Solutions implemented in Scala 3, using ZIO because I suspect it will get handy in the 2nd part of the challenge :)
See https://adventofcode.com/2022/day/10
This rope bridge creaks as you walk along it. You aren't sure how old it is, or whether it can
even support your weight.
It seems to support the Elves just fine, though. The bridge spans a gorge which was carved out
by the massive river far below you.
You step carefully; as you do, the ropes stretch and twist. You decide to distract yourself by
modeling rope physics; maybe you can even figure out where not to step.
Consider a rope with a knot at each end; these knots mark the head and the tail of the rope. If
the head moves far enough away from the tail, the tail is pulled toward the head.
Due to nebulous reasoning involving Planck lengths, you should be able to model the positions of
the knots on a two-dimensional grid. Then, by following a hypothetical series of motions (your
puzzle input) for the head, you can determine how the tail will move.
Due to the aforementioned Planck lengths, the rope must be quite short; in fact, the head (H) and
tail (T) must always be touching (diagonally adjacent and even overlapping both count as touching):
....
.TH.
....
....
.H..
..T.
....
...
.H. (H covers T)
...
If the head is ever two steps directly up, down, left, or right from the tail, the tail must
also move one step in that direction so it remains close enough:
..... ..... .....
.TH.. -> .T.H. -> ..TH.
..... ..... .....
... ... ...
.T. .T. ...
.H. -> ... -> .T.
... .H. .H.
... ... ...
Otherwise, if the head and tail aren't touching and aren't in the same row or column, the
tail always moves one step diagonally to keep up:
..... ..... .....
..... ..H.. ..H..
..H.. -> ..... -> ..T..
.T... .T... .....
..... ..... .....
..... ..... .....
..... ..... .....
..H.. -> ...H. -> ..TH.
.T... .T... .....
..... ..... .....
You just need to work out where the tail goes as the head follows a series of motions.
Assume the head and the tail both start at the same position, overlapping.
For example:
R 4
U 4
L 3
D 1
R 4
D 1
L 5
R 2
This series of motions moves the head right four steps, then up four steps, then left three
steps, then down one step, and so on. After each step, you'll need to update the position
of the tail if the step means the head is no longer adjacent to the tail. Visually, these
motions occur as follows (s marks the starting position as a reference point):
== Initial State ==
......
......
......
......
H..... (H covers T, s)
== R 4 ==
......
......
......
......
TH.... (T covers s)
......
......
......
......
sTH...
......
......
......
......
s.TH..
......
......
......
......
s..TH.
== U 4 ==
......
......
......
....H.
s..T..
......
......
....H.
....T.
s.....
......
....H.
....T.
......
s.....
....H.
....T.
......
......
s.....
== L 3 ==
...H..
....T.
......
......
s.....
..HT..
......
......
......
s.....
.HT...
......
......
......
s.....
== D 1 ==
..T...
.H....
......
......
s.....
== R 4 ==
..T...
..H...
......
......
s.....
..T...
...H..
......
......
s.....
......
...TH.
......
......
s.....
......
....TH
......
......
s.....
== D 1 ==
......
....T.
.....H
......
s.....
== L 5 ==
......
....T.
....H.
......
s.....
......
....T.
...H..
......
s.....
......
......
..HT..
......
s.....
......
......
.HT...
......
s.....
......
......
HT....
......
s.....
== R 2 ==
......
......
.H.... (H covers T)
......
s.....
......
......
.TH...
......
s.....
After simulating the rope, you can count up all of the positions the tail visited at least
once. In this diagram, s again marks the starting position (which the tail also visited)
and # marks other positions the tail visited:
..##..
...##.
.####.
....#.
s###..
So, there are 13 positions the tail visited at least once.
Simulate your complete hypothetical series of motions. How many positions does the tail of
the rope visit at least once?
part two
A rope snaps! Suddenly, the river is getting a lot closer than you remember. The bridge is
still there, but some of the ropes that broke are now whipping toward you as you fall through
the air!
The ropes are moving too quickly to grab; you only have a few seconds to choose how to arch
your body to avoid being hit. Fortunately, your simulation can be extended to support longer
ropes.
Rather than two knots, you now must simulate a rope consisting of ten knots. One knot is still
the head of the rope and moves according to the series of motions. Each knot further down the
rope follows the knot in front of it using the same rules as before.
Using the same series of motions as the above example, but with the knots marked
H, 1, 2, ..., 9, the motions now occur as follows:
== Initial State ==
......
......
......
......
H..... (H covers 1, 2, 3, 4, 5, 6, 7, 8, 9, s)
== R 4 ==
......
......
......
......
1H.... (1 covers 2, 3, 4, 5, 6, 7, 8, 9, s)
......
......
......
......
21H... (2 covers 3, 4, 5, 6, 7, 8, 9, s)
......
......
......
......
321H.. (3 covers 4, 5, 6, 7, 8, 9, s)
......
......
......
......
4321H. (4 covers 5, 6, 7, 8, 9, s)
== U 4 ==
......
......
......
....H.
4321.. (4 covers 5, 6, 7, 8, 9, s)
......
......
....H.
.4321.
5..... (5 covers 6, 7, 8, 9, s)
......
....H.
....1.
.432..
5..... (5 covers 6, 7, 8, 9, s)
....H.
....1.
..432.
.5....
6..... (6 covers 7, 8, 9, s)
== L 3 ==
...H..
....1.
..432.
.5....
6..... (6 covers 7, 8, 9, s)
..H1..
...2..
..43..
.5....
6..... (6 covers 7, 8, 9, s)
.H1...
...2..
..43..
.5....
6..... (6 covers 7, 8, 9, s)
== D 1 ==
..1...
.H.2..
..43..
.5....
6..... (6 covers 7, 8, 9, s)
== R 4 ==
..1...
..H2..
..43..
.5....
6..... (6 covers 7, 8, 9, s)
..1...
...H.. (H covers 2)
..43..
.5....
6..... (6 covers 7, 8, 9, s)
......
...1H. (1 covers 2)
..43..
.5....
6..... (6 covers 7, 8, 9, s)
......
...21H
..43..
.5....
6..... (6 covers 7, 8, 9, s)
== D 1 ==
......
...21.
..43.H
.5....
6..... (6 covers 7, 8, 9, s)
== L 5 ==
......
...21.
..43H.
.5....
6..... (6 covers 7, 8, 9, s)
......
...21.
..4H.. (H covers 3)
.5....
6..... (6 covers 7, 8, 9, s)
......
...2..
..H1.. (H covers 4; 1 covers 3)
.5....
6..... (6 covers 7, 8, 9, s)
......
...2..
.H13.. (1 covers 4)
.5....
6..... (6 covers 7, 8, 9, s)
......
......
H123.. (2 covers 4)
.5....
6..... (6 covers 7, 8, 9, s)
== R 2 ==
......
......
.H23.. (H covers 1; 2 covers 4)
.5....
6..... (6 covers 7, 8, 9, s)
......
......
.1H3.. (H covers 2, 4)
.5....
6..... (6 covers 7, 8, 9, s)
Now, you need to keep track of the positions the new tail, 9, visits. In this example, the
tail never moves, and so it only visits 1 position. However, be careful: more types of motion
are possible than before, so you might want to visually compare your simulated rope to the
one above.
Here's a larger example:
R 5
U 8
L 8
D 3
R 17
D 10
L 25
U 20
These motions occur as follows (individual steps are not shown):
== Initial State ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
...........H.............. (H covers 1, 2, 3, 4, 5, 6, 7, 8, 9, s)
..........................
..........................
..........................
..........................
..........................
== R 5 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
...........54321H......... (5 covers 6, 7, 8, 9, s)
..........................
..........................
..........................
..........................
..........................
== U 8 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
................H.........
................1.........
................2.........
................3.........
...............54.........
..............6...........
.............7............
............8.............
...........9.............. (9 covers s)
..........................
..........................
..........................
..........................
..........................
== L 8 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
........H1234.............
............5.............
............6.............
............7.............
............8.............
............9.............
..........................
..........................
...........s..............
..........................
..........................
..........................
..........................
..........................
== D 3 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
.........2345.............
........1...6.............
........H...7.............
............8.............
............9.............
..........................
..........................
...........s..............
..........................
..........................
..........................
..........................
..........................
== R 17 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
................987654321H
..........................
..........................
..........................
..........................
...........s..............
..........................
..........................
..........................
..........................
..........................
== D 10 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
...........s.........98765
.........................4
.........................3
.........................2
.........................1
.........................H
== L 25 ==
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
...........s..............
..........................
..........................
..........................
..........................
H123456789................
== U 20 ==
H.........................
1.........................
2.........................
3.........................
4.........................
5.........................
6.........................
7.........................
8.........................
9.........................
..........................
..........................
..........................
..........................
..........................
...........s..............
..........................
..........................
..........................
..........................
..........................
Now, the tail (9) visits 36 positions (including s) at least once:
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
..........................
#.........................
#.............###.........
#............#...#........
.#..........#.....#.......
..#..........#.....#......
...#........#.......#.....
....#......s.........#....
.....#..............#.....
......#............#......
.......#..........#.......
........#........#........
.........########.........
Simulate your complete series of motions on a larger rope with ten knots. How many positions
does the tail of the rope visit at least once?
The expedition comes across a peculiar patch of tall trees all planted carefully in a grid.
The Elves explain that a previous expedition planted these trees as a reforestation effort.
Now, they're curious if this would be a good location for a tree house.
First, determine whether there is enough tree cover here to keep a tree house hidden. To do
this, you need to count the number of trees that are visible from outside the grid when
looking directly along a row or column.
The Elves have already launched a quadcopter to generate a map with the height of each tree
(your puzzle input). For example:
30373
25512
65332
33549
35390
Each tree is represented as a single digit whose value is its height, where 0 is the
shortest and 9 is the tallest.
A tree is visible if all of the other trees between it and an edge of the grid are shorter
than it. Only consider trees in the same row or column; that is, only look up, down, left,
or right from any given tree.
All of the trees around the edge of the grid are visible - since they are already on the
edge, there are no trees to block the view. In this example, that only leaves the interior
nine trees to consider:
The top-left 5 is visible from the left and top. (It isn't visible from the right or bottom
since other trees of height 5 are in the way.)
The top-middle 5 is visible from the top and right.
The top-right 1 is not visible from any direction; for it to be visible, there would need to
only be trees of height 0 between it and an edge.
The left-middle 5 is visible, but only from the right.
The center 3 is not visible from any direction; for it to be visible, there would need to be
only trees of at most height 2 between it and an edge.
The right-middle 3 is visible from the right.
In the bottom row, the middle 5 is visible, but the 3 and 4 are not.
With 16 trees visible on the edge and another 5 visible in the interior, a total of 21 trees
are visible in this arrangement.
Consider your map; how many trees are visible from outside the grid?
part two
Content with the amount of tree cover available, the Elves just need to know the best spot to
build their tree house: they would like to be able to see a lot of trees.
To measure the viewing distance from a given tree, look up, down, left, and right from that
tree; stop if you reach an edge or at the first tree that is the same height or taller than the
tree under consideration. (If a tree is right on the edge, at least one of its viewing distances
will be zero.)
The Elves don't care about distant trees taller than those found by the rules above; the
proposed tree house has large eaves to keep it dry, so they wouldn't be able to see higher
than the tree house anyway.
In the example above, consider the middle 5 in the second row:
30373
25512
65332
33549
35390
Looking up, its view is not blocked; it can see 1 tree (of height 3).
Looking left, its view is blocked immediately; it can see only 1 tree (of height 5, right next
to it).
Looking right, its view is not blocked; it can see 2 trees.
Looking down, its view is blocked eventually; it can see 2 trees (one of height 3, then the
tree of height 5 that blocks its view).
A tree's scenic score is found by multiplying together its viewing distance in each of the
four directions. For this tree, this is 4 (found by multiplying 1 * 1 * 2 * 2).
However, you can do even better: consider the tree of height 5 in the middle of the fourth row:
30373
25512
65332
33549
35390
Looking up, its view is blocked at 2 trees (by another tree with a height of 5).
Looking left, its view is not blocked; it can see 2 trees.
Looking down, its view is also not blocked; it can see 1 tree.
Looking right, its view is blocked at 2 trees (by a massive tree of height 9).
This tree's scenic score is 8 (2 * 2 * 1 * 2); this is the ideal spot for the tree house.
Consider each tree on your map. What is the highest scenic score possible for any tree?
You can hear birds chirping and raindrops hitting leaves as the expedition proceeds.
Occasionally, you can even hear much louder sounds in the distance; how big do the animals
get out here, anyway?
The device the Elves gave you has problems with more than just its communication system. You
try to run a system update:
$ system-update --please --pretty-please-with-sugar-on-top
Error: No space left on device
Perhaps you can delete some files to make space for the update?
You browse around the filesystem to assess the situation and save the resulting terminal
output (your puzzle input). For example:
$ cd /
$ ls
dir a
14848514 b.txt
8504156 c.dat
dir d
$ cd a
$ ls
dir e
29116 f
2557 g
62596 h.lst
$ cd e
$ ls
584 i
$ cd ..
$ cd ..
$ cd d
$ ls
4060174 j
8033020 d.log
5626152 d.ext
7214296 k
The filesystem consists of a tree of files (plain data) and directories (which can contain
other directories or files). The outermost directory is called /. You can navigate around
the filesystem, moving into or out of directories and listing the contents of the directory
you're currently in.
Within the terminal output, lines that begin with $ are commands you executed, very much
like some modern computers:
cd means change directory. This changes which directory is the current directory, but the
specific result depends on the argument:
cd x moves in one level: it looks in the current directory for the directory named x and
makes it the current directory.
cd .. moves out one level: it finds the directory that contains the current directory, then
makes that directory the current directory.
cd / switches the current directory to the outermost directory, /.
ls means list. It prints out all of the files and directories immediately contained by the
current directory:
123 abc means that the current directory contains a file named abc with size 123.
dir xyz means that the current directory contains a directory named xyz.
Given the commands and output in the example above, you can determine that the filesystem
looks visually like this:
- / (dir)
- a (dir)
- e (dir)
- i (file, size=584)
- f (file, size=29116)
- g (file, size=2557)
- h.lst (file, size=62596)
- b.txt (file, size=14848514)
- c.dat (file, size=8504156)
- d (dir)
- j (file, size=4060174)
- d.log (file, size=8033020)
- d.ext (file, size=5626152)
- k (file, size=7214296)
Here, there are four directories: / (the outermost directory), a and d (which are in /),
and e (which is in a). These directories also contain files of various sizes.
Since the disk is full, your first step should probably be to find directories that are good
candidates for deletion. To do this, you need to determine the total size of each directory.
The total size of a directory is the sum of the sizes of the files it contains, directly or
indirectly. (Directories themselves do not count as having any intrinsic size.)
The total sizes of the directories above can be found as follows:
The total size of directory e is 584 because it contains a single file i of size 584 and no
other directories.
The directory a has total size 94853 because it contains files f (size 29116), g (size 2557),
and h.lst (size 62596), plus file i indirectly (a contains e which contains i).
Directory d has total size 24933642.
As the outermost directory, / contains every file. Its total size is 48381165, the sum of the
size of every file.
To begin, find all of the directories with a total size of at most 100000, then calculate the
sum of their total sizes. In the example above, these directories are a and e; the sum of
their total sizes is 95437 (94853 + 584). (As in this example, this process can count files
more than once!)
Find all of the directories with a total size of at most 100000. What is the sum of the total
sizes of those directories?
part two
Now, you're ready to choose a directory to delete.
The total disk space available to the filesystem is 70000000. To run the update, you need
unused space of at least 30000000. You need to find a directory you can delete that will
free up enough space to run the update.
In the example above, the total size of the outermost directory (and thus the total amount of
used space) is 48381165; this means that the size of the unused space must currently be
21618835, which isn't quite the 30000000 required by the update. Therefore, the update still
requires a directory with total size of at least 8381165 to be deleted before it can run.
To achieve this, you have the following options:
Delete directory e, which would increase unused space by 584.
Delete directory a, which would increase unused space by 94853.
Delete directory d, which would increase unused space by 24933642.
Delete directory /, which would increase unused space by 48381165.
Directories e and a are both too small; deleting them would not free up enough space. However,
directories d and / are both big enough! Between these, choose the smallest: d, increasing
unused space by 24933642.
Find the smallest directory that, if deleted, would free up enough space on the filesystem
to run the update. What is the total size of that directory?
The preparations are finally complete; you and the Elves leave camp on foot and begin to make
your way toward the star fruit grove.
As you move through the dense undergrowth, one of the Elves gives you a handheld device. He
says that it has many fancy features, but the most important one to set up right now is the
communication system.
However, because he's heard you have significant experience dealing with signal-based systems,
he convinced the other Elves that it would be okay to give you their one malfunctioning
device - surely you'll have no problem fixing it.
As if inspired by comedic timing, the device emits a few colorful sparks.
To be able to communicate with the Elves, the device needs to lock on to their signal. The
signal is a series of seemingly-random characters that the device receives one at a time.
To fix the communication system, you need to add a subroutine to the device that detects a
start-of-packet marker in the datastream. In the protocol being used by the Elves, the start
of a packet is indicated by a sequence of four characters that are all different.
The device will send your subroutine a datastream buffer (your puzzle input); your subroutine
needs to identify the first position where the four most recently received characters were all
different. Specifically, it needs to report the number of characters from the beginning of the
buffer to the end of the first such four-character marker.
For example, suppose you receive the following datastream buffer:
mjqjpqmgbljsphdztnvjfqwrcgsmlb
After the first three characters (mjq) have been received, there haven't been enough characters
received yet to find the marker. The first time a marker could occur is after the fourth
character is received, making the most recent four characters mjqj. Because j is repeated,
this isn't a marker.
The first time a marker appears is after the seventh character arrives. Once it does, the
last four characters received are jpqm, which are all different. In this case, your subroutine
should report the value 7, because the first start-of-packet marker is complete after 7
characters have been processed.
Here are a few more examples:
bvwbjplbgvbhsrlpgdmjqwftvncz: first marker after character 5
nppdvjthqldpwncqszvftbrmjlhg: first marker after character 6
nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg: first marker after character 10
zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw: first marker after character 11
How many characters need to be processed before the first start-of-packet marker
is detected?
part two
Your device's communication system is correctly detecting packets, but still isn't
working. It looks like it also needs to look for messages.
A start-of-message marker is just like a start-of-packet marker, except it consists of
14 distinct characters rather than 4.
Here are the first positions of start-of-message markers for all of the above examples:
mjqjpqmgbljsphdztnvjfqwrcgsmlb: first marker after character 19
bvwbjplbgvbhsrlpgdmjqwftvncz: first marker after character 23
nppdvjthqldpwncqszvftbrmjlhg: first marker after character 23
nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg: first marker after character 29
zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw: first marker after character 26
How many characters need to be processed before the first start-of-message marker
is detected?
The expedition can depart as soon as the final supplies have been unloaded from the ships.
Supplies are stored in stacks of marked crates, but because the needed supplies are buried
under many other crates, the crates need to be rearranged.
The ship has a giant cargo crane capable of moving crates between stacks. To ensure none of
the crates get crushed or fall over, the crane operator will rearrange them in a series of
carefully-planned steps. After the crates are rearranged, the desired crates will be at
the top of each stack.
The Elves don't want to interrupt the crane operator during this delicate procedure, but they
forgot to ask her which crate will end up where, and they want to be ready to unload them
as soon as possible so they can embark.
They do, however, have a drawing of the starting stacks of crates and the rearrangement
procedure (your puzzle input). For example:
[D]
[N] [C]
[Z] [M] [P]
1 2 3
move 1 from 2 to 1
move 3 from 1 to 3
move 2 from 2 to 1
move 1 from 1 to 2
In this example, there are three stacks of crates. Stack 1 contains two crates: crate Z is
on the bottom, and crate N is on top. Stack 2 contains three crates; from bottom to top,
they are crates M, C, and D. Finally, stack 3 contains a single crate, P.
Then, the rearrangement procedure is given. In each step of the procedure, a quantity of
crates is moved from one stack to a different stack. In the first step of the above
rearrangement procedure, one crate is moved from stack 2 to stack 1, resulting
in this configuration:
[D]
[N] [C]
[Z] [M] [P]
1 2 3
In the second step, three crates are moved from stack 1 to stack 3. Crates are moved one at
a time, so the first crate to be moved (D) ends up below the second and third crates:
[Z]
[N]
[C] [D]
[M] [P]
1 2 3
Then, both crates are moved from stack 2 to stack 1. Again, because crates are moved one
at a time, crate C ends up below crate M:
[Z]
[N]
[M] [D]
[C] [P]
1 2 3
Finally, one crate is moved from stack 1 to stack 2:
[Z]
[N]
[D]
[C] [M] [P]
1 2 3
The Elves just need to know which crate will end up on top of each stack; in this example,
the top crates are C in stack 1, M in stack 2, and Z in stack 3, so you should combine
these together and give the Elves the message CMZ.
After the rearrangement procedure completes, what crate ends up on top of each stack?
part two
As you watch the crane operator expertly rearrange the crates, you notice the process isn't
following your prediction.
Some mud was covering the writing on the side of the crane, and you quickly wipe it away. The
crane isn't a CrateMover 9000 - it's a CrateMover 9001.
The CrateMover 9001 is notable for many new and exciting features: air conditioning, leather seats,
an extra cup holder, and the ability to pick up and move multiple crates at once.
Again considering the example above, the crates begin in the same configuration:
[D]
[N] [C]
[Z] [M] [P]
1 2 3
Moving a single crate from stack 2 to stack 1 behaves the same as before:
[D]
[N] [C]
[Z] [M] [P]
1 2 3
However, the action of moving three crates from stack 1 to stack 3 means that those three moved
crates stay in the same order, resulting in this new configuration:
[D]
[N]
[C] [Z]
[M] [P]
1 2 3
Next, as both crates are moved from stack 2 to stack 1, they retain their order as well:
[D]
[N]
[C] [Z]
[M] [P]
1 2 3
Finally, a single crate is still moved from stack 1 to stack 2, but now it's crate C that gets
moved:
[D]
[N]
[Z]
[M] [C] [P]
1 2 3
In this example, the CrateMover 9001 has put the crates in a totally different order: MCD.
Before the rearrangement process finishes, update your simulation so that the Elves know where
they should stand to be ready to unload the final supplies. After the rearrangement procedure
completes, what crate ends up on top of each stack?
Space needs to be cleared before the last supplies can be unloaded from the ships, and so
several Elves have been assigned the job of cleaning up sections of the camp. Every
section has a unique ID number, and each Elf is assigned a range of section IDs.
However, as some of the Elves compare their section assignments with each other, they've
noticed that many of the assignments overlap. To try to quickly find overlaps and reduce
duplicated effort, the Elves pair up and make a big list of the section assignments for
each pair (your puzzle input).
For example, consider the following list of section assignment pairs:
2-4,6-8
2-3,4-5
5-7,7-9
2-8,3-7
6-6,4-6
2-6,4-8
For the first few pairs, this list means:
Within the first pair of Elves, the first Elf was assigned sections 2-4 (sections 2, 3,
and 4), while the second Elf was assigned sections 6-8 (sections 6, 7, 8).
The Elves in the second pair were each assigned two sections.
The Elves in the third pair were each assigned three sections: one got sections 5, 6,
and 7, while the other also got 7, plus 8 and 9.
This example list uses single-digit section IDs to make it easier to draw; your actual
list might contain larger numbers. Visually, these pairs of section assignments
look like this:
.234..... 2-4
.....678. 6-8
.23...... 2-3
...45.... 4-5
....567.. 5-7
......789 7-9
.2345678. 2-8
..34567.. 3-7
.....6... 6-6
...456... 4-6
.23456... 2-6
...45678. 4-8
Some of the pairs have noticed that one of their assignments fully contains the other. For
example, 2-8 fully contains 3-7, and 6-6 is fully contained by 4-6. In pairs where one
assignment fully contains the other, one Elf in the pair would be exclusively cleaning
sections their partner will already be cleaning, so these seem like the most in need of
reconsideration. In this example, there are 2 such pairs.
In how many assignment pairs does one range fully contain the other?
part two
It seems like there is still quite a bit of duplicate work planned. Instead, the Elves would
like to know the number of pairs that overlap at all.
In the above example, the first two pairs (2-4,6-8 and 2-3,4-5) don't overlap, while the
remaining four pairs (5-7,7-9, 2-8,3-7, 6-6,4-6, and 2-6,4-8) do overlap:
5-7,7-9 overlaps in a single section, 7.
2-8,3-7 overlaps all of the sections 3 through 7.
6-6,4-6 overlaps in a single section, 6.
2-6,4-8 overlaps in sections 4, 5, and 6.
So, in this example, the number of overlapping assignment pairs is 4.
In how many assignment pairs do the ranges overlap?
One Elf has the important job of loading all of the rucksacks with supplies for the jungle
journey. Unfortunately, that Elf didn't quite follow the packing instructions, and so a few
items now need to be rearranged.
Each rucksack has two large compartments. All items of a given type are meant to go into exactly
one of the two compartments. The Elf that did the packing failed to follow this rule for exactly
one item type per rucksack.
The Elves have made a list of all of the items currently in each rucksack (your puzzle input),
but they need your help finding the errors. Every item type is identified by a single lowercase
or uppercase letter (that is, a and A refer to different types of items).
The list of items for each rucksack is given as characters all on a single line. A given rucksack
always has the same number of items in each of its two compartments, so the first half of the
characters represent items in the first compartment, while the second half of the characters
represent items in the second compartment.
For example, suppose you have the following list of contents from six rucksacks:
vJrwpWtwJgWrhcsFMMfFFhFp
jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
PmmdzqPrVvPwwTWBwg
wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
ttgJtRGJQctTZtZT
CrZsJsPPZsGzwwsLwLmpwMDw
The first rucksack contains the items vJrwpWtwJgWrhcsFMMfFFhFp, which means its first compartment
contains the items vJrwpWtwJgWr, while the second compartment contains the items hcsFMMfFFhFp. The
only item type that appears in both compartments is lowercase p.
The second rucksack's compartments contain jqHRNqRjqzjGDLGL and rsFMfFZSrLrFZsSL. The only item type
that appears in both compartments is uppercase L.
The third rucksack's compartments contain PmmdzqPrV and vPwwTWBwg; the only common item type is
uppercase P.
The fourth rucksack's compartments only share item type v.
The fifth rucksack's compartments only share item type t.
The sixth rucksack's compartments only share item type s.
To help prioritize item rearrangement, every item type can be converted to a priority:
Lowercase item types a through z have priorities 1 through 26.
Uppercase item types A through Z have priorities 27 through 52.
In the above example, the priority of the item type that appears in both compartments of each
rucksack is 16 (p), 38 (L), 42 (P), 22 (v), 20 (t), and 19 (s); the sum of these is 157.
Find the item type that appears in both compartments of each rucksack. What is the sum of the
priorities of those item types?
part two
As you finish identifying the misplaced items, the Elves come to you with another issue.
For safety, the Elves are divided into groups of three. Every Elf carries a badge that
identifies their group. For efficiency, within each group of three Elves, the badge is the
only item type carried by all three Elves. That is, if a group's badge is item type B, then
all three Elves will have item type B somewhere in their rucksack, and at most two of the
Elves will be carrying any other item type.
The problem is that someone forgot to put this year's updated authenticity sticker on the
badges. All of the badges need to be pulled out of the rucksacks so the new authenticity
stickers can be attached.
Additionally, nobody wrote down which item type corresponds to each group's badges. The
only way to tell which item type is the right one is by finding the one item type that is
common between all three Elves in each group.
Every set of three lines in your list corresponds to a single group, but each group can
have a different badge item type. So, in the above example, the first group's rucksacks
are the first three lines:
vJrwpWtwJgWrhcsFMMfFFhFp
jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
PmmdzqPrVvPwwTWBwg
And the second group's rucksacks are the next three lines:
wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
ttgJtRGJQctTZtZT
CrZsJsPPZsGzwwsLwLmpwMDw
In the first group, the only item type that appears in all three rucksacks is lowercase r;
this must be their badges. In the second group, their badge item type must be Z.
Priorities for these items must still be found to organize the sticker attachment efforts:
here, they are 18 (r) for the first group and 52 (Z) for the second group.
The sum of these is 70.
Find the item type that corresponds to the badges of each three-Elf group.
What is the sum of the priorities of those item types?
Rock Paper Scissors is a game between two players. Each game contains many rounds; in each round,
the players each simultaneously choose one of Rock, Paper, or Scissors using a hand shape. Then,
a winner for that round is selected: Rock defeats Scissors, Scissors defeats Paper, and Paper
defeats Rock. If both players choose the same shape, the round instead ends in a draw.
Appreciative of your help yesterday, one Elf gives you an encrypted strategy guide (your puzzle
input) that they say will be sure to help you win. "The first column is what your opponent is going
to play: A for Rock, B for Paper, and C for Scissors. The second column--" Suddenly, the Elf is
called away to help with someone's tent.
The second column, you reason, must be what you should play in response: X for Rock, Y for Paper,
and Z for Scissors. Winning every time would be suspicious, so the responses must have been
carefully chosen.
The winner of the whole tournament is the player with the highest score. Your total score is the
sum of your scores for each round. The score for a single round is the score for the shape you
selected (1 for Rock, 2 for Paper, and 3 for Scissors) plus the score for the outcome of the round
(0 if you lost, 3 if the round was a draw, and 6 if you won).
Since you can't be sure if the Elf is trying to help you or trick you, you should calculate the
score you would get if you were to follow the strategy guide.
For example, suppose you were given the following strategy guide:
A Y
B X
C Z
This strategy guide predicts and recommends the following:
In the first round, your opponent will choose Rock (A), and you should choose Paper (Y). This ends
in a win for you with a score of 8 (2 because you chose Paper + 6 because you won).
In the second round, your opponent will choose Paper (B), and you should choose Rock (X). This ends
in a loss for you with a score of 1 (1 + 0).
The third round is a draw with both players choosing Scissors, giving you a score of 3 + 3 = 6.
In this example, if you were to follow the strategy guide, you would get a total score of
15 (8 + 1 + 6).
What would your total score be if everything goes exactly according to your strategy guide?
part two
The Elf finishes helping with the tent and sneaks back over to you. "Anyway, the second column says
how the round needs to end: X means you need to lose, Y means you need to end the round in a draw,
and Z means you need to win. Good luck!"
The total score is still calculated in the same way, but now you need to figure out what shape to
choose so the round ends as indicated. The example above now goes like this:
In the first round, your opponent will choose Rock (A), and you need the round to end in a draw (Y),
so you also choose Rock. This gives you a score of 1 + 3 = 4.
In the second round, your opponent will choose Paper (B), and you choose Rock so you lose (X) with
a score of 1 + 0 = 1.
In the third round, you will defeat your opponent's Scissors with Rock for a score of 1 + 6 = 7.
Now that you're correctly decrypting the ultra top secret strategy guide, you would get a total
score of 12.
Following the Elf's instructions for the second column, what would your total score be if everything
goes exactly according to your strategy guide?
part one
The jungle must be too overgrown and difficult to navigate in vehicles or access from
the air; the Elves' expedition traditionally goes on foot. As your boats approach land,
the Elves begin taking inventory of their supplies. One important consideration is food
- in particular, the number of Calories each Elf is carrying (your puzzle input).
The Elves take turns writing down the number of Calories contained by the various meals,
snacks, rations, etc. that they've brought with them, one item per line. Each Elf
separates their own inventory from the previous Elf's inventory (if any) by a blank line.
For example, suppose the Elves finish writing their items' Calories and end up with the
following list:
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
This list represents the Calories of the food carried by five Elves:
The first Elf is carrying food with 1000, 2000, and 3000 Calories, a total of 6000Calories.
The second Elf is carrying one food item with 4000 Calories.
The third Elf is carrying food with 5000 and 6000 Calories, a total of 11000 Calories.
The fourth Elf is carrying food with 7000, 8000, and 9000 Calories, a total of 24000Calories.
The fifth Elf is carrying one food item with 10000 Calories.
In case the Elves get hungry and need extra snacks, they need to know which Elf to ask:
they'd like to know how many Calories are being carried by the Elf carrying the most
Calories. In the example above, this is 24000 (carried by the fourth Elf).
Find the Elf carrying the most Calories. How many total Calories is that Elf carrying?
part two
By the time you calculate the answer to the Elves' question, they've already realized that the
Elf carrying the most Calories of food might eventually run out of snacks.
To avoid this unacceptable situation, the Elves would instead like to know the total Calories
carried by the top three Elves carrying the most Calories. That way, even if one of those
Elves runs out of snacks, they still have two backups.
In the example above, the top three Elves are the fourth Elf (with 24000 Calories), then the
third Elf (with 11000 Calories), then the fifth Elf (with 10000 Calories). The sum of the Calories
carried by these three elves is 45000.
Find the top three Elves carrying the most Calories. How many Calories are those Elves
carrying in total?