Advent of Code 2016

As I did last year, I’m going to solve the Advent of Code problems again this year.

Or that was the plan. It turns out that instead I put down my blog for almost a year and a half and never quite got around to doing these problems. So I’m actually backdating these posts from the early days of 2018 to where they would have been had I solved them on time. They’re still interesting problems, so give them a read.


AoC 2016 Day 23: Assembunny2

Source: Safe Cracking

Part 1: Take the assembunny interpreter from day 12 and add an instruction (tgl X) that modifies the code at an offset of X instructions.

  • inc becomes dec; any other one argument instruction (including tgl) becomes inc
  • jnz becomes cpy; any other two argument instructions become jnz
  • Toggling an instruction outside of the program does nothing (it does not halt execution)
  • If toggling produces an invalid instruction, ignore it

Run the given program with the initial register of a = 7. What is the final value in register a?


AoC 2016 Day 21: Scrambler

Source: Scrambled Letters and Hash

Part 1: Another virtual machine, of sorts. Start with the string abcdefgh and apply a sequence of the following commands to it:

  • swap position X with position Y = swap two positions
  • swap letter X with letter Y = swap to letters, no matter where they are
  • rotate (left|right) X steps = rotate forward or backward
  • rotate based on position of letter X = find X, rotate right based on its position; if the original position was >= 4, rotate one more1
  • reverse positions X through Y = reverse a subset of the string
  • move position X to position Y = take a character at a position out of the string and put it somewhere else specific


AoC 2016 Day 18: Its A Trap

Source: Like a Rogue

Part 1: Starting with a sequence of . and ^, generate additional rows using the rules based on the three characters above the new position.

  • ^^. -> ^
  • .^^ -> ^
  • ^.. -> ^
  • ..^ -> ^
  • Otherwise -> .

How many safe tiles (.) are there after 40 generations?


AoC 2016 Day 17: Md5 Maze

Source: Two Steps Forward

Part 1: Create a 4x4 grid of rooms with doors Up, Down, Left, and Right from each location. To determine if a door is currently open:

  • Calculate MD5(salt + sequence) where sequence is a string containing any combination of UDLR depending on how you got to this room
  • The first four hex values represent the doors Up, Down, Left, and Right respectively: bcdef means open; anything else is closed

Find the shortest path from (0, 0) to (3, 3).