/**
 * External dependencies
 */
import * as Y from 'yjs';
import * as buffer from 'lib0/buffer';

/**
 * Internal dependencies
 */
import {
	CRDT_DOC_META_PERSISTENCE_KEY,
	CRDT_DOC_VERSION,
	CRDT_STATE_MAP_KEY,
	CRDT_STATE_MAP_SAVED_AT_KEY as SAVED_AT_KEY,
	CRDT_STATE_MAP_SAVED_BY_KEY as SAVED_BY_KEY,
	CRDT_STATE_MAP_VERSION_KEY as VERSION_KEY,
} from './config';
import type { CRDTDoc } from './types';

// An object representation of CRDT document metadata.
type DocumentMeta = Record< string, DocumentMetaValue >;
type DocumentMetaValue = boolean | number | string;

/**
 * Creates a new Y.Doc instance with the given document metadata.
 *
 * @param {DocumentMeta} documentMeta Optional metadata to associate with the
 *                                    document. Metadata is not persisted.
 */
export function createYjsDoc( documentMeta: DocumentMeta = {} ): CRDTDoc {
	// Convert the object representation of CRDT document metadata to a map.
	// Document metadata is passed to the Y.Doc constructor and stored in its
	// `meta` property. It is not synced to peers or persisted with the document.
	// It is just a place to store transient information about this doc instance.
	const metaMap = new Map< string, DocumentMetaValue >(
		Object.entries( documentMeta )
	);

	// IMPORTANT: Do not add update the document itself to avoid generating updates
	// before observers are attached. Add initial updates in `initializeYjsDoc`.
	return new Y.Doc( { meta: metaMap } );
}

/**
 * Initializes a Y.Doc instance with the necessary CRDT state for our use case.
 *
 * @param {Y.Doc} ydoc Y.Doc instance to initialize.
 */
export function initializeYjsDoc( ydoc: CRDTDoc ): void {
	const stateMap = ydoc.getMap( CRDT_STATE_MAP_KEY );
	stateMap.set( VERSION_KEY, CRDT_DOC_VERSION );
}

/**
 * Record that the entity was saved (persisted to the database) in the CRDT
 * document record metadata.
 *
 * @param {CRDTDoc} ydoc CRDT document.
 */
export function markEntityAsSaved( ydoc: CRDTDoc ): void {
	const recordMeta = ydoc.getMap( CRDT_STATE_MAP_KEY );
	recordMeta.set( SAVED_AT_KEY, Date.now() );
	recordMeta.set( SAVED_BY_KEY, ydoc.clientID );
}

function pseudoRandomID(): number {
	return Math.floor( Math.random() * 1000000000 );
}

export function serializeCrdtDoc( crdtDoc: CRDTDoc ): string {
	return JSON.stringify( {
		document: buffer.toBase64( Y.encodeStateAsUpdateV2( crdtDoc ) ),
		updateId: pseudoRandomID(), // helps with debugging
	} );
}

export function deserializeCrdtDoc(
	serializedCrdtDoc: string
): CRDTDoc | null {
	try {
		const { document } = JSON.parse( serializedCrdtDoc );

		// Mark this document as from persistence.
		const docMeta: DocumentMeta = {
			[ CRDT_DOC_META_PERSISTENCE_KEY ]: true,
		};

		// Apply the document as an update against a new (temporary) Y.Doc.
		const ydoc = createYjsDoc( docMeta );
		const yupdate = buffer.fromBase64( document );
		Y.applyUpdateV2( ydoc, yupdate );

		// Overwrite the client ID (which is from a previous session) with a random
		// client ID. Deserialized documents should not be used directly. Instead,
		// their state should be applied to another in-use document.
		ydoc.clientID = pseudoRandomID();

		return ydoc;
	} catch {
		return null;
	}
}