2023.09

challenges/2023/09.js

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+// Part 1
+const solvePuzzle1 = (input) => {
+  const histories = parseSequences(input);
+  return addValues(histories, extrapolateForwards);
+};
+
+// Part 2
+const solvePuzzle2 = (input) => {
+  const histories = parseSequences(input);
+  return addValues(histories, extrapolateBackwards);
+};
+
+// Helpers
+
+const addValues = (histories, extraPolate) => {
+  let sum = 0;
+
+  histories.forEach((history) => {
+    const sequences = iterate(history);
+    sum += extraPolate(sequences);
+  });
+
+  return sum;
+};
+
+const getLast = (arr) => arr[arr.length - 1];
+const parseSequences = (input) => input.split('\n').map((l) => l.split(' ').map((n) => Number(n)));
+
+const iterate = (history) => {
+  const currentSequences = [history];
+
+  while (getLast(currentSequences).some((n) => n !== 0)) {
+    const newSeq = [];
+    const lastSeq = getLast(currentSequences);
+
+    lastSeq.forEach((n, idx) => {
+      idx && newSeq.push(n - lastSeq[idx - 1]);
+    });
+
+    currentSequences.push(newSeq);
+  }
+
+  return currentSequences;
+};
+
+const extrapolateBackwards = (sequences) => {
+  getLast(sequences).unshift(0);
+
+  for (let i = sequences.length - 2; i >= 0; i--) {
+    sequences[i].unshift(sequences[i][0] - sequences[i + 1][0]);
+  }
+
+  return sequences[0][0];
+};
+
+const extrapolateForwards = (sequences) => {
+  getLast(sequences).push(0);
+
+  for (let i = sequences.length - 2; i >= 0; i--) {
+    sequences[i].push(getLast(sequences[i]) + getLast(sequences[i + 1]));
+  }
+
+  return getLast(sequences[0]);
+};
+
+require(__dirname + '/../../utils/test.js').test(__filename, __dirname, solvePuzzle1, '1');
+require(__dirname + '/../../utils/test.js').test(__filename, __dirname, solvePuzzle2, '2');