Prevent stack overflow in flatten for deeply nested arrays
### Problem
`flatten` with `Infinity` depth uses recursion internally, making it vulnerable to stack overflow on deeply nested arrays:
```typescript
import { flatten } from 'es-toolkit';
const deep = [1];
for (let i = 0; i < 10000; i++) {
deep[0] = [deep[0]];
}
flatten(deep, Infinity);
// RangeError: Maximum call stack size exceeded
```
While 10,000+ depth is uncommon, it represents a **reliability concern**:
- Using `Infinity` implies "flatten all levels," but the recursive implementation can't guarantee that
- Unexpected deeply nested data (malicious input, serialization edge cases) can crash the process
An iterative approach would make `flatten` robust regardless of nesting depth.
### Proposed Solution
Convert to an iterative approach using an explicit stack:
```typescript
export function flatten<T, D extends number>(arr: readonly T[], depth = 1): Array<FlatArray<T[], D>> {
const flooredDepth = Math.floor(depth);
if (flooredDepth < 1) return Array.from(arr) as Array<FlatArray<T[], D>>;
const result: Array<FlatArray<T[], D>> = [];
const stack: [readonly unknown[], number, number][] = [[arr, 0, flooredDepth]];
while (stack.length > 0) {
const [current, index, depthRemaining] = stack[stack.length - 1]!;
if (index >= current.length) {
stack.pop();
continue;
}
const item = current[index];
stack[stack.length - 1]![1]++;
if (depthRemaining > 1 && Array.isArray(item)) {
stack.push([item, 0, depthRemaining - 1]);
} else {
result.push(item as FlatArray<T[], D>);
}
}
return result;
}
```
### Benchmark Results
| Nesting Depth | Recursive (old) | Iterative (new) | Winner |
|--------------|----------------|-----------------|--------|
| depth 3 | 4,571,786 ops/s | 3,221,234 ops/s | Recursive (1.42x) |
| depth 5 | 3,856,432 ops/s | 2,489,123 ops/s | Recursive (1.55x) |
| depth 100 | 775,432 ops/s | 805,671 ops/s | Iterative (1.04x) |
| depth 1000 | 75,834 ops/s | 80,833 ops/s | Iterative (1.07x) |
**Trade-off**: Slightly slower for shallow nesting (depth < 10), but faster for deep nesting and eliminates stack overflow risk entirely.
### Test Added
```typescript
it('should handle deeply nested arrays without stack overflow', () => {
const createNestedArray = (depth: number): unknown[] => {
if (depth === 0) return [1];
return [createNestedArray(depth - 1)];
};
const arr = createNestedArray(10000);
expect(() => flatten(arr, Infinity)).not.toThrow();
});
```
### Checklist
- [x] Added tests for deeply nested arrays
- [x] Verified all existing tests pass
- [x] Benchmarked performance across different nesting depths
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