https://adventofcode.com/2021/day/1

## \--- Day 1: Sonar Sweep ---

You're minding your own business on a ship at sea when the overboard alarm
goes off! You rush to see if you can help. Apparently, one of the Elves
tripped and accidentally sent the sleigh keys flying into the ocean!

Before you know it, you're inside a submarine the Elves keep ready for
situations like this. It's covered in Christmas lights (because of course it
is), and it even has an experimental antenna that should be able to track the
keys if you can boost its signal strength high enough; there's a little meter
that indicates the antenna's signal strength by displaying 0-50 _stars_.

Your instincts tell you that in order to save Christmas, you'll need to get
all _fifty stars_ by December 25th.

Collect stars by solving puzzles. Two puzzles will be made available on each
day in the Advent calendar; the second puzzle is unlocked when you complete
the first. Each puzzle grants _one star_. Good luck!

As the submarine drops below the surface of the ocean, it automatically
performs a sonar sweep of the nearby sea floor. On a small screen, the sonar
sweep report (your puzzle input) appears: each line is a measurement of the
sea floor depth as the sweep looks further and further away from the
submarine.

For example, suppose you had the following report:

[code]

    199
    200
    208
    210
    200
    207
    240
    269
    260
    263
    
[/code]

This report indicates that, scanning outward from the submarine, the sonar
sweep found depths of `199`, `200`, `208`, `210`, and so on.

The first order of business is to figure out how quickly the depth increases,
just so you know what you're dealing with - you never know if the keys will
get carried into deeper water by an ocean current or a fish or something.

To do this, count _the number of times a depth measurement increases_ from the
previous measurement. (There is no measurement before the first measurement.)
In the example above, the changes are as follows:

[code]

    199 (N/A - no previous measurement)
    200 ( _increased_ )
    208 ( _increased_ )
    210 ( _increased_ )
    200 (decreased)
    207 ( _increased_ )
    240 ( _increased_ )
    269 ( _increased_ )
    260 (decreased)
    263 ( _increased_ )
    
[/code]

In this example, there are _`7`_ measurements that are larger than the
previous measurement.

_How many measurements are larger than the previous measurement?_

Your puzzle answer was `1754`.

## \--- Part Two ---

Considering every single measurement isn't as useful as you expected: there's
just too much noise in the data.

Instead, consider sums of a _three-measurement sliding window_. Again
considering the above example:

[code]

    199  A      
    200  A B    
    208  A B C  
    210    B C D
    200  E   C D
    207  E F   D
    240  E F G  
    269    F G H
    260      G H
    263        H
    
[/code]

Start by comparing the first and second three-measurement windows. The
measurements in the first window are marked `A` (`199`, `200`, `208`); their
sum is `199 + 200 + 208 = 607`. The second window is marked `B` (`200`, `208`,
`210`); its sum is `618`. The sum of measurements in the second window is
larger than the sum of the first, so this first comparison _increased_.

Your goal now is to count _the number of times the sum of measurements in this
sliding window increases_ from the previous sum. So, compare `A` with `B`,
then compare `B` with `C`, then `C` with `D`, and so on. Stop when there
aren't enough measurements left to create a new three-measurement sum.

In the above example, the sum of each three-measurement window is as follows:

[code]

    A: 607 (N/A - no previous sum)
    B: 618 ( _increased_ )
    C: 618 (no change)
    D: 617 (decreased)
    E: 647 ( _increased_ )
    F: 716 ( _increased_ )
    G: 769 ( _increased_ )
    H: 792 ( _increased_ )
    
[/code]

In this example, there are _`5`_ sums that are larger than the previous sum.

Consider sums of a three-measurement sliding window. _How many sums are larger
than the previous sum?_