# doctrine-orm-0002 — ClassMetadata::addSubClass: O(n²) subclass dedup during metadata loading

**Status:** OPEN
**Severity:** MEDIUM
**Component:** `doctrine/orm` — `src/Doctrine/ORM/Mapping/ClassMetadata.php`
**Affects:** Application startup / metadata cache warming; entity class hierarchies with many subclasses

---

## Root Cause

`ClassMetadata.php:2313` — `addSubClass()`:

```php
// DEFECTIVE — O(subclasses) linear scan per addSubClass call
public function addSubClass(string $className): void
{
    if (is_subclass_of($className, $this->name) && ! in_array($className, $this->subClasses, true)) {
        $this->subClasses[] = $className;
    }
}
```

`$this->subClasses` is a plain PHP array (`public array $subClasses = []`, line 314).
`in_array` scans the entire array for every call.

`addSubClass()` is called inside loops in `ClassMetadataFactory`:

- `ClassMetadataFactory.php:152`: `$class->addSubClasses($parent->subClasses)` — bulk add loop
- `ClassMetadataFactory.php:386`: inside `foreach ($parentClasses as $parentClass)` loop inside
  `findAbstractEntityClassesNotListedInDiscriminatorMap()`

The second call site is the worst case: for each discriminator map entry, `getParentClasses()`
returns all ancestor classes, and `addSubClass` is called for each. With S subclasses and
D depth, total comparisons: O(S × D × S) = O(S²×D).

---

## Complexity Analysis

| Metric | Defective | Fixed |
|--------|-----------|-------|
| Single addSubClass | O(current subclass count) | O(1) |
| Full hierarchy load (S subclasses) | O(S²) | O(S) |
| Growth pattern | Quadratic | Linear |

---

## Fix

Replace `$this->subClasses` with a parallel indexed structure, or use a `Set`-equivalent
(array keyed by class name):

```php
// Keep public array $subClasses for backward compat, add private set for O(1) lookup
private array $subClassSet = [];

public function addSubClass(string $className): void
{
    if (is_subclass_of($className, $this->name) && ! isset($this->subClassSet[$className])) {
        $this->subClassSet[$className] = true;
        $this->subClasses[] = $className;
    }
}
```

Alternatively, key `$subClasses` by class name directly (breaking change to indexed access).

---

## Call Sites

| File | Line | Context |
|------|------|---------|
| `src/Mapping/ClassMetadata.php` | 2313 | `addSubClass()` |
| `src/Mapping/ClassMetadataFactory.php` | 152 | bulk inherit from parent |
| `src/Mapping/ClassMetadataFactory.php` | 386 | discriminator map abstract class discovery |

---

## Speedup Estimate

Metadata loading for a hierarchy with 50 subclasses: ~50× reduction in membership check work.
Impact is startup/warm-up only (metadata is cached after first load). Applications without
metadata caching (e.g., dev mode with `auto_generate_proxy_classes`) see this on every request.