feat: Day 1: Chronal Calibration (part 2)

Author: mariotacke

day-01-chronal-calibration/README.md

@@ -19,5 +19,30 @@ Here are other example situations:
 
 Starting with a frequency of zero, **what is the resulting frequency** after all of the changes in frequency have been applied?
 
+## Part Two
+
+You notice that the device repeats the same frequency change list over and over. To calibrate the device, you need to find the first frequency it reaches **twice**.
+
+For example, using the same list of changes above, the device would loop as follows:
+
+- Current frequency ` 0`, change of `+1`; resulting frequency ` 1`.
+- Current frequency ` 1`, change of `-2`; resulting frequency `-1`.
+- Current frequency `-1`, change of `+3`; resulting frequency ` 2`.
+- Current frequency ` 2`, change of `+1`; resulting frequency ` 3`.
+- (At this point, the device continues from the start of the list.)
+- Current frequency ` 3`, change of `+1`; resulting frequency ` 4`.
+- Current frequency ` 4`, change of `-2`; resulting frequency ` 2`, which has already been seen.
+
+In this example, the first frequency reached twice is `2`. Note that your device might need to repeat its list of frequency changes many times before a duplicate frequency is found, and that duplicates might be found while in the middle of processing the list.
+
+Here are other examples:
+
+- `+1, -1` first reaches `0` twice.
+- `+3, +3, +4, -2, -4` first reaches `10` twice.
+- `-6, +3, +8, +5, -6` first reaches `5` twice.
+- `+7, +7, -2, -7, -4` first reaches `14` twice.
+
+**What is the first frequency your device reaches twice?**
+
 ## References
 - https://adventofcode.com/2018/day/1

day-01-chronal-calibration/chronal-calibration2.js

@@ -0,0 +1,33 @@
+const calibration = (input) => {
+  const frequencies = new Set([0]);
+
+  const changes = input
+    .split('\n')
+    .map((x) => parseInt(x));
+
+  let frequency = 0;
+  let i = 0;
+
+  // eslint-disable-next-line no-constant-condition
+  while (true) {
+    if (i === changes.length) {
+      i = 0;
+
+      continue;
+    }
+
+    frequency += changes[i];
+
+    if (frequencies.has(frequency)) {
+      break;
+    }
+
+    frequencies.add(frequency);
+
+    i++;
+  }
+
+  return frequency;
+};
+
+module.exports = calibration;

day-01-chronal-calibration/test.js

@@ -1,6 +1,7 @@
 const assert = require('assert');
 
 const chronalCalibration = require('./chronal-calibration');
+const chronalCalibration2 = require('./chronal-calibration2');
 
 describe('Day 1: Chronal Calibration', () => {
   it('should calculate frequency from +1, -2, +3, +1', () => {
@@ -39,4 +40,57 @@ describe('Day 1: Chronal Calibration', () => {
 
     assert.strictEqual(chronalCalibration(changes), -6);
   });
+
+  describe('Part Two', () => {
+    it('should calculate first duplicate frequency from +1, -2, +3, +1', () => {
+      const changes =
+        `+1
+         -2
+         +3
+         +1`;
+
+      assert.strictEqual(chronalCalibration2(changes), 2);
+    });
+
+    it('should calculate first duplicate frequency from +1, -1', () => {
+      const changes =
+        `+1
+         -1`;
+
+      assert.strictEqual(chronalCalibration2(changes), 0);
+    });
+
+    it('should calculate first duplicate frequency from +3, +3, +4, -2, -4', () => {
+      const changes =
+        `+3
+         +3
+         +4
+         -2
+         -4`;
+
+      assert.strictEqual(chronalCalibration2(changes), 10);
+    });
+
+    it('should calculate first duplicate frequency from -6, +3, +8, +5, -6', () => {
+      const changes =
+        `-6
+         +3
+         +8
+         +5
+         -6`;
+
+      assert.strictEqual(chronalCalibration2(changes), 5);
+    });
+
+    it('should calculate first duplicate frequency from +7, +7, -2, -7, -4', () => {
+      const changes =
+        `+7
+         +7
+         -2
+         -7
+         -4`;
+
+      assert.strictEqual(chronalCalibration2(changes), 14);
+    });
+  });
 });