const fs = require("fs");

let [rules, messages] = fs
  .readFileSync("input", "utf-8")
  .trim()
  .split("\n\n")
  .map((part) => part.split("\n"));

rules = Object.fromEntries(rules.map((line) => line.split(": ")));

// unecessary, but might reveal an overlooked loop
rules["8"] = "42 | 42 8";
rules["11"] = "42 31 | 42 11 31";

// console.log(rules);
// console.log(messages);

function getRegex(idx, visited = []) {
  const rule = rules[idx];
  if (rule.startsWith('"')) {
    return rule.slice(1, rule.length - 1);
  }
  let regex = "";
  let multiple = false;
  for (const symbol of rule.split(" ")) {
    if (symbol === "|") {
      regex = "(" + regex + "|";
      multiple = true;
    } else {
      regex = regex + getRegex(parseInt(symbol), visited.concat(idx));
    }
  }
  regex = multiple ? regex + ")" : regex;
  rules[idx] = '"' + regex + '"';
  return regex;
}

// Rule 0 is "8 11"
// These rules are the only looping rules through self referencing
// rule 8: 42 | 42 8
//   expects rule 42 one or more times
// rule 11: 42 31 | 42 11 31
//   recursively expects R42 R11 R31
//   translated:
//   expects rule 42 one or more times followed by 31 by the same quantity
// combined:
//   rule 0: 8 11
//   start of string matches 42 (one or more)
//   end of string matches 31 (one or more)
//   the quantity of R42 matches has to be more than the quantity of R31

const rule42 = getRegex(42);
const rule31 = getRegex(31);
const possibleEndings = [];

// 10 is a guessed maximum repetition based message length
// if this value is too high the regex wont compile
// for larger messages a recursive approach is necessary
for (let reps = 1; reps <= 10; reps++) {
  possibleEndings.push(`((${rule42.repeat(reps)})(${rule31.repeat(reps)}))`);
}
const patternRaw = `^(${rule42})+(${possibleEndings.join("|")})$`;
// console.log("Pattern:", patternRaw);
console.log("Compiling RegExp...");
const pattern = new RegExp(patternRaw);
console.log("done.");

console.log("Checking messages...");
let sum = 0;
for (let message of messages) {
  if (pattern.test(message)) sum++;
}
console.log(sum);