import { IdentitySet, Value } from 'preserves';

export type Assertion = Value<Ref>;
export type Handle = number;
export type ExitReason = null | { ok: true } | { ok: false, err: Error };
export type LocalAction = (t: Turn) => void;

export interface Entity {
    assert(turn: Turn, assertion: Assertion, handle: Handle): void;
    retract(turn: Turn, handle: Handle): void;
    message(turn: Turn, body: Assertion): void;
    sync(turn: Turn, peer: Ref): void;
}

export interface Ref { readonly relay: Actor, readonly target: Partial<Entity> };

export class Actor {
    readonly outbound: Map<Handle, Ref>;
    exitReason: ExitReason = null;

    constructor(initialAssertions = new Map<Handle, Ref>()) {
        this.outbound = initialAssertions;
    }

    terminateWith(t: Turn, reason: Exclude<ExitReason, null>) {
        if (this.exitReason !== null) return;
        this.exitReason = reason;
        this.outbound.forEach((peer, h) => t._retract(peer, h));
    }

    execute(proc: () => void): void {
        queueMicrotask(() => {
            if (this.exitReason !== null) return;
            try {
                proc();
            } catch (err) {
                console.error(Actor, err);
                Turn.for(this, t => this.terminateWith(t, { ok: false, err }));
            }
        });
    }
}

let nextHandle = 0;

export function _sync(turn: Turn, e: Partial<Entity>, peer: Ref): void {
    e.sync ? e.sync!(turn, peer) : turn.message(peer, true);
}

export class Turn {
    readonly actor: Actor;
    readonly queues = new Map<Actor, LocalAction[]>();

    static for(actor: Actor, f: LocalAction): void {
        const t = new Turn(actor);
        f(t);
        t.queues.forEach((q, a) => a.execute(() => q.forEach(f => Turn.for(a, f))));
    }

    private constructor(actor: Actor) {
        this.actor = actor;
    }

    ref(e: Partial<Entity>): Ref {
        return { relay: this.actor, target: e };
    }

    spawn(bootProc: LocalAction, initialAssertions = new IdentitySet<Handle>()): void {
        this.enqueue(this.actor, () => {
            const newOutbound = new Map<Handle, Ref>();
            initialAssertions.forEach(key => {
                newOutbound.set(key, this.actor.outbound.get(key)!); // we trust initialAssertions
                this.actor.outbound.delete(key);
            });
            const child = new Actor(newOutbound);
            child.execute(() => Turn.for(child, bootProc));
        });
    }

    quit(): void {
        this.enqueue(this.actor, t => this.actor.terminateWith(t, { ok: true }));
    }

    assert(ref: Ref, assertion: Assertion): Handle {
        const h = nextHandle++;
        this.enqueue(ref.relay, t => {
            this.actor.outbound.set(h, ref);
            ref.target.assert?.(t, assertion, h);
        });
        return h;
    }

    retract(h: Handle): void {
        this._retract(this.actor.outbound.get(h)!, h);
    }

    replace(ref: Ref, h: Handle | undefined, assertion: Assertion): Handle {
        const newHandle = this.assert(ref, assertion);
        if (h !== void 0) this.retract(h);
        return newHandle;
    }

    _retract(ref: Ref, handle: Handle): void {
        this.enqueue(ref.relay, t => {
            this.actor.outbound.delete(handle);
            ref.target.retract?.(t, handle);
        });
    }

    sync(ref: Ref): Promise<Turn> {
        return new Promise(resolve => this.enqueue(ref.relay, t =>
            _sync(t, ref.target, this.ref({ message: resolve }))));
    }

    message(ref: Ref, assertion: Assertion): void {
        this.enqueue(ref.relay, t => ref.target.message?.(t, assertion));
    }

    enqueue(relay: Actor, a: LocalAction): void {
        this.queues.get(relay)?.push(a) ?? this.queues.set(relay, [a]);
    }
}

export type AttenuationFilter = (assertion: Assertion) => Assertion | null;

export class Attenuation implements Entity {
    readonly target: Partial<Entity>;
    readonly filter: AttenuationFilter;

    constructor(target: Partial<Entity>, filter: AttenuationFilter) {
        this.target = target;
        this.filter = filter;
    }

    assert(turn: Turn, assertion: Assertion, handle: Handle): void {
        const filtered = this.filter(assertion);
        if (filtered !== null) this.target.assert?.(turn, filtered, handle);
    }

    retract(turn: Turn, handle: Handle): void {
        // TODO: consider whether we want targets to even see blocked handles
        this.target.retract?.(turn, handle);
    }

    message(turn: Turn, body: Assertion): void {
        const filtered = this.filter(body);
        if (filtered !== null) this.target.message?.(turn, filtered);
    }

    sync(turn: Turn, peer: Ref): void {
        _sync(turn, this.target, peer);
    }
}

export function attenuate(ref: Ref, filter: AttenuationFilter): Ref {
    return { relay: ref.relay, target: new Attenuation(ref.target, filter) };
}