const CANCEL = Symbol();

export interface CancellationToken {
    readonly IsCancellationRequested: boolean;
    ThrowIfCancellationRequested(): void;
}

export class CancellationTokenSource {
    readonly Token: CancellationToken;

    constructor() {
        this.Token = new CancellationTokenImpl();
    }

    Cancel() {
        this.Token[CANCEL]();
    }
}

export class TaskCancelledException extends Error {
    constructor() {
        super("Task Cancelled");
        Reflect.setPrototypeOf(this, TaskCancelledException.prototype);
    }
}

class CancellationTokenImpl implements CancellationToken {
    IsCancellationRequested: boolean;

    constructor() {
        this.IsCancellationRequested = false;
    }

    ThrowIfCancellationRequested() {
        if (this.IsCancellationRequested) {
            throw new TaskCancelledException();
        }
    }

    [CANCEL]() {
        this.IsCancellationRequested = true;
    }
}