Source: Day 25: Code Chronicle

Full solution for today (spoilers!).

Part 1

You are given a series of locks and keys (see below). How many unique (lock, key) are there that do not overlap (they do not have to fit perfectly).

A lock starts from the top. The entire top row is # and the entire bottom row is ..

#####
.####
.####
.####
.#.#.
.#...
.....

A key is the opposite:

.....
.....
.....
#....
#.#..
#.#.#
#####

read more...

Source: Day 24: Crossed Wires

Full solution for today (spoilers!).

Part 1

Given a collection of gates of the form arg0 (AND|OR|XOR) arg2 -> out and input values of form x** and y**, what is the value of z** interpreted as a binary number?

read more...

Source: Day 23: LAN Party

Full solution for today (spoilers!).

Part 1

You are given the edges of an undirected graph. Count how many complete subgraphs of size three exist that contain one or more starting with the letter t.

Aside: Games with local (but not hotseat) multiplayer have gotten rather rarer over the years… how many people still know what a LAN party is/was?

read more...

Source: Day 22: Monkey Market

Full solution for today (spoilers!).

Part 1

Implement a PRNG with the following update function:

1. Multiply by 64, xor with the previous value, modulo 16777216
2. Divide by 32, xor with the previous value (from step 1), modulo 16777216
3. Multiply by 2048, xor with the previous value (from step 2), module 16777216

For each of a series of seeds, sum the 2000th generated number.

read more...

Source: Day 21: Keypad Conundrum

Full solution for today (spoilers!).

Part 1

You are trying to type a code on a keypad:

+---+---+---+
| 7 | 8 | 9 |
+---+---+---+
| 4 | 5 | 6 |
+---+---+---+
| 1 | 2 | 3 |
+---+---+---+
    | 0 | A |
    +---+---+

But you cannot type directly. Instead, you can control a pointer on the keypad with arrow keys:

    +---+---+
    | ^ | A |
+---+---+---+
| < | v | > |
+---+---+---+

Whenever you type a ^ on the arrow keys, the pointer on the keypad will move up one, etc. When you type A, then the pointer on the keypad will type whatever it is pointing at.

But that’s not enough either. Add a second keypad. And then a third, that is the one you are actually controlling.

For each output sequence multiple the length of the minimum input sequence needed to generate it by the numeric value of the input sequence (ignoring any A); sum these.

Note: Moving off any keypad or into the blank spaces is an error.

read more...

Source: Day 20: Race Condition

Full solution for today (spoilers!).

Part 1

Given a maze with exactly one path, find how many single walls you can walk through (remove) that shorten the best path by at least 100 units.

read more...

Source: Day 19: Linen Layout

Full solution for today (spoilers!).

Part 1

Given a comma delimited list of substrings and a list of strings, count how many of the latter strings can be made up of any (repeating) combination of the former.

read more...

Source: Day 18: RAM Run

Full solution for today (spoilers!).

Part 1

You are given a series of points on a 71x71 grid. Taking only the first 1024 points, how long is the shortest path from (0, 0) to (70, 70)?

read more...

Source: Day 17: Chronospatial Computer

Full solution for today (spoilers!).

Part 1

Implement a virtual machine. The machine will have 3 unbounded signed registers, 8 opcodes (see below), a variable parameter scheme (see below that). You will be given the initial values of the 3 registers and a program. Find the final output.

Instructions

Parameter specification

For instructions that can take reg/val, 0 to 3 (inclusive) are treated as literal values, 4 is register A, 5 is B, 6, is C, and 7 is an error (should never happen).

For instructions that only take val, it’s always a literal value in the range 0 to 7 (inclusive).

read more...

Source: Day 16: Reindeer Maze

Full solution for today (spoilers!).

Part 1

Given a maze, what is the shortest path between S and E where walking straight costs one and turning costs 1000.

read more...