var Transform = require('stream').Transform;
var allocBuffer = require('./alloc-buffer').allocBuffer;

/** @type {Transform} */
function EventMessageChunkerStream(options) {
    Transform.call(this, options);

    this.currentMessageTotalLength = 0;
    this.currentMessagePendingLength = 0;
    /** @type {Buffer} */
    this.currentMessage = null;

    /** @type {Buffer} */
    this.messageLengthBuffer = null;
}

EventMessageChunkerStream.prototype = Object.create(Transform.prototype);

/**
 *
 * @param {Buffer} chunk
 * @param {string} encoding
 * @param {*} callback
 */
EventMessageChunkerStream.prototype._transform = function(chunk, encoding, callback) {
    var chunkLength = chunk.length;
    var currentOffset = 0;

    while (currentOffset < chunkLength) {
        // create new message if necessary
        if (!this.currentMessage) {
            // working on a new message, determine total length
            var bytesRemaining = chunkLength - currentOffset;
            // prevent edge case where total length spans 2 chunks
            if (!this.messageLengthBuffer) {
                this.messageLengthBuffer = allocBuffer(4);
            }
            var numBytesForTotal = Math.min(
                4 - this.currentMessagePendingLength, // remaining bytes to fill the messageLengthBuffer
                bytesRemaining // bytes left in chunk
            );

            chunk.copy(
                this.messageLengthBuffer,
                this.currentMessagePendingLength,
                currentOffset,
                currentOffset + numBytesForTotal
            );

            this.currentMessagePendingLength += numBytesForTotal;
            currentOffset += numBytesForTotal;

            if (this.currentMessagePendingLength < 4) {
                // not enough information to create the current message
                break;
            }
            this.allocateMessage(this.messageLengthBuffer.readUInt32BE(0));
            this.messageLengthBuffer = null;
        }

        // write data into current message
        var numBytesToWrite = Math.min(
            this.currentMessageTotalLength - this.currentMessagePendingLength, // number of bytes left to complete message
            chunkLength - currentOffset // number of bytes left in the original chunk
        );
        chunk.copy(
            this.currentMessage, // target buffer
            this.currentMessagePendingLength, // target offset
            currentOffset, // chunk offset
            currentOffset + numBytesToWrite // chunk end to write
        );
        this.currentMessagePendingLength += numBytesToWrite;
        currentOffset += numBytesToWrite;

        // check if a message is ready to be pushed
        if (this.currentMessageTotalLength && this.currentMessageTotalLength === this.currentMessagePendingLength) {
            // push out the message
            this.push(this.currentMessage);
            // cleanup
            this.currentMessage = null;
            this.currentMessageTotalLength = 0;
            this.currentMessagePendingLength = 0;
        }
    }

    callback();
};

EventMessageChunkerStream.prototype._flush = function(callback) {
    if (this.currentMessageTotalLength) {
        if (this.currentMessageTotalLength === this.currentMessagePendingLength) {
            callback(null, this.currentMessage);
        } else {
            callback(new Error('Truncated event message received.'));
        }
    } else {
        callback();
    }
};

/**
 * @param {number} size Size of the message to be allocated.
 * @api private
 */
EventMessageChunkerStream.prototype.allocateMessage = function(size) {
    if (typeof size !== 'number') {
        throw new Error('Attempted to allocate an event message where size was not a number: ' + size);
    }
    this.currentMessageTotalLength = size;
    this.currentMessagePendingLength = 4;
    this.currentMessage = allocBuffer(size);
    this.currentMessage.writeUInt32BE(size, 0);
};

/**
 * @api private
 */
module.exports = {
    EventMessageChunkerStream: EventMessageChunkerStream
};