// handlerGenerator returns ProtocolHandlers that interact with our application.
// In this example, we are only using Holding Registers.
func handlerGenerator(name string) modbusone.ProtocolHandler {
    return &modbusone.SimpleHandler{
        ReadHoldingRegisters: func(address, quantity uint16) ([]uint16, error) {
            fmt.Printf("%v ReadHoldingRegisters from %v, quantity %v\n",
                name, address, quantity)
            r := make([]uint16, quantity)
            // application code that fills in r here
            return r, nil
        },
        WriteHoldingRegisters: func(address uint16, values []uint16) error {
            fmt.Printf("%v WriteHoldingRegisters from %v, quantity %v\n",
                name, address, len(values))
            // application code here
            return nil
        },
        OnErrorImp: func(req modbusone.PDU, errRep modbusone.PDU) {
            fmt.Printf("%v received error:%x in request:%x", name, errRep, req)
        },
    }
}

func Example_serialPort() {
    // Server id and baudRate, for Modbus over serial port.
    id := byte(1)
    baudRate := int64(19200)

    // Open serial connections:
    clientSerial, serverSerial := newInternalSerial()
    // Normally we want to open a serial connection from serial.OpenPort
    // such as github.com/tarm/serial. modbusone can take any io.ReadWriteCloser,
    // so we created two that talks to each other for demonstration here.

    // SerialContext adds baudRate information to calculate
    // the duration that data transfers should takes.
    // It also records Stats of read and dropped packets.
    clientSerialContext := modbusone.NewSerialContext(clientSerial, baudRate)
    serverSerialContext := modbusone.NewSerialContext(serverSerial, baudRate)

    // You can create either a client or a server from a SerialContext and an id.
    client := modbusone.NewRTUClient(clientSerialContext, id)
    server := modbusone.NewRTUServer(serverSerialContext, id)

    // Create Handler to handle client and server actions.
    handler := handlerGenerator

    termChan := make(chan error)

    // Now we are ready to serve!
    // Serve is blocking until the serial connection has io errors or is closed.
    // So we use a goroutine to start it and continue setting up our demo.
    go client.Serve(handler("client"))
    go func() {
        //A server is Started to same way as a client
        err := server.Serve(handler("server"))
        // Do something with the err here.
        // For a command line app, you probably want to terminate.
        // For a service, you probably want to wait until you can open the serial port again.
        termChan <- err
    }()
    defer client.Close()
    defer server.Close()

    // If you only need to support server side, then you are done.
    // If you need to support client side, then you need to make requests.
    startAddress := uint16(0)
    quantity := uint16(200)
    reqs, err := modbusone.MakePDURequestHeaders(modbusone.FcReadHoldingRegisters,
        startAddress, quantity, nil)
    if err != nil {
        fmt.Println(err) //if what you asked for is not possible.
    }
    // Larger than allowed requests are split to many packets.
    fmt.Println("reqs count:", len(reqs))

    // We can add more requests, even of different types.
    // The last nil is replaced by the reqs to append to.
    startAddress = uint16(1000)
    quantity = uint16(100)
    reqs, err = modbusone.MakePDURequestHeaders(modbusone.FcWriteMultipleRegisters,
        startAddress, quantity, reqs)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println("reqs count:", len(reqs))

    // Range over the requests to handle each individually,
    for _, r := range reqs {
        err = client.DoTransaction(r)
        if err != nil {
            fmt.Println(err, "on", r) // The server timed out, or the connection was closed.
        }
    }
    // or just do them all at once. Notice that reqs can be reused.
    n, err := modbusone.DoTransactions(client, id, reqs)
    if err != nil {
        fmt.Println(err, "on", reqs[n])
    }

    // Clean up
    server.Close()
    err = <-termChan
    fmt.Println("serve terminated:", err)

    //Output:
    //reqs count: 2
    //reqs count: 3
    //server ReadHoldingRegisters from 0, quantity 125
    //client WriteHoldingRegisters from 0, quantity 125
    //server ReadHoldingRegisters from 125, quantity 75
    //client WriteHoldingRegisters from 125, quantity 75
    //client ReadHoldingRegisters from 1000, quantity 100
    //server WriteHoldingRegisters from 1000, quantity 100
    //server ReadHoldingRegisters from 0, quantity 125
    //client WriteHoldingRegisters from 0, quantity 125
    //server ReadHoldingRegisters from 125, quantity 75
    //client WriteHoldingRegisters from 125, quantity 75
    //client ReadHoldingRegisters from 1000, quantity 100
    //server WriteHoldingRegisters from 1000, quantity 100
    //serve terminated: io: read/write on closed pipe
} //end readme example