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?

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Source: Grid Computing

Part 1: You are given a grid of nodes and the output of the df command telling you how much space is on each. How many pairs of nodes are there where the data from some node A would fit entirely on B?

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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

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Source: Firewall Rules

Part 1: Given a list of integer ranges (a la 5-8), what is the first value not in any given range?

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Source: An Elephant Named Joseph

Part 1: Create a circular list of the numbers 1 through n. Going around the list, each currently remaining number removes the number after it. What is the last remaining number?

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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?

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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).

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Source: Dragon Checksum

Part 1: Generate noise using a modified dragon curve:

• Start with data a
• Create a copy of the data b, reverse and invert it (0 <-> 1)
• Create the string a0b

Repeat until you have enough data, truncate at the end if needed.

From this string calculate a checksum as follows:

• xor each pair of bits, concatenate the results
• If the resulting string has an even length, repeat; if it’s odd, stop

Calculate the checksum of a given initial state expanded to 272 bits.

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Source: Timing is Everything

Part 1: Given a series of openings one second apart, each with n positions that advance one position per second, what is the first time you can start the simulation so that you pass each in position 0.

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Source: One-Time Pad

Part 1: Calculate a series of MD5 hashes (the same as Day 5). A hash is considered valid if it contains a triple (three characters in a row) and somewhere in the next 1000 hashes there is a quintuple of that same character.

What index produces the 64th key?

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