require 'serialport'

class DMX
    START_OF_MESSAGE = "\x7E"
    SEND_DMX_LABEL   = "\x06"
    START_OF_DATA    = "\x00"
    END_OF_MESSAGE   = "\xE7"

    SERIAL_PORT_PARAMS = {
        'baud' => 115_200,
        'data_bits' => 8,
        'stop_bits' => 2,
        'parity' => SerialPort::NONE
    }

    #
    # Wikipedia says "A maximum-sized packet, which has 512 channels, takes
    # approximately 23 ms to send, corresponding to a max refresh rate of ~44 Hz.
    # For higher refresh rates, packets having fewer than 512 channels can be sent.
    #
    def initialize(path, channel_count)
        raise ArgumentError.new("channel_count must be between 25 and 512") if (channel_count < 25 or channel_count > 512)

        @path, @channel_count = path, channel_count

        @port = SerialPort.new(@path, SERIAL_PORT_PARAMS)

        # Allocate a buffer once, so we don't continually produce garbage
        @packet = "\0" * (@channel_count + 6)       # 6 bytes of START_OF_MESSAGE, etc.

        # Set the static values
        payload_size = (@channel_count + 1)     # +1 for the start code

        @packet[0] = START_OF_MESSAGE
        @packet[1] = SEND_DMX_LABEL
        @packet[2] = (payload_size & 255).chr                   # LSB of size
        @packet[3] = ((payload_size >> 8) & 255).chr    # MSB of size
        @packet[4] = START_OF_DATA
        # @channel_count bytes of DMX channels
        @packet[4 + @channel_count + 1] = END_OF_MESSAGE
    end

    def set(index, value)
        raise ArgumentError.new("index out of range") if (index < 0 or index >= @channel_count)
        @packet[5 + index] = value.chr
    end

    def send
        @port.write(@packet)
    end
end