Prompt: Can you write an industrial strength quicksort algorithm in JavaScript?
___

Below is an **industrial-strength Quicksort** implementation in JavaScript.
It includes the techniques normally required for high-performance, production-grade behavior:

### ✔️ Features

* **Median-of-three pivot selection** (reduces worst-case risk)
* **Cutoff to insertion sort** for small partitions (improves speed)
* **Tail-recursion elimination / iterative partitioning** (prevents deep recursion)
* **In-place sorting**
* **Stable performance across partially sorted / adversarial data**

---

# Industrial-Strength Quicksort (JavaScript)

```js
/**
 * Industrial-strength in-place quicksort.
 * Uses:
 *  - Median-of-three pivoting
 *  - Insertion sort for small partitions
 *  - Tail recursion elimination
 */

function quicksort(arr) {
  if (!Array.isArray(arr) || arr.length < 2) return arr;
  quicksortInternal(arr, 0, arr.length - 1);
  return arr;
}

const INSERTION_SORT_THRESHOLD = 16;

// ------------------ Helper: Insertion Sort ------------------

function insertionSort(arr, left, right) {
  for (let i = left + 1; i <= right; i++) {
    const tmp = arr[i];
    let j = i - 1;
    while (j >= left && arr[j] > tmp) {
      arr[j + 1] = arr[j];
      j--;
    }
    arr[j + 1] = tmp;
  }
}

// ------------------ Helper: Median-of-Three Pivot ------------------

function medianOfThree(arr, a, b, c) {
  const A = arr[a], B = arr[b], C = arr[c];
  if (A < B) {
    if (B < C) return b;
    return A < C ? c : a;
  } else {
    if (A < C) return a;
    return B < C ? c : b;
  }
}

// ------------------ Main Quicksort Logic ------------------

function quicksortInternal(arr, left, right) {
  while (left < right) {

    // Use insertion sort for small segments
    if (right - left < INSERTION_SORT_THRESHOLD) {
      insertionSort(arr, left, right);
      return;
    }

    // Median-of-three pivot selection
    const mid = left + ((right - left) >> 1);
    const pivotIndex = medianOfThree(arr, left, mid, right);
    const pivot = arr[pivotIndex];

    // Move pivot out of the way
    swap(arr, pivotIndex, right);

    // Partitioning (Lomuto variant)
    let i = left;
    for (let j = left; j < right; j++) {
      if (arr[j] <= pivot) {
        swap(arr, i, j);
        i++;
      }
    }
    swap(arr, i, right);

    // Tail recursion elimination:
    // Recurse into smaller part; iterate on the larger.
    const leftSize = i - 1 - left;
    const rightSize = right - (i + 1);

    if (leftSize < rightSize) {
      quicksortInternal(arr, left, i - 1);
      left = i + 1;               // eliminate tail-recursion
    } else {
      quicksortInternal(arr, i + 1, right);
      right = i - 1;              // eliminate tail-recursion
    }
  }
}

// ------------------ Utility ------------------

function swap(arr, i, j) {
  const tmp = arr[i];
  arr[i] = arr[j];
  arr[j] = tmp;
}

// ------------------ Usage ------------------

let data = [9, 2, 7, 4, 1, 3, 8, 6, 5];
console.log(quicksort(data));  // sorted output
```

---

# Why This Is “Industrial-Strength”

### 1. **Median-of-three pivoting**

Greatly reduces the chance that nearly sorted or reverse-sorted data cause O(n²).

### 2. **Insertion sort threshold**

For small partitions (≤16 elements), insertion sort is faster than recursion + partitioning.

### 3. **Tail recursion elimination**

Ensures the algorithm never blows the call stack on large arrays; maximum recursion depth is O(log n).

### 4. **In-place**

No extra allocations, cache-friendly, and suitable for large datasets.

---

If you'd like, I can also provide:

✅ A stable version (slow but stable)
✅ A version optimized for V8 (Chrome/Node) performance quirks
✅ A fully iterative (stack-based) version
✅ A TypeScript version

Just tell me!
____
Prompt: Could you generate test cases for this code?
____
Here are **comprehensive, industrial-strength test cases** for the Quicksort implementation you asked for.
They cover correctness, edge cases, randomized stress testing, adversarial inputs, and comparisons with the built-in `.sort()` for verification.

You can run these in Node.js, a browser, or a test framework like Jest/Mocha.

---

# ✅ **Test Suite (Plain JavaScript)**

```js
// Import or paste your quicksort() implementation here

function deepEqual(a, b) {
  return JSON.stringify(a) === JSON.stringify(b);
}

function runTest(name, fn) {
  try {
    fn();
    console.log(`✔️  ${name}`);
  } catch (err) {
    console.error(`❌  ${name}`);
    console.error(err);
  }
}

/* ===============================
 * BASIC FUNCTIONALITY TESTS
 * =============================== */

runTest("Sort already sorted list", () => {
  const arr = [1, 2, 3, 4, 5];
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, arr)) throw new Error("Mismatch");
});

runTest("Sort reverse sorted list", () => {
  const arr = [5, 4, 3, 2, 1];
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, [1,2,3,4,5])) throw new Error("Mismatch");
});

runTest("Sort random small list", () => {
  const arr = [9, 1, 5, 3, 2, 8];
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});

runTest("Sort with duplicates", () => {
  const arr = [5, 3, 5, 2, 2, 9, 1, 5];
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});

runTest("Sort an array with all identical elements", () => {
  const arr = new Array(20).fill(7);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, arr)) throw new Error("Mismatch");
});

runTest("Sort array with negative numbers", () => {
  const arr = [-5, -1, 0, 3, -2, 8];
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});

runTest("Sort array with floats", () => {
  const arr = [1.1, 0.1, 3.5, 2.2, -1.8];
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});


/* ===============================
 * EDGE CASE TESTS
 * =============================== */

runTest("Empty array", () => {
  const arr = [];
  const sorted = quicksort([]);
  if (!deepEqual(sorted, [])) throw new Error("Mismatch");
});

runTest("Single-element array", () => {
  const arr = [42];
  const sorted = quicksort([42]);
  if (!deepEqual(sorted, arr)) throw new Error("Mismatch");
});

runTest("Two-element array", () => {
  const arr = [2, 1];
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, [1, 2])) throw new Error("Mismatch");
});

runTest("Handles extremely large numbers", () => {
  const arr = [Number.MAX_SAFE_INTEGER, -999999999999, 0, 500, 42];
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});


/* ===============================
 * STRESS + ADVERSE CASES
 * =============================== */

runTest("Random large array (10k elements)", () => {
  const arr = Array.from({ length: 10000 }, () => Math.floor(Math.random() * 1e6));
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});

runTest("Adversarial input: already sorted with repetition", () => {
  const arr = [];
  for (let i = 0; i < 5000; i++) arr.push(i % 50);
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});

runTest("Adversarial input: descending order with duplicates", () => {
  const arr = [];
  for (let i = 5000; i >= 0; i--) arr.push(i % 10);
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});


/* ===============================
 * VERY LARGE STRESS TEST (optional)
 * =============================== */

// Only run this if you want to test for real industrial loads
/*
runTest("Very large array (100k elements)", () => {
  const arr = Array.from({ length: 100000 }, () => Math.floor(Math.random() * 1e6));
  const expected = [...arr].sort((a, b) => a - b);
  const sorted = quicksort([...arr]);
  if (!deepEqual(sorted, expected)) throw new Error("Mismatch");
});
*/
```

---

# 📌 What These Tests Cover

### ✔ Correctness

Comparisons against JavaScript’s built-in `.sort()` ensure accuracy.

### ✔ Edge Cases

* Empty arrays
* Single element
* Two elements
* All elements identical
* Negative / floating numbers
* Very large values

### ✔ Adversarial Cases

Designed to trigger worst-case patterns in naive quicksort:

* Already sorted arrays
* Reverse sorted arrays
* Duplicates-heavy arrays

### ✔ Stress Tests

10,000–100,000 item random arrays to ensure performance and stack stability.

---

# Want a version for Jest / Mocha?

I can generate a fully structured Jest or Mocha test file with `describe()` and `it()` blocks as well — just tell me your preferred framework.