## Example

```typescript before
/** @derive(PartialEq, Hash) */
class User {
  id: number;
  name: string;

  @partialEq(skip) // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

Generated output:

```typescript before
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

Generated output:

```typescript before
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

Generated output:

```typescript before
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

Generated output:

```typescript before
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

Generated output:

```typescript before
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

```typescript after
class User {
  id: number;
  name: string;

  // Don't compare cached values
  /** @hash({ skip: true }) */
  cachedScore: number;

  equals(other: unknown): boolean {
    if (this === other) return true;
    if (!(other instanceof User)) return false;
    const typedOther = other as User;
    return this.id === typedOther.id && this.name === typedOther.name;
  }

  hashCode(): number {
    let hash = 17;
    hash = (hash * 31 +
      (Number.isInteger(this.id) ? this.id | 0 : this.id
        .toString()
        .split("")
        .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    hash = (hash * 31 +
      (this.name ?? "").split("").reduce(
        (h, c) => (h * 31 + c.charCodeAt(0)) | 0,
        0,
      )) |
      0;
    hash = (hash * 31 +
      (Number.isInteger(this.cachedScore)
        ? this.cachedScore | 0
        : this.cachedScore
          .toString()
          .split("")
          .reduce((h, c) => (h * 31 + c.charCodeAt(0)) | 0, 0))) |
      0;
    return hash;
  }
}
```

## Equality Contract

When implementing `PartialEq`, consider also implementing `Hash`:

- **Reflexivity**: `a.equals(a)` is always true
- **Symmetry**: `a.equals(b)` implies `b.equals(a)`
- **Hash consistency**: Equal objects must have equal hash codes