boarding
• The instructions further suggest going one of two directions based on the character encountered
• The solution seems clear

Given this boarding pass

FBFBBFFRLR

Splitting FB from RL

FBFBBFF RLR

Replacing Fs and Rs with 1s, Bs and Ls with 0s

1010011 101

Parsing each one as a decimal

44       5

Generating the ID

44 * 8 + 5 = 357

Writing a working algorithm

Split the input at each new-line character into an array of strings

For each string
  Change the string according to the following instructions:
    Split the string into groups containing exclusively FBs and RLs
    Replace FBs with 1 and RLs with 0
    Parse the modified string as a decimal
    Calculate the sum of:
      The product of the first decimal and 8
      And the second decimal
  Return the sum

Return the highest number in the modified list of decimals

Here's a visualization of my algorithm for Part 1

Building the simulator

• Since these are boarding pass IDs, I wanted to simulate the population of a plane's seating chart
• By the end of running Part 1, there should be one seat left unfilled: that's my seat!

Part 2

1. Writing a slightly modified, working algorithm
2. Finishing the simulator

Writing a slightly modified, working algorithm

• I still need to generate the list of boarding pass IDs
• But now I need to sort them in ascending order
• And I need to find the only gap: where the number increments by 2 instead of 1

Split the input at each new-line character into an array of strings

For each string
  Change the string according to the following instructions:
    Split the string into groups containing exclusively FBs and RLs
    Replace FBs with 1 and RLs with 0
    Parse the modified string as a decimal
    Calculate the sum of:
      The product of the first decimal and 8
      And the second decimal
  Return the sum

Sort the list of decimals in ascending order

Filter the list to exclude all but one number:
  Find the number where the number at the very next location is not equal to this number plus one

Since the number found is the seat next to mine
  Return that number plus one

Here's a visualization of my algorithm for Part 2

Finishing the simulator

As foreshadowed above, I wanted Part 2 to reveal the seat that in Part 1 was the only gap

Try the simulator!

This puzzle felt easy.

That made me feel great about my progress.

Bring on the next one!