package eventstream

import (

	"bytes"

	"encoding/binary"

	"hash"

	"hash/crc32"

	"io"

)

// Encoder provides EventStream message encoding.

type Encoder struct {

	w io.Writer

	headersBuf *bytes.Buffer

}

// NewEncoder initializes and returns an Encoder to encode Event Stream

// messages to an io.Writer.

func NewEncoder(w io.Writer) *Encoder {

	return &Encoder{

		w:          w,

		headersBuf: bytes.NewBuffer(nil),

	}

}

// Encode encodes a single EventStream message to the io.Writer the Encoder

// was created with. An error is returned if writing the message fails.

func (e *Encoder) Encode(msg Message) error {

	e.headersBuf.Reset()

	err := encodeHeaders(e.headersBuf, msg.Headers)

	if err != nil {

		return err

	}

	crc := crc32.New(crc32IEEETable)

	hashWriter := io.MultiWriter(e.w, crc)

	headersLen := uint32(e.headersBuf.Len())

	payloadLen := uint32(len(msg.Payload))

	if err := encodePrelude(hashWriter, crc, headersLen, payloadLen); err != nil {

		return err

	}

	if headersLen > 0 {

		if _, err := io.Copy(hashWriter, e.headersBuf); err != nil {

			return err

		}

	}

	if payloadLen > 0 {

		if _, err := hashWriter.Write(msg.Payload); err != nil {

			return err

		}

	}

	msgCRC := crc.Sum32()

	return binary.Write(e.w, binary.BigEndian, msgCRC)

}

func encodePrelude(w io.Writer, crc hash.Hash32, headersLen, payloadLen uint32) error {

	p := messagePrelude{

		Length:     minMsgLen + headersLen + payloadLen,

		HeadersLen: headersLen,

	}

	if err := p.ValidateLens(); err != nil {

		return err

	}

	err := binaryWriteFields(w, binary.BigEndian,

		p.Length,

		p.HeadersLen,

	)

	if err != nil {

		return err

	}

	p.PreludeCRC = crc.Sum32()

	err = binary.Write(w, binary.BigEndian, p.PreludeCRC)

	if err != nil {

		return err

	}

	return nil

}

func encodeHeaders(w io.Writer, headers Headers) error {

	for _, h := range headers {

		hn := headerName{

			Len: uint8(len(h.Name)),

		}

		copy(hn.Name[:hn.Len], h.Name)

		if err := hn.encode(w); err != nil {

			return err

		}

		if err := h.Value.encode(w); err != nil {

			return err

		}

	}

	return nil

}

func binaryWriteFields(w io.Writer, order binary.ByteOrder, vs ...interface{}) error {

	for _, v := range vs {

		if err := binary.Write(w, order, v); err != nil {

			return err

		}

	}

	return nil

}