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143 lines
4.1 KiB
Forth
143 lines
4.1 KiB
Forth
# Day 2: Password Philosophy
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Your flight departs in a few days from the coastal
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airport; the easiest way down to the coast from
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here is via toboggan.
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The shopkeeper at the North Pole Toboggan Rental
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Shop is having a bad day. "Something's wrong with
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our computers; we can't log in!" You ask if you can
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take a look.
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Their password database seems to be a little
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corrupted: some of the passwords wouldn't have been
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allowed by the Official Toboggan Corporate Policy
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that was in effect when they were chosen.
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To try to debug the problem, they have created a
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list (your puzzle input) of passwords (according
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to the corrupted database) and the corporate
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policy when that password was set.
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For example, suppose you have the following list:
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1-3 a: abcde
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1-3 b: cdefg
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2-9 c: ccccccccc
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Each line gives the password policy and then the
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password. The password policy indicates the lowest
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and highest number of times a given letter must
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appear for the password to be valid. For example,
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`1-3 a` means that the password must contain `a` at
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least 1 time and at most 3 times.
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In the above example, 2 passwords are valid. The
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middle password, cdefg, is not; it contains no
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instances of b, but needs at least 1. The first and
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third passwords are valid: they contain one a or
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nine c, both within the limits of their respective
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policies.
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How many passwords are valid according to their
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policies?
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----
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This is an easy problem. Just a little parsing and reduction.
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`range` is defined to parse the limits of the character. This
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just means splitting the string and converting the pieces to
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a number.
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The `reduce` word is slighly more complex. It takes the
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password and the character and constructs a combinator for
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use in filtering out other characters.
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The remaining characters will be counted and compared to
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the range.
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Basically, an input line like:
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1-3 a: abcde
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Becomes:
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'abcde [ $a eq? ] s:filter s:length #1 #3 n:between?
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At this point, I just add up the flags (-1 for TRUE), and
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use `n:abs` to get the final count.
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~~~
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:range
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#0 a:fetch $- s:tokenize [ s:to-number ] a:for-each ;
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:reduce
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[ #2 a:fetch ] [ #1 a:fetch fetch &eq? curry ] bi s:filter ;
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:process (na-n)
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[ reduce s:length ] [ range ] bi n:between? + ;
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#0 'input-day-2 [ ASCII:SPACE s:tokenize process ] file:for-each-line
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n:abs
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~~~
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----
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# Part 2
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While it appears you validated the passwords
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correctly, they don't seem to be what the Official
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Toboggan Corporate Authentication System is expecting.
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The shopkeeper suddenly realizes that he just
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accidentally explained the password policy rules from
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his old job at the sled rental place down the street!
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The Official Toboggan Corporate Policy actually works
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a little differently.
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Each policy actually describes two positions in the
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password, where 1 means the first character, 2 means
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the second character, and so on. (Be careful; Toboggan
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Corporate Policies have no concept of "index zero"!)
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Exactly one of these positions must contain the given
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letter. Other occurrences of the letter are irrelevant
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for the purposes of policy enforcement.
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Given the same example list from above:
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1-3 a: abcde is valid: position 1 contains a and position 3 does not.
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1-3 b: cdefg is invalid: neither position 1 nor position 3 contains b.
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2-9 c: ccccccccc is invalid: both position 2 and position 9 contain c.
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How many passwords are valid according to the new
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interpretation of the policies?
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----
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So in this variation, I need to check two positions per
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password.
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Being lazy here, I'm using `reorder` a couple of times
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to restructure the stack. The only thing to remember is
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that indexing is one based, so the word that extracts
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the positions needs to decrement to adjust for Retro's
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zero based indexing.
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~~~
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:grab-characters
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'abc 'abac reorder + fetch [ + fetch ] dip ;
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:check
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'abc 'abac reorder eq? [ eq? ] dip xor + ;
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:positions
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#0 a:fetch $- s:tokenize [ s:to-number n:dec ] a:for-each ;
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:process (na-n)
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[ #1 a:fetch fetch ] [ #2 a:fetch ] [ positions ] tri
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grab-characters check ;
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#0 'input-day-2 [ ASCII:SPACE s:tokenize process ] file:for-each-line
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n:abs
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~~~
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