Advent of Code 2021

Solutions for 2021

Day 1 ⭐ ⭐

How many numbers are bigger than the previous number?
How many sums of three numbers are bigger than the previous three?

diff function is very nice
diff function has a lag argument :)

Day 2 ⭐ ⭐

Follow the forward, down, up instructions. Where do you end up?
Actually, the instructions are different. Where do you end up?

Use a tibble rather than a loop, and sum the columns, because it’s linear
Similar, but a couple of extra steps, because it’s cumulative

Day 3 ⭐ ⭐

Find the ‘gamma’ and ‘epsilon’, calculating the most common bits in the columns.
Finding the ‘oxygen’ and ‘carbon dioxide’ is a bit more difficult.

strtoi is the perfect function for this.
Determine each column sequentially, so, recursive function with colnum as input.

Day 4 ⭐ ⭐

Which bingo card will win first?
We should let the giant squid win, which card will win last?

Lots of preparation and calculation and checking functions for this
We have to go through all the numbers to see which one is last
I don’t think there’s any way around this, there’s no secret

Day 5 ⭐ ⭐

Calculate what is happening with some hydrothermal vents
Calculate what is happening with all the hydrothermal vents

Create a matrix and add 1 to all the plumes
Do the same thing with all the diagonals as well

Day 6 ⭐ ⭐

Find how many lanternfish there are after 80 days
How many lanternfish are there after 256 days?

Loop, apply the rules
Again, loop again

Day 7 ⭐ ⭐

Day 8 ⭐ ⭐

Day 9 ⭐ ⭐

Day 10 ⭐ ⭐

Day 11 ⭐ ⭐

Day 13 ⭐ ⭐

Day 17 ⭐ ⭐

Find the highest trajectory that lands in the box
How many trajectories land in the box?

This could probably be solved in a nicer way than what I did, but I got there
Three of the x velocities gave the same max height which is cool but also probably mathematical
Part one you were only shooting up but part two you can shoot down as well