2020 day 19 part 1 (C) - won't work for part 2 !!
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2020/day19/README.org
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2020/day19/README.org
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** --- Day 19: Monster Messages ---
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You land in an airport surrounded by dense forest. As you walk to your
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high-speed train, the Elves at the Mythical Information Bureau contact
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you again. They think their satellite has collected an image of a /sea
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monster/! Unfortunately, the connection to the satellite is having
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problems, and many of the messages sent back from the satellite have
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been corrupted.
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They sent you a list of /the rules valid messages should obey/ and a
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list of /received messages/ they've collected so far (your puzzle
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input).
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The /rules for valid messages/ (the top part of your puzzle input) are
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numbered and build upon each other. For example:
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#+BEGIN_EXAMPLE
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0: 1 2
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1: "a"
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2: 1 3 | 3 1
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3: "b"
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#+END_EXAMPLE
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Some rules, like =3: "b"=, simply match a single character (in this
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case, =b=).
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The remaining rules list the sub-rules that must be followed; for
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example, the rule =0: 1 2= means that to match rule =0=, the text being
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checked must match rule =1=, and the text after the part that matched
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rule =1= must then match rule =2=.
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Some of the rules have multiple lists of sub-rules separated by a pipe
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(=|=). This means that /at least one/ list of sub-rules must match. (The
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ones that match might be different each time the rule is encountered.)
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For example, the rule =2: 1 3 | 3 1= means that to match rule =2=, the
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text being checked must match rule =1= followed by rule =3= /or/ it must
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match rule =3= followed by rule =1=.
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Fortunately, there are no loops in the rules, so the list of possible
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matches will be finite. Since rule =1= matches =a= and rule =3= matches
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=b=, rule =2= matches either =ab= or =ba=. Therefore, rule =0= matches
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=aab= or =aba=.
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Here's a more interesting example:
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#+BEGIN_EXAMPLE
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0: 4 1 5
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1: 2 3 | 3 2
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2: 4 4 | 5 5
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3: 4 5 | 5 4
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4: "a"
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5: "b"
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#+END_EXAMPLE
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Here, because rule =4= matches =a= and rule =5= matches =b=, rule =2=
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matches two letters that are the same (=aa= or =bb=), and rule =3=
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matches two letters that are different (=ab= or =ba=).
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Since rule =1= matches rules =2= and =3= once each in either order, it
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must match two pairs of letters, one pair with matching letters and one
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pair with different letters. This leaves eight possibilities: =aaab=,
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=aaba=, =bbab=, =bbba=, =abaa=, =abbb=, =baaa=, or =babb=.
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Rule =0=, therefore, matches =a= (rule =4=), then any of the eight
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options from rule =1=, then =b= (rule =5=): =aaaabb=, =aaabab=,
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=abbabb=, =abbbab=, =aabaab=, =aabbbb=, =abaaab=, or =ababbb=.
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The /received messages/ (the bottom part of your puzzle input) need to
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be checked against the rules so you can determine which are valid and
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which are corrupted. Including the rules and the messages together, this
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might look like:
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#+BEGIN_EXAMPLE
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0: 4 1 5
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1: 2 3 | 3 2
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2: 4 4 | 5 5
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3: 4 5 | 5 4
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4: "a"
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5: "b"
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ababbb
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bababa
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abbbab
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aaabbb
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aaaabbb
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#+END_EXAMPLE
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Your goal is to determine /the number of messages that completely match
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rule =0=/. In the above example, =ababbb= and =abbbab= match, but
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=bababa=, =aaabbb=, and =aaaabbb= do not, producing the answer /=2=/.
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The whole message must match all of rule =0=; there can't be extra
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unmatched characters in the message. (For example, =aaaabbb= might
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appear to match rule =0= above, but it has an extra unmatched =b= on the
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end.)
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/How many messages completely match rule =0=?/
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Your puzzle answer was =285=.
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** --- Part Two ---
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As you look over the list of messages, you realize your matching rules
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aren't quite right. To fix them, completely replace rules =8: 42= and
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=11: 42 31= with the following:
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#+BEGIN_EXAMPLE
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8: 42 | 42 8
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11: 42 31 | 42 11 31
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#+END_EXAMPLE
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This small change has a big impact: now, the rules /do/ contain loops,
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and the list of messages they could hypothetically match is infinite.
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You'll need to determine how these changes affect which messages are
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valid.
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Fortunately, many of the rules are unaffected by this change; it might
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help to start by looking at which rules always match the same set of
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values and how /those/ rules (especially rules =42= and =31=) are used
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by the new versions of rules =8= and =11=.
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(Remember, /you only need to handle the rules you have/; building a
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solution that could handle any hypothetical combination of rules would
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be [[https://en.wikipedia.org/wiki/Formal_grammar][significantly more
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difficult]].)
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For example:
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#+BEGIN_EXAMPLE
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42: 9 14 | 10 1
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9: 14 27 | 1 26
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10: 23 14 | 28 1
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1: "a"
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11: 42 31
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5: 1 14 | 15 1
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19: 14 1 | 14 14
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12: 24 14 | 19 1
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16: 15 1 | 14 14
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31: 14 17 | 1 13
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6: 14 14 | 1 14
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2: 1 24 | 14 4
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0: 8 11
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13: 14 3 | 1 12
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15: 1 | 14
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17: 14 2 | 1 7
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23: 25 1 | 22 14
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28: 16 1
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4: 1 1
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20: 14 14 | 1 15
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3: 5 14 | 16 1
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27: 1 6 | 14 18
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14: "b"
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21: 14 1 | 1 14
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25: 1 1 | 1 14
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22: 14 14
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8: 42
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26: 14 22 | 1 20
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18: 15 15
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7: 14 5 | 1 21
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24: 14 1
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abbbbbabbbaaaababbaabbbbabababbbabbbbbbabaaaa
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bbabbbbaabaabba
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babbbbaabbbbbabbbbbbaabaaabaaa
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aaabbbbbbaaaabaababaabababbabaaabbababababaaa
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bbbbbbbaaaabbbbaaabbabaaa
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bbbababbbbaaaaaaaabbababaaababaabab
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ababaaaaaabaaab
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ababaaaaabbbaba
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baabbaaaabbaaaababbaababb
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abbbbabbbbaaaababbbbbbaaaababb
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aaaaabbaabaaaaababaa
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aaaabbaaaabbaaa
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aaaabbaabbaaaaaaabbbabbbaaabbaabaaa
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babaaabbbaaabaababbaabababaaab
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aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba
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#+END_EXAMPLE
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Without updating rules =8= and =11=, these rules only match three
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messages: =bbabbbbaabaabba=, =ababaaaaaabaaab=, and =ababaaaaabbbaba=.
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However, after updating rules =8= and =11=, a total of /=12=/ messages
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match:
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- =bbabbbbaabaabba=
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- =babbbbaabbbbbabbbbbbaabaaabaaa=
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- =aaabbbbbbaaaabaababaabababbabaaabbababababaaa=
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- =bbbbbbbaaaabbbbaaabbabaaa=
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- =bbbababbbbaaaaaaaabbababaaababaabab=
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- =ababaaaaaabaaab=
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- =ababaaaaabbbaba=
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- =baabbaaaabbaaaababbaababb=
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- =abbbbabbbbaaaababbbbbbaaaababb=
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- =aaaaabbaabaaaaababaa=
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- =aaaabbaabbaaaaaaabbbabbbaaabbaabaaa=
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- =aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba=
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/After updating rules =8= and =11=, how many messages completely match
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rule =0=?/
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Your puzzle answer was =412=.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, all that is left is for you to [[/2020][admire your
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Advent calendar]].
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If you still want to see it, you can [[file:19/input][get your puzzle
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input]].
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You can also [Shareon
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[[https://twitter.com/intent/tweet?text=I%27ve+completed+%22Monster+Messages%22+%2D+Day+19+%2D+Advent+of+Code+2020&url=https%3A%2F%2Fadventofcode%2Ecom%2F2020%2Fday%2F19&related=ericwastl&hashtags=AdventOfCode][Twitter]]
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[[javascript:void(0);][Mastodon]]] this puzzle.
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