AoC 2018 Day 14: Functionally Circular Elfs

Source: Chocolate Charts

Part 1: Create an infinite stream of numbers, by starting with [3, 7] with two pointers: 0 and 1. To add values to the list:

  • Add the current values of the two pointers
    • If the value is less than ten, add that value to the end of the list
    • If the value is greater or equal to ten, add 1 and then the ones digits to the end of the list
  • Update each pointer by adding the value it is pointing at to its current index plus one

With that algorithm, find the ten digits after a given index.


AoC 2018 Day 13: Mine Cart Madness

Source: Mine Cart Madness

Part 1: Load a minecart track that looks like this:

/->-\        
|   |  /----\
| /-+--+-\  |
| | |  | v  |
\-+-/  \-+--/
  \------/

Assuming minecarts follow the tracks and alternate turning left, going straight, and turning right on each intersection (+), where does the first collision occur?

NOTE: Update carts top to bottom, left to right. Carts can collide mid update.


AoC 2018 Day 11: Gridlocked Fuel

Source: Chronal Charge

Part 1: Define a grid as follows (x,y coordinates + a constant C):

  • r(x) = x + 10
  • G(x, y) = hundreds(r(x) * (r(x) * y + C)) - 5

Find the 3x3 area in a 300x300 grid with the highest total G(x, y) .


AoC 2018 Day 9: Marble Madness

Source: Marble Mania

Part 1: Place marbles in a circle such that each marble is placed by skipping one place except for marbles divisible by 23. For those, don’t place them, skip back 7 places, and remove that marble as well. Add these two marbles to your current score.

Given a specific player count and last marble, what’s the highest score?