/*
--- Day 19: Monster Messages ---

You land in an airport surrounded by dense forest. As you walk to your high-speed train, the Elves
at the Mythical Information Bureau contact you again. They think their satellite has collected an
image of a sea monster! Unfortunately, the connection to the satellite is having problems, and many
of the messages sent back from the satellite have been corrupted.

They sent you a list of the rules valid messages should obey and a list of received messages they've
collected so far (your puzzle input).

The rules for valid messages (the top part of your puzzle input) are numbered and build upon each
other. For example:

0: 1 2
1: "a"
2: 1 3 | 3 1
3: "b"

Some rules, like 3: "b", simply match a single character (in this case, b).

The remaining rules list the sub-rules that must be followed; for example, the rule 0: 1 2 means
that to match rule 0, the text being checked must match rule 1, and the text after the part that
matched rule 1 must then match rule 2.

Some of the rules have multiple lists of sub-rules separated by a pipe (|). This means that at least
one list of sub-rules must match. (The ones that match might be different each time the rule is
encountered.) For example, the rule 2: 1 3 | 3 1 means that to match rule 2, the text being checked
must match rule 1 followed by rule 3 or it must match rule 3 followed by rule 1.

Fortunately, there are no loops in the rules, so the list of possible matches will be finite. Since
rule 1 matches a and rule 3 matches b, rule 2 matches either ab or ba. Therefore, rule 0 matches aab
or aba.

Here's a more interesting example:

0: 4 1 5
1: 2 3 | 3 2
2: 4 4 | 5 5
3: 4 5 | 5 4
4: "a"
5: "b"

Here, because rule 4 matches a and rule 5 matches b, rule 2 matches two letters that are the same
(aa or bb), and rule 3 matches two letters that are different (ab or ba).

Since rule 1 matches rules 2 and 3 once each in either order, it must match two pairs of letters,
one pair with matching letters and one pair with different letters. This leaves eight possibilities:
aaab, aaba, bbab, bbba, abaa, abbb, baaa, or babb.

Rule 0, therefore, matches a (rule 4), then any of the eight options from rule 1, then b (rule 5):
aaaabb, aaabab, abbabb, abbbab, aabaab, aabbbb, abaaab, or ababbb.

The received messages (the bottom part of your puzzle input) need to be checked against the rules so
you can determine which are valid and which are corrupted. Including the rules and the messages
together, this might look like:

0: 4 1 5
1: 2 3 | 3 2
2: 4 4 | 5 5
3: 4 5 | 5 4
4: "a"
5: "b"

ababbb
bababa
abbbab
aaabbb
aaaabbb

Your goal is to determine the number of messages that completely match rule 0. In the above example,
ababbb and abbbab match, but bababa, aaabbb, and aaaabbb do not, producing the answer 2. The whole
message must match all of rule 0; there can't be extra unmatched characters in the message. (For
example, aaaabbb might appear to match rule 0 above, but it has an extra unmatched b on the end.)

How many messages completely match rule 0?
*/

package com.curtislb.adventofcode.year2020.day19.part1

import com.curtislb.adventofcode.common.io.forEachSection
import com.curtislb.adventofcode.year2020.day19.rule.parseRule
import java.nio.file.Path
import java.nio.file.Paths

/**
 * Returns the solution to the puzzle for 2020, day 19, part 1.
 *
 * @param inputPath The path to the input file for this puzzle.
 */
fun solve(inputPath: Path = Paths.get("..", "input", "input.txt")): Int {
    val file = inputPath.toFile()
    val ruleStrings = mutableMapOf<Int, String>()
    val messages = mutableListOf<String>()
    file.forEachSection { lines ->
        if (ruleStrings.isEmpty()) {
            for (line in lines) {
                val (ruleKeyString, ruleString) = line.split(':').map { it.trim() }
                ruleStrings[ruleKeyString.toInt()] = ruleString
            }
        } else {
            messages.addAll(lines)
        }
    }

    val primaryRule = parseRule(ruleStrings[0]!!, ruleStrings)
    val primaryRuleRegex = primaryRule.toRegex()
    return messages.count { primaryRuleRegex.matches(it) }
}

fun main() {
    println(solve())
}
