import {Awaitable, ErrorWithContext, JSONState, Maybe, Rehydratable, Cache} from '../../util'
import {CanonicalItemClass} from '../CanonicalReceiver'
import {Container, Inject, Injectable, isInstantiable} from '../../di'
import {Canon} from '../../service/Canon'

/** Type annotation for a Queueable that should be pushed onto a queue. */
export type ShouldQueue<T> = T & Queueable

/**
 * Base class for an object that can be pushed to/popped from a queue.
 */
export abstract class Queueable extends CanonicalItemClass implements Rehydratable {
    abstract dehydrate(): Awaitable<JSONState>

    abstract rehydrate(state: JSONState): Awaitable<void>

    /**
     * When the item is popped from the queue, this method is called.
     */
    public abstract execute(): Awaitable<void>

    /**
     * Determine whether the object should be pushed to the queue or not.
     */
    public shouldQueue(): boolean {
        return true
    }

    /**
     * The name of the queue where this object should be pushed by default.
     */
    public defaultQueue(): string {
        return 'default'
    }

    /**
     * Get the canonical resolver so we can re-instantiate this class from the queue.
     * Throw an error if it could not be determined.
     */
    public getFullyQualifiedCanonicalResolver(): string {
        const resolver = (this.constructor as typeof Queueable).getFullyQualifiedCanonicalResolver()
        if ( !resolver ) {
            throw new ErrorWithContext('Cannot push Queueable onto queue: missing canonical resolver.')
        }

        return resolver
    }
}

/**
 * Truth function that returns true if an object implements the same interface as Queueable.
 * This is done in case some external library needs to be incorporated as the base class for
 * a Queueable, and cannot be made to extend Queueable.
 * @param something
 */
export function isQueueable(something: unknown): something is Queueable {
    if ( something instanceof Queueable ) {
        return true
    }

    return (
        typeof something === 'function'
        && typeof (something as any).dehydrate === 'function'
        && typeof (something as any).rehydrate === 'function'
        && typeof (something as any).shouldQueue === 'function'
        && typeof (something as any).defaultQueue === 'function'
        && typeof (something as any).getFullyQualifiedCanonicalResolver === 'function'
    )
}

/**
 * Truth function that returns true if the given object is Queueable and wants to be
 * pushed onto the queue.
 * @param something
 */
export function shouldQueue<T>(something: T): something is ShouldQueue<T> {
    return isQueueable(something) && something.shouldQueue()
}

/**
 * A multi-node queue that accepts & reinstantiates Queueables.
 *
 * @example
 * There are several queue backends your application may use. These are
 * configured via the `queue` config. To get the default queue, however,
 * use this class as a DI token:
 * ```ts
 * this.container().make<Queue>(Queue)
 * ```
 *
 * This will resolve the concrete implementation configured by your app.
 */
@Injectable()
export class Queue {
    @Inject()
    protected readonly cache!: Cache

    @Inject()
    protected readonly canon!: Canon

    @Inject('injector')
    protected readonly injector!: Container

    constructor(
        public readonly name: string,
    ) { }

    public get queueIdentifier(): string {
        return `extollo__queue__${this.name}`
    }

    /** Get the number of items waiting in the queue. */
    // public abstract length(): Awaitable<number>

    /** Push a new queueable onto the queue. */
    public async push(item: ShouldQueue<Queueable>): Promise<void> {
        const data = {
            q: true,
            r: item.getFullyQualifiedCanonicalResolver(),
            d: await item.dehydrate(),
        }

        await this.cache.arrayPush(this.queueIdentifier, JSON.stringify(data))
    }

    /** Remove and return a queueable from the queue. */
    public async pop(): Promise<Maybe<Queueable>> {
        const item = await this.cache.arrayPop(this.queueIdentifier)
        if ( !item ) {
            return
        }

        const data = JSON.parse(item)
        if ( !data.q || !data.r ) {
            throw new ErrorWithContext('Cannot pop Queueable: payload is invalid.', {
                data,
                queueName: this.name,
                queueIdentifier: this.queueIdentifier,
            })
        }

        const canonicalItem = this.canon.getFromFullyQualified(data.r)
        if ( !canonicalItem ) {
            throw new ErrorWithContext('Cannot pop Queueable: canonical name is not resolvable', {
                data,
                queueName: this.name,
                queueIdentifier: this.queueIdentifier,
                canonicalName: data.r,
            })
        }

        if ( !isInstantiable(canonicalItem) ) {
            throw new ErrorWithContext('Cannot pop Queueable: canonical item is not instantiable', {
                data,
                canonicalItem,
                queueName: this.name,
                queueIdentifier: this.queueIdentifier,
                canonicalName: data.r,
            })
        }

        const instance = this.injector.make(canonicalItem)
        if ( !isQueueable(instance) ) {
            throw new ErrorWithContext('Cannot pop Queueable: canonical item instance is not Queueable', {
                data,
                canonicalItem,
                instance,
                queueName: this.name,
                queueIdentifier: this.queueIdentifier,
                canonicalName: data.r,
            })
        }

        await instance.rehydrate(data.d)
        return instance
    }

    /** Push a raw payload onto the queue. */
    public async pushRaw(item: JSONState): Promise<void> {
        await this.cache.arrayPush(this.queueIdentifier, JSON.stringify(item))
    }

    /** Remove and return a raw payload from the queue. */
    public async popRaw(): Promise<Maybe<JSONState>> {
        const item = await this.cache.arrayPop(this.queueIdentifier)
        if ( item ) {
            return JSON.parse(item)
        }
    }
}