Advent of Code
--- Day 7: The Sum of Its Parts ---
You find yourself standing on a snow-covered coastline; apparently, you
landed a little off course. The region is too hilly to see the North Pole
from here, but you do spot some Elves that seem to be trying to unpack
something that washed ashore. It's quite cold out, so you decide to risk
creating a paradox by asking them for directions.
"Oh, are you the search party?" Somehow, you can understand whatever Elves
from the year 1018 speak; you assume it's Ancient Nordic Elvish. Could the
device on your wrist also be a translator? "Those clothes don't look very
warm; take this." They hand you a heavy coat.
"We do need to find our way back to the North Pole, but we have higher
priorities at the moment. You see, believe it or not, this box contains
something that will solve all of Santa's transportation problems - at least,
that's what it looks like from the pictures in the instructions." It doesn't
seem like they can read whatever language it's in, but you can: "Sleigh kit.
Some assembly required."
"'Sleigh'? What a wonderful name! You must help us assemble this 'sleigh' at
once!" They start excitedly pulling more parts out of the box.
The instructions specify a series of steps and requirements about which
steps must be finished before others can begin (your puzzle input). Each
step is designated by a single letter. For example, suppose you have the
following instructions:
Step C must be finished before step A can begin.
Step C must be finished before step F can begin.
Step A must be finished before step B can begin.
Step A must be finished before step D can begin.
Step B must be finished before step E can begin.
Step D must be finished before step E can begin.
Step F must be finished before step E can begin.
Visually, these requirements look like this:
-->A--->B--
/ \ \
C -->D----->E
\ /
---->F-----
Your first goal is to determine the order in which the steps should be
completed. If more than one step is ready, choose the step which is first
alphabetically. In this example, the steps would be completed as follows:
• Only C is available, and so it is done first.
• Next, both A and F are available. A is first alphabetically, so it is
done next.
• Then, even though F was available earlier, steps B and D are now also
available, and B is the first alphabetically of the three.
• After that, only D and F are available. E is not available because only
some of its prerequisites are complete. Therefore, D is completed next.
• F is the only choice, so it is done next.
• Finally, E is completed.
So, in this example, the correct order is CABDFE.
In what order should the steps in your instructions be completed?
Your puzzle answer was GKRVWBESYAMZDPTIUCFXQJLHNO.
--- Part Two ---
As you're about to begin construction, four of the Elves offer to help. "The
sun will set soon; it'll go faster if we work together." Now, you need to
account for multiple people working on steps simultaneously. If multiple
steps are available, workers should still begin them in alphabetical order.
Each step takes 60 seconds plus an amount corresponding to its letter: A=1,
B=2, C=3, and so on. So, step A takes 60+1=61 seconds, while step Z takes
60+26=86 seconds. No time is required between steps.
To simplify things for the example, however, suppose you only have help from
one Elf (a total of two workers) and that each step takes 60 fewer seconds
(so that step A takes 1 second and step Z takes 26 seconds). Then, using the
same instructions as above, this is how each second would be spent:
Second Worker 1 Worker 2 Done
0 C .
1 C .
2 C .
3 A F C
4 B F CA
5 B F CA
6 D F CAB
7 D F CAB
8 D F CAB
9 D . CABF
10 E . CABFD
11 E . CABFD
12 E . CABFD
13 E . CABFD
14 E . CABFD
15 . . CABFDE
Each row represents one second of time. The Second column identifies how
many seconds have passed as of the beginning of that second. Each worker
column shows the step that worker is currently doing (or . if they are
idle). The Done column shows completed steps.
Note that the order of the steps has changed; this is because steps now take
time to finish and multiple workers can begin multiple steps simultaneously.
In this example, it would take 15 seconds for two workers to complete these
steps.
With 5 workers and the 60+ second step durations described above, how long
will it take to complete all of the steps?
Your puzzle answer was 903.
Both parts of this puzzle are complete! They provide two gold stars: **
References
Visible links
. https://adventofcode.com/
. https://adventofcode.com/2018/about
. https://adventofcode.com/2018/events
. https://adventofcode.com/2018/settings
. https://adventofcode.com/2018/auth/logout
. Advent of Code Supporter
https://adventofcode.com/2018/support
. https://adventofcode.com/2018
. https://adventofcode.com/2018
. https://adventofcode.com/2018/support
. https://adventofcode.com/2018/sponsors
. https://adventofcode.com/2018/leaderboard
. https://adventofcode.com/2018/stats
. https://adventofcode.com/2018/sponsors
. https://adventofcode.com/2015/day/6
. https://adventofcode.com/2018
. https://adventofcode.com/2018/day/7/input