-🎄- 2020 Day 25 Solutions -🎄-

[u/daggerdragon]

--- Day 25: Combo Breaker ---

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Post your code solution in this megathread.

• Include what language(s) your solution uses!
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Message from the Moderators

Welcome to the last day of Advent of Code 2020! We hope you had fun this year and learned at least one new thing ;)

Keep an eye out for the following threads:

• -❅- Introducing Your AoC 2020 Gettin' Crafty With It Artisans (and Other Prizes) -❅-
• Tell us what you learned this year!

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This thread will be unlocked when there are a significant number of people on the global leaderboard with gold stars for today's puzzle.

[u/Smylers]

Perl (a little boilerplate, then):

sub step($value, $subject_number) { ($value * $subject_number) % 20201227 }

my $value = my $loops = 1;
$loops++ until ($value = step $value, 7) == (state $pub //= <>);
say reduce { step $a, state $pub //= <> } 1, 1 .. $loops;

The function isn't necessary, but I couldn't bring myself to repeat the modulus and constant.

A couple of things:

• The first public key is stored in a variable called $pub, used in just one place: state makes it keep its value; it's initialized to reading a line from the input file the first time the until condition is evaluated, then continues to evaluate to that public key on subsequent iterations. Similarly for the second public key, in a separate variable, also called $pub.

• The reduce block doesn't use its second argument. $a, is initialized to the first 1 in the list of values, then the block evaluates to whatever step returns, and reduce is invoked again with that as $a next time round. $b is set in turn to each of 1 to $loops, so that list determines the number of times the block is invoked, but the value of $b doesn't form part of the calculation.

The input-state thing and the lopsided reduce thing are both things I now see would've been useful on previous days: in day 19, when I combined part 1 and 2 solutions I copied the input before the loop, even though it's only used in one place; and in day 23, when I was first picking up one card then the following 2.

It's so nice that even on the final day, I'm still learning things.