2018 day 12 complete

2018/12/part2.js

@@ -0,0 +1,70 @@
+const fs = require('fs'); // file system
+const input = fs.readFileSync(`${__dirname}/input.txt`, 'utf-8')	// read the input file
+	.split('\n');	// split on lines
+
+// get the initial state from the input
+let state = input[0].slice('initial state: '.length);
+
+let rules = {};
+
+// extract all rules from input
+for (const line of input.splice(2)) {
+	rules[line.slice(0, 5)] = line.slice(9);
+}
+
+// to add extra pots as if there were endless pots
+function expand (s) {
+	let r = s.replace(/^(\.+)?/, '.....');
+	r = r.replace(/(\.+)?$/, '.....');
+	return r;
+}
+
+let zero = 5;
+// add empty pots
+state = expand(state);
+
+let done = false;
+for (var i = 0; i < 250; i++){
+
+
+	if (done) {
+		var gen = i;
+		break;
+	}
+
+	// create 'empty' array of '.'s
+	const pots = Array(3).join('.').split('');
+
+	state.split('').forEach((_, n) => {
+		// get the section to look at of prev gen
+		const sect = state.slice(n-2, n+3);
+		if (sect.length !== 5) { return }
+
+		// get the value of the matching rule
+		let plant = rules[sect];
+		if (!plant) { return }
+
+		// replace the value in the next gen
+		pots.push(plant);
+	});
+	// move the '0th pot' marker
+	zero -= pots.indexOf('#') - 5;
+
+	// if the pattern is repeating. process is done.
+	done = expand(pots.join('')) === state;
+
+	// store the generation with extra pots.
+	state = expand(pots.join(''));
+
+
+	// console.log(Array(20-zero).join(' ')+state);	// output with pot 0 aligned
+	// console.log(state);	// output misaligned
+
+}
+
+// calculate the value of last gen
+const result = state.split('').reduce((val, pot, index) => {
+	return (Number(val) || 0) + (pot === '#' ? index - zero : 0);
+})
+
+console.log(result + (50000000000 - gen) * state.match(/#/g).length)