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74 lines
2.1 KiB
JavaScript
74 lines
2.1 KiB
JavaScript
4 years ago
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const fs = require("fs");
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let [rules, messages] = fs
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.readFileSync("input", "utf-8")
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.trim()
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.split("\n\n")
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.map((part) => part.split("\n"));
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rules = Object.fromEntries(rules.map((line) => line.split(": ")));
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// unecessary, but might reveal an overlooked loop
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rules["8"] = "42 | 42 8";
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rules["11"] = "42 31 | 42 11 31";
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// console.log(rules);
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// console.log(messages);
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function getRegex(idx, visited = []) {
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const rule = rules[idx];
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if (rule.startsWith('"')) {
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return rule.slice(1, rule.length - 1);
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}
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let regex = "";
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let multiple = false;
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for (const symbol of rule.split(" ")) {
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if (symbol === "|") {
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regex = "(" + regex + "|";
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multiple = true;
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} else {
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regex = regex + getRegex(parseInt(symbol), visited.concat(idx));
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}
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}
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regex = multiple ? regex + ")" : regex;
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rules[idx] = '"' + regex + '"';
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return regex;
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}
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// Rule 0 is "8 11"
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// These rules are the only looping rules through self referencing
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// rule 8: 42 | 42 8
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// expects rule 42 one or more times
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// rule 11: 42 31 | 42 11 31
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// recursively expects R42 R11 R31
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// translated:
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// expects rule 42 one or more times followed by 31 by the same quantity
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// combined:
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// rule 0: 8 11
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// start of string matches 42 (one or more)
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// end of string matches 31 (one or more)
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// the quantity of R42 matches has to be more than the quantity of R31
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const rule42 = getRegex(42);
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const rule31 = getRegex(31);
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const possibleEndings = [];
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// 10 is a guessed maximum repetition based message length
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// if this value is too high the regex wont compile
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// for larger messages a recursive approach is necessary
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for (let reps = 1; reps <= 10; reps++) {
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possibleEndings.push(`((${rule42.repeat(reps)})(${rule31.repeat(reps)}))`);
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}
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const patternRaw = `^(${rule42})+(${possibleEndings.join("|")})$`;
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// console.log("Pattern:", patternRaw);
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console.log("Compiling RegExp...");
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const pattern = new RegExp(patternRaw);
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console.log("done.");
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console.log("Checking messages...");
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let sum = 0;
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for (let message of messages) {
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if (pattern.test(message)) sum++;
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}
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console.log(sum);
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