adventofcode/2018/day07/problem

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: **

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