AoC Day 5: Alchemical Reduction

Alright! We got through Day 4. It's done. We completed the tasks given to us. For most of us, it is dead to us and we will speak of it no more.

Day 5 is upon us! Today is all about the chemistry. We've got long strings of "polymers" that may or may not cancel each other out.

I'm hoping that people are more satisfied with their solutions this go-around. 🎄😬

Good luck!

Comments

[Matt Morgis]

Late, but very happy with this one and had a lot of fun

Node.js

const isUpperCase = letter => letter === letter.toUpperCase();
const isLowerCase = letter => letter === letter.toLowerCase();
const lettersAreEqual = (a, b) => a.toUpperCase() === b.toUpperCase();
const last = array => array[array.length - 1];

const unique = array => [
  ...new Map(array.map(s => [s.toLowerCase(), s])).values()
];

const doesReact = (a, b) => {
  let reacts = false;
  if (
    (isLowerCase(a) && isUpperCase(b)) ||
    (isUpperCase(a) && isLowerCase(b))
  ) {
    if (lettersAreEqual(a, b)) {
      reacts = true;
    }
  }
  return reacts;
};

const removePolarity = polymer => {
  polymer = [...polymer];
  const output = [""];

  for (const char of polymer) {
    if (doesReact(char, last(output))) {
      output.pop();
    } else {
      output.push(char);
    }
  }

  // minus one for the emptry string at the start
  return output.length - 1;
};

const bestPolarity = polymer => {
  polymer = [...polymer];
  const uniqueLetters = unique(polymer);
  const results = uniqueLetters.map(letter => {
    const strippedPolymer = polymer.filter(c => !lettersAreEqual(c, letter));
    return removePolarity(strippedPolymer);
  });

  return Math.min.apply(null, results);
};

const fs = require("fs").promises;
const {removePolarity, bestPolarity} = require("./remove-polarity");

const main = async () => {
  try {
    const input = await fs.readFile(__dirname + "/input.txt", {
      encoding: "utf-8"
    });

    const part1 = removePolarity(input);
    console.log({part1});

    const part2 = bestPolarity(input);
    console.log({part2});
  } catch (e) {
    console.log(e.message);
    process.exit(-1);
  }
};

main();

Full code: github.com/MattMorgis/Advent-Of-Co...

[Martin Himmel]

I'm a bit behind on these, but catching up this weekend! Day 5 is my favorite so far. 😁

<?php
$input = trim(file_get_contents('./input.txt'));

function reduce_polymer($str) {
    $index = strlen($str);
    while ($index > 0) {
        $index--;
        if ($str[$index] == $str[$index + 1]) {
            continue;
        }
        if (strtolower($str[$index]) == strtolower($str[$index + 1])) {
            $str = substr_replace($str, '', $index, 2);
        }
    }
    return strlen($str);
}

echo 'Initial reduction: ' . reduce_polymer($input) . PHP_EOL;

$counts = [];
foreach (range('a', 'z') as $letter) {
    $copy = $input;
    $copy = str_ireplace($letter, '', $copy);
    $counts[$letter] = reduce_polymer($copy);
}

echo 'Shortest polymer length: ' . min($counts);

[Ben Sinclair]

This is the first one of these I've tried and I'm a lazy person at best, so I did it in Vim:

let @q=':s/aA\|bB\|cC\|dD\|eE\|fF\|gG\|hH\|iI\|jJ\|kK\|lL\|mM\|nN\|oO\|pP\|qQ\|rR\|sS\|tT\|uU\|vV\|wW\|xX\|yY\|zZ\|Aa\|Bb\|Cc\|Dd\|Ee\|Ff\|Gg\|Hh\|Ii\|Jj\|Kk\|Ll\|Mm\|Nn\|Oo\|Pp\|Qq\|Rr\|Ss\|Tt\|Uu\|Vv\|Ww\|Xx\|Yy\|Zz//g
@qq'

There are only 52 combinations after all. Takes about 2 minutes (with nohlsearch...)

Stage 2? Eh, my lunchbreak is really for lunch.

[Patrick Tingen]

Wow, thanks for the idea to use a stack. I revised my first solution in OpenEdge 4GL and it went down from 39 second (yes, seconds) to 240 msec. A-ma-zing!

[Christopher Kruse]

Probably won't get an explanatory blog post on this one today, and need to catch up to finish/submit yesterday's, but here's my Clojure solution for Day 5 (see gist.github.com/ballpointcarrot/7e...

(ns aoc.aoc5)

(defn polymer-drop [[c1 c2]]
  (cond
    (= c1 c2) false
    (or (nil? c1) (nil? c2)) false
    (= (Character/toLowerCase c1) (Character/toLowerCase c2)) true
    :else false))

(defn shrink [input]
  (loop [shrunk [] chars-to-test (take 2 input) left (drop 2 input)]
    (cond
      (and (empty? left) (every? nil? chars-to-test)) (apply str shrunk)
      (nil? (first chars-to-test)) (recur shrunk [(last chars-to-test) (first left)] (rest left))
      (polymer-drop chars-to-test) (if (empty? shrunk)
                                     (recur shrunk (take 2 left) (drop 2 left))
                                     (recur (pop shrunk) [(last shrunk) (first left)] (rest left)))
      :else (recur (conj shrunk (first chars-to-test)) [(last chars-to-test) (first left)] (rest left)))))

(defn remove-char
  "Remove all instances of a character (case-insensitive)
   from a string"
  [string chr]
  (apply str (remove #(or (= % (Character/toUpperCase chr)) (= % (Character/toLowerCase chr))) string)))

(defn char-range
  [start end]
  (map char (range (int start) (inc (int end)))))

(defn find-shortest-polymer [input-string]
  (apply min (pmap #(-> input-string
                        (remove-char %)
                        (shrink)
                        (count)) (char-range \a \z))))

[Shahor]

I wrote this solution pretty quickly... and then kept having the wrong value.

I went down a rabbit hole involving hexdump and such because I was pretty confident in what the code was supposed to do and didn't see a valid result come up. It drove me crazy, but finally after way more time digging than I'd like to admit, here's my solution:

import Fs from "fs"
import Path from "path"

const CASE_ASCII_OFFSET = 32

let input = Fs.readFileSync(Path.join(__dirname, "input.txt"))
    .toString()
    .trim()

for (let i = 0; i < input.length; i++) {
    const currentValue = input.charAt(i)
    const nextValue = input.charAt(i + 1)

    // reached the end
    if (nextValue === undefined) {
        continue
    }

    const isSameTypeAndOppositePolarity =
        Math.abs(currentValue.charCodeAt(0) - nextValue.charCodeAt(0)) ===
        CASE_ASCII_OFFSET

    if (isSameTypeAndOppositePolarity) {
        input = input.slice(0, i) + input.slice(i + 2)
        // Coming back to previous position but since it's going to be
        // incremented by the for loop, let's take a supplementary step
        i = Math.max(-1, i - 2)
    }
}

console.log(input.length)

I'll try and find some time to write up what problem I encountered

[Neil Gall]

Ugh, I hated this one, mainly because I took a completely wrong approach at first in an effort to be very fast. It was fast alright but could I get it to produce the right answer?!

Ali Spittel's stack solution is beautiful.

[Yordi Verkroost]

My day 5 solutions in Elixir. I'm actually pretty happy with how the code turned out this time, might be the cleanest of all days up until now.

The main idea of the implementation of part one is to put characters on a stack, and then compare the head of the stack with the next element to determine if the head and the new element should stay or go.

Part two is a wrapper around the first part, where the input is preprocessed (e.g. units are removed) before being fed to the implementation of the first part. The units to remove are based on unicode numbers.

Common:

defmodule AoC.DayFive.Common do
  @lower_upper_unicode_difference 32

  def read_input(path) do
    path
    |> File.read!()
    |> String.graphemes()
  end

  def process(polymer) do
    polymer
    |> Enum.reduce([], fn unit, result -> process_unit(unit, result) end)
  end

  def get_lower_upper_unicode_difference() do
    @lower_upper_unicode_difference
  end

  defp process_unit(unit, []) do
    [unit]
  end

  defp process_unit(unit, polymer) do
    [head | rest] = polymer
    <<h::utf8>> = head
    <<u::utf8>> = unit

    case abs(h - u) do
      @lower_upper_unicode_difference -> rest
      _ -> [unit | polymer]
    end
  end
end

Part one:

defmodule AoC.DayFive.PartOne do
  alias AoC.DayFive.Common

  def main() do
    "lib/day5/input.txt"
    |> Common.read_input()
    |> Common.process()
    |> Enum.count()
  end
end

Part two:

defmodule AoC.DayFive.PartTwo do
  alias AoC.DayFive.Common

  @lower_a 97
  @lower_z 122

  def main() do
    "lib/day5/input.txt"
    |> Common.read_input()
    |> process()
  end

  defp process(polymer) do
    Enum.reduce(@lower_a..@lower_z, Enum.count(polymer), fn lower_unicode, length ->
      higher_unicode = lower_unicode - Common.get_lower_upper_unicode_difference()
      lower_character = <<lower_unicode::utf8>>
      higher_character = <<higher_unicode::utf8>>

      polymer_length =
        polymer
        |> Enum.filter(fn x -> x != lower_character and x != higher_character end)
        |> Common.process()
        |> Enum.count()

      min(polymer_length, length)
    end)
  end
end

[Carly Ho 🌈]

PHP

Second part looks kind of long because I figured it would be faster to just declare the letters rather than generate the letters programmatically. I'm pretty sure there's a way to improve performance here, since this has a long-ish runtime, but I wasn't up to optimizing performance at midnight, haha. I might fiddle with this some more to see if I can do better.

Part 1:

<?php
$polymer = str_split(trim(file_get_contents($argv[1])));
do {
  $destruction = false;
  for ($i = 0; $i < count($polymer)-1; $i++) {
    $val = $polymer[$i];
    $next = $polymer[$i+1];
    if (($val == strtoupper($next) && strtoupper($next) != $next) || ($val == strtolower($next) && strtolower($next) != $next)) {
      $destruction = true;
      array_splice($polymer, $i, 2);
      break;
    }
  }
} while ($destruction);
echo count($polymer);
die(1);

Part 2:

<?php
$basepolymer = str_split(trim(file_get_contents($argv[1])));
$polymer = $basepolymer;
$shortest;
$units = array(
  array('a', 'A'),
  array('b', 'B'),
  array('c', 'C'),
  array('d', 'D'),
  array('e', 'E'),
  array('f', 'F'),
  array('g', 'G'),
  array('h', 'H'),
  array('i', 'I'),
  array('j', 'J'),
  array('k', 'K'),
  array('l', 'L'),
  array('m', 'M'),
  array('n', 'N'),
  array('o', 'O'),
  array('p', 'P'),
  array('q', 'Q'),
  array('r', 'R'),
  array('s', 'S'),
  array('t', 'T'),
  array('u', 'U'),
  array('v', 'V'),
  array('w', 'W'),
  array('x', 'X'),
  array('y', 'Y'),
  array('z', 'Z')
);
foreach ($units as $unit) {
  $polymer = array_filter($polymer, function($c) {
    global $unit;
    if (in_array($c, $unit)) {
      return false;
    }
    return true;
  });
  do {
    $destruction = false;
    for ($i = 0; $i < count($polymer)-1; $i++) {
      $val = $polymer[$i];
      $next = $polymer[$i+1];
      if (($val == strtoupper($next) && strtoupper($next) != $next) || ($val == strtolower($next) && strtolower($next) != $next)) {
        $destruction = true;
        array_splice($polymer, $i, 2);
        break;
      }
    }
  } while ($destruction);
  if (!$shortest || count($polymer) < $shortest) {
    $shortest = count($polymer);
  }
  echo $shortest;
  $polymer = $basepolymer;
}

echo $shortest;
die(1);

[Bjarne Magnussen]

I am using Advent of Code to learn Golang, and here is the solution I came up with. Suggestions for improvements are always welcome!

I was first using a list which would go from left to right and remove pairs as they were found reacting, shifting one step to the left and continue finding pairs that react. However, this could be simplified by using Ali Spittel's stack solution in Python.

With this inspiration, here is my Golang solution. I have added timing functions just to print out the running time as I have compared them to Python it is finally a speed up using Golang for the solution in this case!

package main

import (
    "bufio"
    "fmt"
    "os"
    "strings"
    "time"
    "unicode"
)

// readLines reads a whole file into memory
// and returns a slice of its lines.
func readLines(path string) ([]string, error) {
    file, err := os.Open(path)
    if err != nil {
        return nil, err
    }
    defer file.Close()

    var lines []string
    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        lines = append(lines, scanner.Text())
    }
    return lines, scanner.Err()
}

func invertChar(c rune) rune {
    if unicode.ToUpper(c) == c {
        return unicode.ToLower(c)
    }
    return unicode.ToUpper(c)
}

func react(polymer string) int {
    stack := []rune{}
    var top rune
    for _, unit := range polymer {
        if len(stack) == 0 {
            // Initialise stack:
            stack = append(stack, unit)
            continue
        }
        top = stack[len(stack)-1]
        if top == invertChar(unit) {
            // Consume last unit:
            stack = stack[:len(stack)-1]
        } else {
            stack = append(stack, unit)
        }
    }
    return len(stack)
}

func main() {
    data, err := readLines("input")
    if err != nil {
        panic(err)
    }

    polymerString := data[0]

    // Part 1
    start := time.Now()
    fmt.Println(react(polymerString))
    elapsed := time.Since(start)
    fmt.Println(elapsed)

    // Part 2:
    start = time.Now()
    var reduced string
    minReact := -1
    charSet := "ABCDEFGHIJKLMNOPQRSTUVXYZ"
    for _, c := range charSet {
        reduced = strings.Replace(polymerString, string(c), "", -1)
        reduced = strings.Replace(reduced, string(unicode.ToLower(c)), "", -1)
        r := react(reduced)
        if minReact == -1 || r < minReact {
            minReact = r
        }
    }
    fmt.Println(minReact)
    elapsed = time.Since(start)
    fmt.Println(elapsed)

}

[Ryan Palo]

Nice!

[Ben Lovy]

...welp, that was a solid "d'oh" moment, opening this thread. Hindsight is 20/20.

Here's the brute force way in F#. I'll have to come back to it after work to implement the smart way.

module util =
  let getInput inputFile =
    System.IO.File.ReadAllText(inputFile)

  let doesReact first second =
    (System.Char.ToUpper first = System.Char.ToUpper second) && first <> second

  let rec reactString altered result input =
    match input with
    | [] -> if altered then reactString false "" (string result |> List.ofSeq) else result
    | [a] -> if altered then reactString false "" (string result + string a |> List.ofSeq) else result + string a
    | head::next::tail ->
      if doesReact head next then
        reactString true result tail
      else
        reactString altered (result + string head) ([next] @ tail)

module part1 =
  open util

  let execute inputFile =
    getInput inputFile
    |> List.ofSeq
    |> reactString false ""
    |> String.length

module part2 =
  open util
  let cleanInput targetPair input =
    List.filter (fun el -> el <> System.Char.ToUpper targetPair && el <> System.Char.ToLower targetPair) input
  let execute inputFile =
     let input = getInput inputFile |> List.ofSeq
     let allPairs = [ for l in 'a' .. 'z' do yield l ]
     let res = List.map (fun el -> (el, reactString false "" (cleanInput el input))) allPairs
     List.sortBy (fun el -> String.length (snd el)) res
     |> List.head
     |> snd
     |> String.length

[Ben Lovy]

Yep, stack-based trick worked just fine:

  let reactQuickly input =
    Seq.fold (fun s c ->
      let last = if Array.length s > 0 then Some (Array.last s) else None
      match last with
      | Some x ->
        if c <> x && (x = System.Char.ToUpper c || x = System.Char.ToLower c) then
          Array.sub s 0 (Array.length s - 1)
        else Array.append s [| c |]
      | None -> Array.append s [| c |]) [| |] input
        |> Array.length

Replacing reactString with reactQuickly was enough to bring the total runtime from several (many) minutes to around 3 seconds.

Using the tip from @choroba and reducing everything first would probably speed it up even more.

[sneens]

Hey, here's my solution with ruby.

DATA = File.open('input.txt')
main_data = DATA.read
char_hash = Hash.new { |h, k| h[k] = [] }

def reduce_string(string)
  loop do
    last_length = string.size
    ('a'..'z').each do |letter|
      string.gsub!("#{letter}#{letter.upcase}", '')
      string.gsub!("#{letter.upcase}#{letter}", '')
    end
    break if last_length == string.size
  end
  string.size
end

def remove_letters(string, letter)
  string = string.dup
  string.gsub!(letter, '')
  string.gsub!(letter.upcase, '')
end

('a'..'z').each do |letter|
  char_hash[letter] = reduce_string(remove_letters(main_data, letter))
end

puts reduce_string(main_data)
puts char_hash.values.min

[Ryan Palo]

Nice and clean!

[E. Choroba]

I reached for regular expressions in Perl. I wasn't sure about the performance, but 0.4s for the part 2 seemed enough.

The regex is constructed dynamically and simply lists all the possible pairs to remove.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $regex = join '|', map { +"$_\u$_", "\u$_$_" } 'a' .. 'z';
                                                  # fix highlighting bug: '
chomp( my $input = <> );

1 while $input =~ s/$regex//g;
say length $input;

For the part 2, I discovered you can start from the already reduced polymer, which improved the performance more than 10 times.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $regex = join '|', map { +"$_\u$_", "\u$_$_" } 'a' .. 'z';
                                                  # fix highlighting bug: '
chomp( my $input = <> );

1 while $input =~ s/$regex//g;
my $min = length $input;
for my $letter ('a' .. 'z') {
    next unless $input =~ /$letter/i;

    my $removed = $input =~ s/$letter//gri;
    1 while $removed =~ s/$regex//g;
    $min = length $removed if length $removed < $min;
}

say $min;

[Ryan Palo]

I originally had an iterative, while-loopy, pointer-based solution, and that ran OK, but like a lot of people, I realized that it's basically an extended version of the "Matching Parentheses" problem, and that a stack would let me iterate through the list more with less backward steps. Now it runs pretty quick!

Rust's lack of easy handling of upper- and lowercase characters kind of bummed me out.

Part 1

/// Day 5: Alchemical Reduction

use std::collections::HashSet;

// Part 1: How many units remain after fully reacting a polymer

/// Reduce down a polymer by dropping any mer pairs
/// 
/// A mer pair is any lowercase letter and its corresponding uppercase
/// letter, adjacent to each other
/// Optionally, provide a "drop_mer" to drop unconditionally, case insenitive
/// pair or not
pub fn reduce(polymer: &str, drop_mer: Option<char>) -> String {
    let mut result: Vec<char> = Vec::new();
    for c in polymer.chars() {
        if matches_drop_mer(c, drop_mer) { continue; }
        if result.last().is_some() && is_polymer_pair(*result.last().unwrap(), c) {
            result.pop();
        } else {
            result.push(c);
        }
    }
    result.into_iter().collect()
}

fn is_polymer_pair(first: char, second: char) -> bool {
    (first.is_lowercase() && second.is_uppercase() && 
        second.to_lowercase().next().unwrap() == first) ||
    (first.is_uppercase() && second.is_lowercase() &&
        second.to_uppercase().next().unwrap() == first)
}

fn matches_drop_mer(c: char, drop_mer: Option<char>) -> bool {
    drop_mer.is_some()
    && c.to_lowercase().next().unwrap() == drop_mer.unwrap().to_lowercase().next().unwrap()
}

Part 2

For part 2, I didn't iterate over all 26 letters of the alphabet no matter what, I used a set to figure out what characters were in the input and only loop over those. In this case, it doesn't help, because the input has all 26 anyhow. Oh well. I learned a lot about .collect() today.

// Part 2: Figure out which polymer, when removed, allows the most
// compacting, remove it, and return the length of the shortest polymer
// after compaction.

/// Optimizes a polymer by figuring out which *one* mer is inhibiting
/// reduction the most and removing it.  The reduced string is returned
pub fn optimize(polymer: &str) -> String {
    let possible_mers: HashSet<char> = polymer.to_lowercase().chars().collect();
    let mut result_candidates: Vec<String> = Vec::new();

    for mer in possible_mers.into_iter() {
        result_candidates.push(reduce(polymer, Some(mer)));
    }

    result_candidates.into_iter()
        .min_by_key(|candidate| candidate.len())
        .expect("No result candidates found.")
}

[Jon Bristow]

Kotlin Solution

A nice easy one! Just fold into a stack and bada-bing-bada-boom!

Part 1

I started with String and String.last. This got me the solution, but it turned out to be really slow. Here's my updated code using a Java LinkedList instead, which is multiple orders of magnitude faster.

fun String.react() =
    fold(LinkedList<Char>()) { acc, c ->
        when {
            acc.isEmpty() -> acc.push(c)
            acc.peek() == c -> acc.push(c)
            acc.peek().equals(c, ignoreCase = true) -> acc.push(c)
            else -> acc.pop()
        }
        acc
    }

fun answer1(input: String) = input.react().count()

Part 2

This one seemed a little too simple at first, but my brute force implementation worked in about a 1500ms (still noticeably slow). After checking that my answer was correct, I did the stack optimization above which pulled me back down to 80ms or so.

fun answer2(input: String) =
    ('a'..'z').zip('A'..'Z')
        .map { (lc, uc) ->
            input.filterNot { it == lc || it == uc }
                .react()
                .count()
        }.min()

Overall, this was a nice mid-week breather, and I'm happy to spend more of my thought cycles doing my OSCP lab work.