Source: Spinlock¹

Part 1: Start with a circular buffer containing [0] and current_position = 0. For n from 1 up to 2017:

1. Step forward steps (puzzle input)
2. Input the next value for n, set current_position to n, increment n
3. Repeat

What is the value after 2017?

It’s a bit weird to describe, but the given example helps (assume steps = 3):

(0)
0 (1)
0 (2) 1
0  2 (3) 1
0  2 (4) 3  1
0 (5) 2  4  3  1
0  5  2  4  3 (6) 1
0  5 (7) 2  4  3  6  1
0  5  7  2  4  3 (8) 6  1
0 (9) 5  7  2  4  3  8  6  1

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Source: Permutation Promenade

Part 1: Running on the string a...p apply a series of the following commands:

• sX rotates the string right by X positions
• xX/Y swaps positions X and Y
• pA/B swaps the letters A and B no matter their positions

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Source: Dueling Generators

Part 1: Create a pair of generators A and B where:

• A_n = 16807 A_{n-1} \mod 2147483647
• B_n = 48271 B_{n-1} \mod 2147483647

How many of the first 40 million values have matching values for the low 16 bits of each generator?

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Source: Disk Defragmentation

Part 1: Create a 128x128 grid. Generate each row by taking the knot hash of salt-{index}. The bits of the hash represent if a tile in the grid is free (0) or used (1).

Given your salt as input, how many squares are used?

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Source: Packet Scanners

Part 1: Multiple layers are defined with rules of the form:

• {index}: {depth}

Each layer will start at position 0, then once per tick will advance towards depth. Once it hits depth-1, it will return to position 0, taking 2*depth-1 per full cycle.

Calculate the sum of index * depth for any scanners that are at position 0 when you pass through them given an initial starting time.

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Source: Digital Plumber

Part 1: A network of nodes is defined by a list of lines formatted as such:

2 <-> 0, 3, 4

In this case, node 2 is connected to 0, 3, and 4 and vice versa.

How many nodes are in the group that contains the node 0?

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Source: Hex Ed¹

Part 1: Work on a hex grid:

  \ n  /
nw +--+ ne
  /    \
-+      +-
  \    /
sw +--+ se
  / s  \

Given a series of steps (n, se, ne) etc, how many steps away from the origin do you end up?

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Source: Knot Hash

Part 1: Starting with a list of the numbers from 1 to n and a list of lengths (as input):

1. Initialize current_position and skip_size to 0
2. For each length element in the lengths list:
3. Reverse the first length elements of the list (starting at current_position)
4. Move forward by length plus skip_size
5. Increment skip_size by 1

After applying the above algorithm, what is the product of the first two elements in the list (from the original first position, not the current_position)?

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Source: Stream Processing

Part 1: An input stream can contain:

• groups are delimited by { and }, groups are nestable and may contain garbage or data (objects within a group are comma delimited)
• garbage is delimited by < and >, groups cannot be nested within garbage, a ! within garbage is an escape character: !> does not end a garbage segment

The score of a single group is equal to how many times it is nested (the innermost group of {{{}}} has score 3).

The score of a stream is the sum of the scores of all groups in that stream.

What is the total score of your input?

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Source: I Heard You Like Registers¹

Part 1: Given a set of registers initialized to 0, interpret a series of instruction of the form:

• {register} (inc|dec) {number|register} if {number|register} (<|<=|=|!=|=>|>) {number|register}

What is the largest value in any register?

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