# PartialOrd

The `PartialOrd` macro generates a `compareTo()` method for **partial ordering** comparison. This is
analogous to Rust's `PartialOrd` trait, enabling comparison between values where some pairs may be
incomparable.

## Generated Output

| Type       | Generated Code                                             | Description                                    |
| ---------- | ---------------------------------------------------------- | ---------------------------------------------- |
| Class      | `classNamePartialCompare(a, b)` + `static compareTo(a, b)` | Standalone function + static wrapper method    |
| Enum       | `enumNamePartialCompare(a, b): number \| null`             | Standalone function returning `number \| null` |
| Interface  | `ifaceNamePartialCompare(a, b): number \| null`            | Standalone function returning `number \| null` |
| Type Alias | `typeNamePartialCompare(a, b): number \| null`             | Standalone function returning `number \| null` |

Names use **camelCase** conversion (e.g., `Temperature` → `temperaturePartialCompare`).

## Return Values

Unlike `Ord`, `PartialOrd` returns `number | null` to handle incomparable values:

- **negative**: `a` is less than `b`
- **0**: `a` is equal to `b`
- **positive**: `a` is greater than `b`
- **null**: Values are incomparable

Note: results are **not** clamped to -1/0/1. String fields return the raw `localeCompare()` value,
which can be any negative or positive number (unlike `Ord`, which clamps `localeCompare` results).
Only check the sign of the result.

## When to Use PartialOrd vs Ord

- **PartialOrd**: When some values may not be comparable
  - Example: Floating-point NaN values
  - Example: Mixed-type unions
  - Example: Type mismatches between objects

- **Ord**: When all values are guaranteed comparable (total ordering)

## Comparison Strategy

Fields are compared **lexicographically** in declaration order:

1. Compare first field
2. If incomparable, return `null`
3. If not equal, return that result
4. Otherwise, compare next field
5. Continue until a difference is found or all fields are equal

## Type-Specific Comparisons

| Type              | Comparison Method                                         |
| ----------------- | --------------------------------------------------------- |
| `number`/`bigint` | Ternary comparison (`a < b ? -1 : a > b ? 1 : 0`)         |
| `string`          | `localeCompare()` (raw, unclamped result)                 |
| `boolean`         | `false < true` (cast to number)                           |
| null/undefined    | Returns `null` for mismatched nullability                 |
| Arrays            | Lexicographic, propagates `null` on incomparable elements |
| `Date`            | Timestamp comparison, `null` if invalid                   |
| Objects           | Delegates to `compareTo()` if available                   |

## Field-Level Options

The `@ord` decorator supports:

- `skip` - Exclude the field from ordering comparison

## Example

```typescript before
/** @derive(PartialOrd) */
class Temperature {
    value: number | null;
    unit: string;
}
```

```typescript after
class Temperature {
    value: number | null;
    unit: string;

    static compareTo(a: Temperature, b: Temperature): number | null {
        return temperaturePartialCompare(a, b);
    }
}

export function temperaturePartialCompare(a: Temperature, b: Temperature): number | null {
    if (a === b) return 0;
    const cmp0 = (() => {
        if (typeof (a.value as any)?.compareTo === 'function') {
            const optResult = (a.value as any).compareTo(b.value);
            return optResult === null ? null : optResult;
        }
        return a.value === b.value ? 0 : null;
    })();
    if (cmp0 === null) return null;
    if (cmp0 !== 0) return cmp0;
    const cmp1 = a.unit.localeCompare(b.unit);
    if (cmp1 === null) return null;
    if (cmp1 !== 0) return cmp1;
    return 0;
}
```

## Return Type

The generated functions return `number | null` where `null` indicates incomparable values.