#pragma once

#include <array>

#include <cstdint>

#include <vector>

#include "envoy/buffer/buffer.h"

namespace Envoy {

namespace Grpc {

// Last bit for an expanded message without compression.

const uint8_t GRPC_FH_DEFAULT = 0b0u;

// Last bit for a compressed message.

const uint8_t GRPC_FH_COMPRESSED = 0b1u;

// Bit specifies end-of-stream response in Connect.

const uint8_t CONNECT_FH_EOS = 0b10u;

constexpr uint64_t GRPC_FRAME_HEADER_SIZE = sizeof(uint8_t) + sizeof(uint32_t);

enum class CompressionAlgorithm { None, Gzip };

struct Frame {

  uint8_t flags_;

  uint32_t length_;

  Buffer::InstancePtr data_;

};

class Encoder {

public:

  Encoder();

  // Creates a new GRPC data frame with the given flags and length.

  // @param flags supplies the GRPC data frame flags.

  // @param length supplies the GRPC data frame length.

  // @param output the buffer to store the encoded data. Its size must be 5.

  void newFrame(uint8_t flags, uint64_t length, std::array<uint8_t, 5>& output);

  // Prepend the gRPC frame into the buffer.

  // @param flags supplies the GRPC data frame flags.

  // @param buffer the full buffer with the message payload.

  void prependFrameHeader(uint8_t flags, Buffer::Instance& buffer);

  // Prepend the gRPC frame into the buffer.

  // @param flags supplies the GRPC data frame flags.

  // @param buffer the buffer with the first part of the message payload.

  // @param message_length the total length of the message, which may be longer

  // than buffer.

  void prependFrameHeader(uint8_t flags, Buffer::Instance& buffer, uint32_t message_length);

};

// Wire format (http://www.grpc.io/docs/guides/wire.html) of GRPC data frame

// header:

//

// ---------------------------------------------------------------------

// |R|R|R|R|R|R|R|C|      L     |      L     |      L     |      L     |

// ---------------------------------------------------------------------

//    Flag (1 byte)                Message Length (4 bytes)

//

// A fixed header consists of five bytes.

// The first byte is the Flag. The last one "C" bit indicates if the message

// is compressed or not (0 is uncompressed, 1 is compressed). The other seven

// "R" bits are reserved for future use.

// The next four "L" bytes represent the message length in BigEndian format.

enum class State {

  // Waiting for decoding the flags (1 byte) of the GRPC data frame.

  FhFlag,

  // Waiting for decoding the 1st byte of the length (4 bytes in total) of the

  // GRPC data frame.

  FhLen0,

  // Waiting for decoding the 2nd byte of the length (4 bytes in total) of the

  // GRPC data frame.

  FhLen1,

  // Waiting for decoding the 3rd byte of the length (4 bytes in total) of the

  // GRPC data frame.

  FhLen2,

  // Waiting for decoding the 4th byte of the length (4 bytes in total) of the

  // GRPC data frame.

  FhLen3,

  // Waiting for decoding the data.

  Data,

};

class FrameInspector {

public:

  // Inspects the given buffer with GRPC data frame and updates the frame count.

  // Invokes visitor callbacks for each frame in the following sequence:

  //   "frameStart frameDataStart frameData* frameDataEnd"

  // If frameStart returns false, then the inspector aborts.

  // Returns the increase in the frame count.

  uint64_t inspect(const Buffer::Instance& input);

  // Returns the current frame count, corresponding to the request/response

  // message count. Counter is incremented on a frame start.

  // Returns the current state in the frame parsing.

protected:

  State state_{State::FhFlag};

  union {

    // Note that this union does not rely on bytes being arranged accurately for a

    // uint32_t, it merely shares the storage. absl::big_endian is used to deserialize

    // the bytes correctly once they are populated.

    uint32_t length_{0};

    uint8_t length_as_bytes_[4];

  };

  uint64_t count_{0};

  // Default value 0 means there is no limitation on maximum frame length.

  uint32_t max_frame_length_{0};

  // When `max_frame_length_` is configured, this flag will be true if frame length is larger than

  // `max_frame_length_`.

  bool is_frame_oversized_{false};

};

class Decoder : public FrameInspector {

public:

  // Decodes the given buffer with GRPC data frame. Drains the input buffer when

  // decoding succeeded (returns true). If the input is not sufficient to make a

  // complete GRPC data frame, it will be buffered in the decoder. If a decoding

  // error happened, the input buffer remains unchanged.

  // @param input supplies the binary octets wrapped in a GRPC data frame.

  // @param output supplies the buffer to store the decoded data.

  // @return absl::status whether the decoding succeeded or not.

  absl::Status decode(Buffer::Instance& input, std::vector<Frame>& output);

  // Determine the length of the current frame being decoded. This is useful when supplying a

  // partial frame to decode() and wanting to know how many more bytes need to be read to complete

  // the frame.

  // Indicates whether it has buffered any partial data.

  // Configures the maximum frame length.

protected:

  bool frameStart(uint8_t) override;

  void frameDataStart() override;

  void frameData(uint8_t*, uint64_t) override;

  void frameDataEnd() override;

private:

  Frame frame_;

  std::vector<Frame>* output_{nullptr};

  bool decoding_error_{false};

};

} // namespace Grpc

} // namespace Envoy