Add Day 2

day-2/README.md

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+# Day 2: Red-Nosed Reports
+
+<br>
+
+## Part 1
+
+Fortunately, the first location The Historians want to search isn't a long walk from the Chief Historian's office.
+
+While the [Red-Nosed Reindeer nuclear fusion/fission plant](https://adventofcode.com/2015/day/19) appears to contain no sign of the Chief
+Historian, the engineers there run up to you as soon as they see you. Apparently, they **still** talk about the time Rudolph was saved
+through molecular synthesis from a single electron.
+
+They're quick to add that - since you're already here - they'd really appreciate your help analyzing some unusual data from the Red-Nosed
+reactor. You turn to check if The Historians are waiting for you, but they seem to have already divided into groups that are currently
+searching every corner of the facility. You offer to help with the unusual data.
+
+The unusual data (your puzzle input) consists of many **reports**, one report per line. Each report is a list of numbers called **levels**
+that are separated by spaces. For example:
+
+```txt
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9
+```
+
+This example data contains six reports each containing five levels.
+
+The engineers are trying to figure out which reports are **safe**. The Red-Nosed reactor safety systems can only tolerate levels that are
+either gradually increasing or gradually decreasing. So, a report only counts as safe if both of the following are true:
+
+- The levels are either **all increasing** or **all decreasing**.
+- Any two adjacent levels differ by **at least one** and **at most three**.
+
+In the example above, the reports can be found safe or unsafe by checking those rules:
+
+- `7 6 4 2 1`: **Safe** because the levels are all decreasing by 1 or 2.
+- `1 2 7 8 9`: **Unsafe** because `2 7` is an increase of 5.
+- `9 7 6 2 1`: **Unsafe** because `6 2` is a decrease of 4.
+- `1 3 2 4 5`: **Unsafe** because `1 3` is increasing but `3 2` is decreasing.
+- `8 6 4 4 1`: **Unsafe** because `4 4` is neither an increase or a decrease.
+- `1 3 6 7 9`: **Safe** because the levels are all increasing by 1, 2, or 3.
+
+So, in this example, `2` reports are **safe**.
+
+Analyze the unusual data from the engineers. **How many reports are safe?**
+
+<br>
+
+## Part 2
+
+The engineers are surprised by the low number of safe reports until they realize they forgot to tell you about the Problem Dampener.
+
+The Problem Dampener is a reactor-mounted module that lets the reactor safety systems **tolerate a single bad level** in what would
+otherwise be a safe report. It's like the bad level never happened!
+
+Now, the same rules apply as before, except if removing a single level from an unsafe report would make it safe, the report instead counts
+as safe.
+
+More of the above example's reports are now safe:
+
+- `7 6 4 2 1`: **Safe** without removing any level.
+- `1 2 7 8 9`: **Unsafe** regardless of which level is removed.
+- `9 7 6 2 1`: **Unsafe** regardless of which level is removed.
+- `1 3 2 4 5`: **Safe** by removing the second level, `3`.
+- `8 6 4 4 1`: **Safe** by removing the third level, `4`.
+- `1 3 6 7 9`: **Safe** without removing any level.
+
+Thanks to the Problem Dampener, `4` reports are actually **safe**!
+
+Update your analysis by handling situations where the Problem Dampener can remove a single level from unsafe reports. **How many reports are
+now safe?**

day-2/day-2.test.ts

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+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import path from 'node:path';
+
+import { determineNumberOfSafeReports, determineNumberOfSafeReportsWithProblemDampener } from './day-2';
+
+describe('Day 2: Red-Nosed Reports', () => {
+  const reportsFilePath = path.join(__dirname, 'reports.txt');
+
+  it('Part 1: should determine number of safe reports', async () => {
+    const expectedNumberOfSafeReports = 660; // Verified for this dataset
+
+    const numberOfSafeReports = await determineNumberOfSafeReports(reportsFilePath);
+
+    assert.strictEqual(numberOfSafeReports, expectedNumberOfSafeReports);
+  });
+
+  it('Part 2: should determine number of safe reports with problem dampener', async () => {
+    const expectedNumberOfSafeReportsWithProblemDampener = 689; // Verified for this dataset
+
+    const numberOfSafeReports = await determineNumberOfSafeReportsWithProblemDampener(reportsFilePath);
+
+    assert.strictEqual(numberOfSafeReports, expectedNumberOfSafeReportsWithProblemDampener);
+  });
+});

day-2/day-2.ts

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+import fs from 'node:fs/promises';
+
+/**
+ * Read file
+ */
+const readFile = async (filePath: string): Promise<string> => {
+  const fileContents = await fs.readFile(filePath, {
+    encoding: 'utf8',
+  });
+  const normalizedFileContents = fileContents.trim().split(/\r?\n/).join('\n');
+  return normalizedFileContents;
+};
+
+/**
+ * Parse reports
+ */
+const parseReports = (reportsFileContent: string) => {
+  return reportsFileContent.split('\n').map((levelsAsString) => {
+    return levelsAsString.split(' ').map((levelAsString) => {
+      return parseInt(levelAsString, 10);
+    });
+  });
+};
+
+/**
+ * Check whether the given levels are safe
+ */
+const areLevelsSafe = (levels: Array<number>) => {
+  // Calculate difference between each adjacent level
+  const levelDifferences: Array<number> = [];
+  for (let levelIndex = 0; levelIndex < levels.length - 1; levelIndex++) {
+    levelDifferences.push(levels[levelIndex] - levels[levelIndex + 1]);
+  }
+
+  // Check whether all level differences are within the safe range
+  const safeLevelRange = [1, 3];
+  const areLevelDifferencesWithinSafeRange = levelDifferences.every((levelDifference) => {
+    return Math.abs(levelDifference) >= safeLevelRange[0] && Math.abs(levelDifference) <= safeLevelRange[1];
+  });
+  if (!areLevelDifferencesWithinSafeRange) {
+    return false; // Early exit
+  }
+
+  // Check whether all level differences continue in one direction (increase or decrease)
+  const hasAnyPositiveLevelDifferences = levelDifferences.some((levelDiff) => {
+    return levelDiff > 0;
+  });
+  const hasAnyNegativeLevelDifferences = levelDifferences.some((levelDiff) => {
+    return levelDiff < 0;
+  });
+  const doAllLevelsContinueInOneDirection =
+    (hasAnyPositiveLevelDifferences && !hasAnyNegativeLevelDifferences) ||
+    (hasAnyNegativeLevelDifferences && !hasAnyPositiveLevelDifferences);
+  if (!doAllLevelsContinueInOneDirection) {
+    return false; // Early exit
+  }
+
+  // All good :)
+  return true;
+};
+
+/**
+ * Part 1: Determine number of safe reports
+ */
+export const determineNumberOfSafeReports = async (reportsFilePath: string) => {
+  // Get data
+  const reportsFileContent = await readFile(reportsFilePath);
+  const reports = parseReports(reportsFileContent);
+
+  // Determine number of safe reports
+  const numberOfSafeReports = reports.filter((levels) => {
+    return areLevelsSafe(levels);
+  }).length;
+
+  // Done
+  return numberOfSafeReports;
+};
+
+/**
+ * Part 2: Determine number of safe reports with problem dampener
+ */
+export const determineNumberOfSafeReportsWithProblemDampener = async (reportsFilePath: string) => {
+  // Get data
+  const reportsFileContent = await readFile(reportsFilePath);
+  const reports = parseReports(reportsFileContent);
+
+  // Determine number of safe reports with problem dampener
+  const numberOfSafeReportsWithProblemDampener = reports.filter((levels) => {
+    // Construct levels in all variants (#bruteforce)
+    const levelVariants = [levels];
+    for (let levelIndex = 0; levelIndex < levels.length; levelIndex++) {
+      levelVariants.push([...levels.slice(0, levelIndex), ...levels.slice(levelIndex + 1)]);
+    }
+
+    // Check every variant
+    let isAnyLevelVariantSafe = false;
+    for (let levelVariantIndex = 0; levelVariantIndex < levelVariants.length; levelVariantIndex++) {
+      const isLevelVariantSafe = areLevelsSafe(levelVariants[levelVariantIndex]);
+      if (isLevelVariantSafe) {
+        isAnyLevelVariantSafe = true;
+        break; // Early exit
+      }
+    }
+
+    // Done
+    return isAnyLevelVariantSafe;
+  }).length;
+
+  // Done
+  return numberOfSafeReportsWithProblemDampener;
+};