adventofcode-rb-2018

For the fourth year in a row, it's the time of the year to do Advent of Code again.

When will it end?

The solutions are written with the following goals, with the most important goal first:

1. Speed. Where possible, use efficient algorithms for the problem. Solutions that take more than a second to run are treated with high suspicion. This need not be overdone; micro-optimisation is not necessary.
2. Readability.
3. Less is More. Whenever possible, write less code. Especially prefer not to duplicate code. This helps keeps solutions readable too.

All solutions are written in Ruby. Features from 2.5.x will be used, with no regard for compatibility with past versions.

Input

In general, all solutions can be invoked in both of the following ways:

• Without command-line arguments, takes input on standard input.
• With command-line arguments, reads input from the named files (- indicates standard input).

Some may additionally support other ways:

• Day 9 (Marble Mania): Pass the number of players and the last marble in ARGV
• Day 11 (Max Square / Chronal Charge): Pass the serial in ARGV
• Day 14 (Chocolate Charts): Pass the input in ARGV
• Day 22 (Mode Maze): Pass the depth, target x, and target y in ARGV in that order

Highlights

Favourite problems:

• Day 17 (Reservoir Research): Novel concept, reasonably challenging, fun to visualise.

Interesting approaches:

• Day 01 (Frequency Deltas / Chronal Calibration): See https://www.reddit.com/r/adventofcode/comments/a20646/2018_day_1_solutions/eaukxu5/ .
• Day 02 (Inventory Management): Interesting O(k^2 * n) solution rather than O(k * n^2), where k is length of strings. See some others which may achieve O(kn) time:
  • https://www.reddit.com/r/adventofcode/comments/a2damm/2018_day2_part_2_a_linear_time_solution/
  • https://www.reddit.com/r/adventofcode/comments/a2mavh/day_2_part_2_under_n2_time_show_off_your_solution/
  • https://www.reddit.com/r/adventofcode/comments/a2rt9s/2018_day_2_part_2_here_are_some_big_inputs_to/
• Day 03 (Grid Claims / No Matter How You Slice It): Use a single-pass approach with a tri-state (free, single, clash) which only needs to iterate the claims once, and doesn't need to iterate the grid after. Would consider a sweep-line approach, but too much work.
• Day 05 (Alchemical Reduction): Since characters can only react with their immediate neighbours, use a stack.
• Day 06 (Chronal Coordinates):
  • Part 1: Simultaneous flood-fill out from all points.
  • Part 2: X distances and Y distances are independent of one another.
• Day 09 (Marble Mania):
  • The marble to be removed is the one to the right of the 18th marble in a cycle of 23. Thus, we only need a singly-linked list, not a doubly-linked list.
  • Marbles to be removed are at indices 19, 35, 51, 67.... Once we have enough marbles in the list, we can stop adding new ones.
• Day 11 (Max Square / Chronal Charge):
  • https://en.wikipedia.org/wiki/Summed-area_table
  • We can calculate a maximum upper bound on the size of a square with length 2N by using four squares of length N.
  • We can calculate a maximum upper bound on the size of a square with length 2N+1 by using two squares of length N plus two squares of length N+1 minus one square.
• Day 14 (Chocolate Charts): When wrapping around, you must land on one of the first ten elements of the array. All sequences converge on index 23. From that point forward, we only need to store the scores that elves will land on. All other scores can be immediately discarded after updating the state of "how many charactes have I seen".
• Day 15 (Beverage Bandits): A single BFS seems like the way to go here. It's easy to keep track of the layers of the BFS, so as soon as a layer finds a nonzero number of goals, we can stop. In addition, implement the "and so none of the units can move until a unit dies" optimisation noted in the problem text.
• Day 18 (Tri-State Automata / Settlers of The North Pole): Bit-shifting implementation, like http://dotat.at/prog/life/life.html, but with an 18-bit index instead of 9-bit. Unfortunately, the "compact representation" did not prove to be faster here.
• Day 22 (Mode Maze): A*, of course.
• Day 23 (Emergency Teleportation): 4D coordinates. The bots' ranges can be represented as eight inequalities, one for each element of the product of [+x, -x], [+y, -y], [+z, -z]. That can also be represented as four intervals by pairing up opposite coordinates. askalski's idea: https://www.reddit.com/r/adventofcode/comments/a9co1u/day_23_part_2_adversarial_input_for_recursive/ecmpxad By using a 4D coordinate system, we can easily calculate bot intersections and additionally know exactly where the number of intersecting bots can possibly change. Subdivide the space at these interesting points.
• Day 25 (Four-Dimensional Adventure): Still an O(n^2) affair, but we can make it slightly better by grouping the points by their first coordinate and only comparing the ones that are wihtin 3 on that dimension.

Takeaways

• Day 06 (Chronal Coordinates): Assumed I needed to be clever about which coordinates I check, wrote incorrect code that attempted to scan every row for the start/end column of the safe region on that row, but was actually an infinite loop. Changed to a flood-fill solution to get a spot on the leaderboard. Later discovered that using only the points in the bounding box would have been fine, given how far away the points are.
• Day 09 (Marble Mania): Assumed there would be some pattern to be found within the sequence and wasted time trying to find it, rather than just brute-forcing it with a better data structure.
• Day 11 (Max Square / Chronal Charge): Attempted to cache (only add new edges and corners) which still ends up taking 2 minutes to run because it's O(n^4), and was bug-prone and took a long time to write. For getting on the leaderboard, consider a completely different approach: Just use the O(n^5) approach, but with size as the outermost loop. Print out the largest square found for each size and submit when they start decreasing. In other words, try asymptotically-slow approaches that can nevertheless give an answer reasonably fast, rather than waiting for an asymptotically-fast approach to finish.
• Day 14 (Chocolate Charts): As I recall, I tried just taking a subarray of the last 6 scores at any time and comparing this against the desired sequence, but even this was too slow; I switched to only incrementing a counter when I had a match, which worked fast enough but was a bit error prone. I saw others just check every 1000 scores or so, only checking the last 1000+6 scores.
• Day 16 (Chronal Classification): A small reading mistake here; I checked for == 3 instead of >= 3.
• Day 19 (Go With the Flow): This one was interesting since my part 1 input was a prime number, so I assumed it was 1 + that number, which got a wrong answer. Then I decided to dump all registers every time when r3 (my register holding a number that might be added to r0), and noticed that my r0 increased from 1 to 12... then the light bulb turned on, because I saw that my part 2 number was divisible by 11.
• Day 21 (Chronal Conversion): I can and should have taken the first value that r0 got compared to, but I made a mistake because of bad variable names. My ipreg variable was named ip at the time, so I was checking ip == 29 when it needed to be regs[ip] == 29. Better variable names will solve this problem. I wonder if statically-checked types would have helped here, but it's hard to imagine that: I would have had to make a register address a different type from a register value, and this would have complicated instruction parsing, I believe.
• Day 22 (Mode Maze): Finally I have an excuse to use A*. I did have an implementation lying around, but it was not battle-tested for proper performance, so I missed my leaderboard chance for this. Note that Ruby SortedSet only gets good insert performance if rbtree is imported; that's a little too magic for me. Also note that Ruby SortedSet also doesn't have good delete performance in either case, so I ended up having to implement my own priority queue.
• Day 23 (Emergency Teleportation): Got lucky with my input because I managed to find a local maximum that was the same distance from the origin as my actual answer. So my leaderboard spot was a bit undeserved.

Posting schedule and policy

Before I post my day N solution, the day N leaderboard must be full. No exceptions.

Waiting any longer than that seems generally not useful since at that time discussion starts on the subreddit anyway.

Solutions posted will be cleaned-up versions of code I use to get leaderboard times (if I even succeed in getting them), rather than the exact code used. This is because leaderboard-seeking code is written for programmer speed (whatever I can come up with in the heat of the moment). This often produces code that does not meet any of the goals of this repository (seen in the introductory paragraph).

Past solutions

The index lists all years/languages I've ever done (or will ever do).