Naive-Then-Optimize Skill

Trigger

Use when implementing algorithms, data transformations, or any code where correctness is critical.

The Insight

Karpathy: "Write the naive algorithm that is very likely correct first, then ask it to optimize it while preserving correctness."

Why This Works

1. •Naive implementations are easier to verify as correct
2. •They serve as a reference/oracle for testing optimized versions
3. •Optimization bugs are caught by comparing to naive version
4. •You might find the naive version is fast enough

Process

Step 1: Implement the Obvious Solution

Don't think about performance. Think about correctness. Write the dumbest, most straightforward code that works.

// Naive: O(n²) but obviously correct
function findDuplicates(arr: number[]): number[] {
  const duplicates: number[] = [];
  for (let i = 0; i < arr.length; i++) {
    for (let j = i + 1; j < arr.length; j++) {
      if (arr[i] === arr[j] && !duplicates.includes(arr[i])) {
        duplicates.push(arr[i]);
      }
    }
  }
  return duplicates;
}

Step 2: Write Tests Against Naive Implementation

describe('findDuplicates', () => {
  it('finds duplicates', () => {
    expect(findDuplicates([1, 2, 2, 3])).toEqual([2]);
  });
  it('handles multiple duplicates', () => {
    expect(findDuplicates([1, 1, 2, 2, 3])).toEqual([1, 2]);
  });
  it('handles no duplicates', () => {
    expect(findDuplicates([1, 2, 3])).toEqual([]);
  });
  it('handles empty array', () => {
    expect(findDuplicates([])).toEqual([]);
  });
});

Step 3: Verify Naive Implementation Passes

Run the tests. They should all pass. If they don't, fix the naive implementation first.

Step 4: Measure Performance

Is the naive version fast enough? Profile it with realistic data.

console.time('naive');
findDuplicates(largeArray);
console.timeEnd('naive');

If it's fast enough, stop here. Don't optimize code that doesn't need it.

Step 5: Optimize While Preserving Behavior

Now optimize, but keep the same tests passing:

// Optimized: O(n) using a Set
function findDuplicates(arr: number[]): number[] {
  const seen = new Set<number>();
  const duplicates = new Set<number>();
  for (const num of arr) {
    if (seen.has(num)) {
      duplicates.add(num);
    }
    seen.add(num);
  }
  return Array.from(duplicates);
}

Step 6: Property-Based Testing (Optional)

For critical code, test that naive and optimized produce same results:

it('optimized matches naive for random inputs', () => {
  for (let i = 0; i < 1000; i++) {
    const input = generateRandomArray();
    const naiveResult = findDuplicatesNaive(input);
    const optimizedResult = findDuplicates(input);
    expect(optimizedResult.sort()).toEqual(naiveResult.sort());
  }
});

When to Use This

• •Algorithm implementations
• •Data transformations
• •Parsers
• •Anything where correctness matters more than cleverness

When to Skip This

• •Simple CRUD operations
• •Code that's obviously correct
• •When the naive version would be identical to the optimized version