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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Source: A Maze of Twisty Little Cubicles¹

Part 1: Generate a procedurally generated maze using the following equation:

• x^2 + 3x + 2xy + y + y^2 + c

x and y are the coordinates of a point and c is a constant.

Count the number of bits for each point. Even is open spaces, odd is walls.

What is the shortest route from (0, 0) to (31, 39)?

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Source: Leonardo’s Monorail

Part 1: Create a virtual machine that has four registers (a, b, c, and d) and can process the following instructions:

• cpy x y - copies x into y (x can be an integer or a register)
• inc x - increases register x by one
• dec x - decreases register x by one
• jnz x y - jumps over y instructions if x is not zero (x can be an integer or a register)

What is the final value in register a?

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Source: Radioisotope Thermoelectric Generators

Part 1: Input will be a list of the following form:

• The first floor contains a hydrogen-compatible microchip and a lithium-compatible microchip.
• The second floor contains a hydrogen generator.
• The third floor contains a lithium generator.
• The fourth floor contains nothing relevant.

You have an elevator that can move exactly 1 or 2 items. You can only leave a microchip on a floor with a non-matching generator if a matching generator is also present.

Move all items to the top (4th) floor.

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Source: Balance Bots

Part 1: Create a sorting machine using input of the following form:

• value X goes to bot A - an input to bot A
• bot A gives low to (bot|output) B and high to (bot|output) C - a sorter that takes two inputs and sends them to the specified bots or output channels

Find the bot that compares the values 17 and 61.

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Source: Explosives in Cyberspace

Part 1: A file is compressed by including compression markers of the form (#x#)... where the first number tells how many characters to copy and the second is how many times to copy them. So A(2x3)HA! becomes AHAHAHA!.

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Source: Two-Factor Authentication

Part 1: Implement a 50x6 pixel screen with the following commands:

• rect AxB turn on a rectangle of pixels in the top left corner
• rotate row y=A by B rotates row A right by B pixels
• rotate column x=A by B rotates column A down by B pixels

After a given sequence of commands, how many pixels are on?

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Source: Internet Protocol Version 7

Part 1: The input is a list of strings, potentially containing sequences in square brackets. Find all strings that have an ABBA sequence (two characters followed by the same two in reverse order) outside of any square brackets, but no ABBA sequences in square brackets.

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Source: Signals and Noise

Part 1: Given a list of strings, find the most common character in each position.

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