retroforth/example/advent-of-code-2020-day-2.retro

# Day 2: Password Philosophy

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:

    1-3 a: abcde
    1-3 b: cdefg
    2-9 c: ccccccccc

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?

----

This is an easy problem. Just a little parsing and reduction.

`range` is defined to parse the limits of the character. This
just means splitting the string and converting the pieces to
a number.

The `reduce` word is slighly more complex. It takes the
password and the character and constructs a combinator for
use in filtering out other characters.

The remaining characters will be counted and compared to
the range.

Basically, an input line like:

    1-3 a: abcde

Becomes:

    'abcde [ $a eq? ] s:filter s:length #1 #3 n:between?

At this point, I just add up the flags (-1 for TRUE), and
use `n:abs` to get the final count.

~~~
:range
  #0 a:fetch $- s:tokenize [ s:to-number ] a:for-each ;

:reduce
  [ #2 a:fetch ] [ #1 a:fetch fetch &eq? curry ] bi s:filter ;

:process (na-n)
  [ reduce s:length ] [ range ] bi n:between? + ;

#0 'input-day-2 [ ASCII:SPACE s:tokenize process ] file:for-each-line
n:abs
~~~

----

# 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?

----

So in this variation, I need to check two positions per
password.

Being lazy here, I'm using `reorder` a couple of times
to restructure the stack. The only thing to remember is
that indexing is one based, so the word that extracts
the positions needs to decrement to adjust for Retro's
zero based indexing.

~~~
:grab-characters
  'abc 'abac reorder + fetch [ + fetch ] dip  ;

:check
  'abc 'abac reorder eq? [ eq? ] dip xor + ;

:positions
  #0 a:fetch $- s:tokenize [ s:to-number n:dec ] a:for-each ;

:process (na-n)
  [ #1 a:fetch fetch ] [ #2 a:fetch ] [ positions ] tri
  grab-characters check ;

#0 'input-day-2 [ ASCII:SPACE s:tokenize process ] file:for-each-line
n:abs
~~~