"use strict";

import os from 'os';

import log from 'log';
const l = log.get("framedtransport");

import { EventEmitter, once } from 'events';

import { write_safe, end_safe } from '../io/StreamHelpers.mjs';

/**
 * Manages a TCP socket using a simple binary frame system.
 * Frames look like this:
 *
 * <LENGTH: Uint32 big-endian> <DATA: Uint8Array>
 */
class FramedTransport extends EventEmitter {
	constructor(socket) {
		super();
		
		this.socket = socket;
		this.socket.on("data", this.handle_chunk.bind(this));
		this.buffer = null;
		
		/** The length of a uint in bytes @type {number} */
		this.uint_length = 4;
		
		this.writing = false;
	}
	
	/**
	 * Handles a single chunk of incoming data.
	 * May or may not emit a frame event.
	 * @param	{Buffer|TypedArray}	chunk	The incoming chunk of data.
	 * @return	{void}
	 */
	handle_chunk(chunk) {
		l.debug(`CHUNK length`, chunk.length, `buffer length`, this.buffer === null ? 0 : this.buffer.length);
		if(this.buffer instanceof Buffer) {
			this.buffer = Buffer.concat([ this.buffer, chunk ]);
			
		}
		else
			this.buffer = chunk;
		
		l.debug(`CHUNK buffer`, this.buffer);
		
		let next_frame_length = this.buffer2uint(this.buffer, 0);
		
		l.debug(`CHUNK total length`, this.buffer.length, `next_frame_length`, next_frame_length);
		
		
		// We have enough data! Emit a frame and then start again.
		if(this.buffer.length - this.uint_length >= next_frame_length) {
			this.emit("frame", this.buffer.slice(this.uint_length, next_frame_length));
			if(this.buffer.length - (this.uint_length + next_frame_length) > 0)
				this.buffer = this.buffer.slice(this.uint_length + next_frame_length);
			else
				this.buffer = null;
			l.info(`FRAME length`, next_frame_length, `length_remaining`, this.buffer === null ? 0 : this.buffer.length);
		}
	}
	
	/**
	 * Converts the number at a given position in a buffer from Uint32 with
	 * big-endian encoding to a normal number.
	 * @param	{Buffer|TypedArray}	buffer	The source buffer.
	 * @param	{number}			pos32	The position in the buffer to read from.
	 * @return	{number}	The parsed number from the buffer.
	 */
	buffer2uint(buffer, pos32) {
		l.debug(`BUFFER2UINT buffer`, buffer, `pos32`, pos32);
		// DataView doesn't work as expected here, because Node.js optimises small buffer to be views of 1 larger buffer.
		let u8 = new Uint8Array(buffer).slice(pos32, this.uint_length);
		// Convert from network byte order if necessary
		if(os.endianness() == "LE") u8.reverse();
		l.debug(`BUFFER2UINT u8`, u8, `u32`, new Uint32Array(u8.buffer));
		return new Uint32Array(u8.buffer)[0];
	}
	
	/**
	 * Converts a positive integer into a Buffer using big-endian encoding.
	 * @param	{number}	uint	The positive integer to convert.
	 * @return	{Buffer}	A new buffer representing the given number.
	 */
	uint2buffer(uint) {
		// DataView doesn't work as expected here, because Node.js optimises small buffer to be views of 1 larger buffer.
		let array = new ArrayBuffer(this.uint_length);
		const u8 = new Uint8Array(array);
		const u32 = new Uint32Array(array);
		u32[0] = uint;
		// Host to network byte order, if appropriate
		if(os.endianness() == "LE") u8.reverse();
		return Buffer.from(array);
	}
	
	/**
	 * Writes a given Buffer or TypedArray to the underlying socket inn a new frame.
	 * @param	{Buffer|TypedArray}	frame	The data to send in a new frame.
	 * @return	{Promise}			A promise that resolves when writing is complete.
	 */
	async write(frame) {
		if(this.writing) await once(this, "write-end");
		this.emit("write-start");
		this.writing = true;
		l.info(`SEND length`, frame.length);
		await write_safe(this.socket, this.uint2buffer(frame.length));
		await write_safe(this.socket, frame);
		this.writing = false;
		this.emit("write-end")
	}
	
	/**
	 * Gracefulls closes this socket.
	 * @return {Promise} A Promise that resolves when the socket is properly closed.
	 */
	async destroy() {
		// Calling socket.end() is important as it closes the stream properly
		await end_safe(this.socket);
		await this.socket.destroy();
	}
}

export default FramedTransport;