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#+HTML_LINK_UP: ../../index.html#2020
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#+TITLE: Day 02 - Password Philosophy

* Puzzle
- This puzzle is taken from: https://adventofcode.com/2020/day/2

Your flight departs in a few days from the coastal airport; the easiest
way down to the coast from here is via toboggan.

The shopkeeper at the North Pole Toboggan Rental Shop is having a bad
day. "Something's wrong with our computers; we can't log in!" You ask if
you can take a look.

Their password database seems to be a little corrupted: some of the
passwords wouldn't have been allowed by the Official Toboggan Corporate
Policy that was in effect when they were chosen.

To try to debug the problem, they have created a list (your puzzle
input) of passwords (according to the corrupted database) and the
corporate policy when that password was set.

For example, suppose you have the following list:
#+BEGIN_SRC
1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc
#+END_SRC

Each line gives the password policy and then the password. The password
policy indicates the lowest and highest number of times a given letter
must appear for the password to be valid. For example, 1-3 a means that
the password must contain a at least 1 time and at most 3 times.

In the above example, 2 passwords are valid. The middle password, cdefg,
is not; it contains no instances of b, but needs at least 1. The first
and third passwords are valid: they contain one a or nine c, both within
the limits of their respective policies.

How many passwords are valid according to their policies?
** Part 2
While it appears you validated the passwords correctly, they don't seem
to be what the Official Toboggan Corporate Authentication System is
expecting.

The shopkeeper suddenly realizes that he just accidentally explained the
password policy rules from his old job at the sled rental place down the
street! The Official Toboggan Corporate Policy actually works a little
differently.

Each policy actually describes two positions in the password, where 1
means the first character, 2 means the second character, and so on. (Be
careful; Toboggan Corporate Policies have no concept of "index zero"!)
Exactly one of these positions must contain the given letter. Other
occurrences of the letter are irrelevant for the purposes of policy
enforcement.

Given the same example list from above:

- =1-3 a: abcde= is valid: position 1 contains a and position 3 does not.
- =1-3 b: cdefg= is invalid: neither position 1 nor position 3 contains b.
- =2-9 c: ccccccccc= is invalid: both position 2 and position 9 contain c.

How many passwords are valid according to the new interpretation of the
policies?
* Solution
Raku grammar is used to parse the file. I took the grammar from [[https://git.tyil.nl/advent-of-code/tree/2020/02.raku][tyil]]'s
solution.
#+BEGIN_SRC raku
# Password grammar was taken from tyil's solution.
grammar Password {
    rule TOP { <num> '-' <num> <letter> ':' <password> }
    token num { \d+ }
    token letter { \w }
    token password { \w+ }
}
#+END_SRC

The program takes a positional input =$part= which instructs it on which
part it should solve. The default is to solve for part 1.

=$valid_passwords= stores the number of passwords that are considered
valid. The checks are made inside the =if/elsif= loop & it continues from
there only if the password is considered valid.
#+BEGIN_SRC raku
sub MAIN (
    # Part to run.
    Int $part where * == 1|2 = 1
) {
    my $valid_passwords = 0;

    for "input".IO.lines -> $entry {
        if Password.parse($entry) -> $match {
            if $part == 1 {
                ...
            } elsif $part == 2 {
                ...
            }
            # All checks were completed & this is a valid password.
            $valid_passwords++;
        }
    }
    say "Part $part: " ~ $valid_passwords;
}
#+END_SRC

Nothing fancy is done, we just count the number of times each character
is used & then perform the check.
#+BEGIN_SRC raku
my %chars;
$match<password>.comb.map({ %chars{$_}++ });

# Check if the letter exists in the password.
next unless %chars{$match<letter>};

# Check for length.
next unless $match<num>[0].Int <= %chars{$match<letter>};
next unless %chars{$match<letter>} <= $match<num>[1].Int;
#+END_SRC
** Part 2
This part too just performs the check.
#+BEGIN_SRC raku
elsif $part == 2 {
    my $combed = $match<password>.comb;
    my $first = $match<num>[0].Int - 1;
    my $second = $match<num>[1].Int - 1;

    next if $combed[$first] eq $combed[$second];
    next unless $combed[$first] eq $match<letter> || $combed[$second] eq $match<letter>;
}
#+END_SRC