# two-p-set

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## Purpose

Provides `TwoPSet<T>`, a Two-Phase Set CRDT. Elements can be added and removed exactly once; removed elements can never be re-added. The set value is the difference between the add-set and the remove-set. This guarantees eventual consistency at the cost of restricting re-addition.

## Features

- `add(element)` — adds to the A-set; throws `OPERATION_INVALID` if element was previously removed
- `remove(element)` — moves element to the R-set; no-op if not in A-set
- `has(element)` — true if in A-set but not R-set
- `value()` — returns a `Set<T>` of live elements
- `merge(state)` — union-merges both A-set and R-set
- `toJSON()` / `fromJSON()` — serialization round-trip
- `clone()` — deep copy

## Usage: Basic Example

```ts
import { TwoPSet } from "@suaveplan/crdt";

const set = new TwoPSet<string>();
set.add("alice");
set.remove("alice");
console.log(set.has("alice")); // false
```

## Usage: Advanced Example

```ts
import { TwoPSet } from "@suaveplan/crdt";

const a = new TwoPSet<number>();
a.add(1);
const b = new TwoPSet<number>();
b.add(2);
a.merge(b.state());
console.log([...a.value()]); // [1, 2]
```

## API Reference

### `TwoPSet<T>`

- `add(element)` — throws `OPERATION_INVALID` if previously removed
- `remove(element)` — no-op if element not in A-set
- `has(element): boolean`
- `value(): Set<T>`
- `merge(state: TwoPSetState<T>): void`

## Implementation Notes

Elements are serialised to JSON for key comparison, enabling support for object elements with structural equality. The restriction that removed elements cannot be re-added is a fundamental invariant of 2P-Set that enables simple merge logic.