use thiserror::Error;

pub const OVERLAY_MAGIC: u32 = 0x534c_414e; // "SLAN"
pub const OVERLAY_VERSION: u8 = 1;
pub const OVERLAY_HEADER_LEN: usize = 22;
pub const OVERLAY_FLAGS_NONE: u16 = 0;

#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Deserialize, serde::Serialize)]
#[repr(u8)]
pub enum FrameType {
    Ethernet = 1,
    Control = 2,
    Keepalive = 3,
}

impl FrameType {
    #[must_use]
    pub const fn as_u8(self) -> u8 {
        self as u8
    }

    pub const fn from_u8(value: u8) -> Result<Self, ProtoError> {
        match value {
            1 => Ok(Self::Ethernet),
            2 => Ok(Self::Control),
            3 => Ok(Self::Keepalive),
            other => Err(ProtoError::UnknownFrameType(other)),
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Deserialize, serde::Serialize)]
pub struct OverlayHeader {
    frame_type: FrameType,
    room_id: u64,
    peer_id: u32,
    flags: u16,
    payload_len: u16,
}

impl OverlayHeader {
    pub fn new(
        frame_type: FrameType,
        room_id: u64,
        peer_id: u32,
        flags: u16,
        payload_len: usize,
    ) -> Result<Self, ProtoError> {
        validate_overlay_flags(flags)?;
        let payload_len = u16::try_from(payload_len).map_err(|_| ProtoError::PayloadTooLarge {
            len: payload_len,
            max: u16::MAX as usize,
        })?;

        Ok(Self {
            frame_type,
            room_id,
            peer_id,
            flags,
            payload_len,
        })
    }

    #[must_use]
    pub const fn frame_type(self) -> FrameType {
        self.frame_type
    }

    #[must_use]
    pub const fn room_id(self) -> u64 {
        self.room_id
    }

    #[must_use]
    pub const fn peer_id(self) -> u32 {
        self.peer_id
    }

    #[must_use]
    pub const fn flags(self) -> u16 {
        self.flags
    }

    #[must_use]
    pub const fn payload_len(self) -> u16 {
        self.payload_len
    }

    #[must_use]
    pub fn encode(self) -> [u8; OVERLAY_HEADER_LEN] {
        let mut bytes = [0; OVERLAY_HEADER_LEN];
        bytes[0..4].copy_from_slice(&OVERLAY_MAGIC.to_be_bytes());
        bytes[4] = OVERLAY_VERSION;
        bytes[5] = self.frame_type.as_u8();
        bytes[6..14].copy_from_slice(&self.room_id.to_be_bytes());
        bytes[14..18].copy_from_slice(&self.peer_id.to_be_bytes());
        bytes[18..20].copy_from_slice(&self.flags.to_be_bytes());
        bytes[20..22].copy_from_slice(&self.payload_len.to_be_bytes());
        bytes
    }

    pub fn decode(bytes: &[u8]) -> Result<Self, ProtoError> {
        if bytes.len() < OVERLAY_HEADER_LEN {
            return Err(ProtoError::DatagramTooShort {
                actual: bytes.len(),
                minimum: OVERLAY_HEADER_LEN,
            });
        }

        let magic = u32::from_be_bytes(bytes[0..4].try_into().expect("header magic slice length"));
        if magic != OVERLAY_MAGIC {
            return Err(ProtoError::BadMagic { actual: magic });
        }

        let version = bytes[4];
        if version != OVERLAY_VERSION {
            return Err(ProtoError::UnsupportedVersion { actual: version });
        }

        let flags = u16::from_be_bytes(bytes[18..20].try_into().expect("flags slice length"));
        validate_overlay_flags(flags)?;

        Ok(Self {
            frame_type: FrameType::from_u8(bytes[5])?,
            room_id: u64::from_be_bytes(bytes[6..14].try_into().expect("room id slice length")),
            peer_id: u32::from_be_bytes(bytes[14..18].try_into().expect("peer id slice length")),
            flags,
            payload_len: u16::from_be_bytes(
                bytes[20..22].try_into().expect("payload len slice length"),
            ),
        })
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct OverlayPacket<'a> {
    header: OverlayHeader,
    payload: &'a [u8],
}

impl<'a> OverlayPacket<'a> {
    pub fn new(header: OverlayHeader, payload: &'a [u8]) -> Result<Self, ProtoError> {
        let declared = usize::from(header.payload_len);

        if payload.len() != declared {
            return Err(ProtoError::PayloadLengthMismatch {
                declared,
                actual: payload.len(),
            });
        }

        Ok(Self { header, payload })
    }

    #[must_use]
    pub const fn header(self) -> OverlayHeader {
        self.header
    }

    #[must_use]
    pub const fn payload(self) -> &'a [u8] {
        self.payload
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Error)]
pub enum ProtoError {
    #[error("datagram is too short: got {actual} bytes, need at least {minimum}")]
    DatagramTooShort { actual: usize, minimum: usize },
    #[error("bad overlay magic 0x{actual:08x}")]
    BadMagic { actual: u32 },
    #[error("unsupported overlay version {actual}")]
    UnsupportedVersion { actual: u8 },
    #[error("unknown overlay frame type {0}")]
    UnknownFrameType(u8),
    #[error("unsupported overlay flags 0x{actual:04x}")]
    UnsupportedFlags { actual: u16 },
    #[error("payload length {len} exceeds wire maximum {max}")]
    PayloadTooLarge { len: usize, max: usize },
    #[error("encoded datagram length {len} exceeds negotiated QUIC datagram budget {max}")]
    DatagramExceedsBudget { len: usize, max: usize },
    #[error("declared payload length {declared} does not match actual length {actual}")]
    PayloadLengthMismatch { declared: usize, actual: usize },
    #[error("Ethernet frame is too short: got {actual} bytes, need at least {minimum}")]
    EthernetFrameTooShort { actual: usize, minimum: usize },
}

fn validate_overlay_flags(flags: u16) -> Result<(), ProtoError> {
    if flags != OVERLAY_FLAGS_NONE {
        return Err(ProtoError::UnsupportedFlags { actual: flags });
    }

    Ok(())
}

pub fn encode_datagram(
    frame_type: FrameType,
    room_id: u64,
    peer_id: u32,
    flags: u16,
    payload: &[u8],
) -> Result<Vec<u8>, ProtoError> {
    let header = OverlayHeader::new(frame_type, room_id, peer_id, flags, payload.len())?;
    let mut datagram = Vec::with_capacity(OVERLAY_HEADER_LEN + payload.len());
    datagram.extend_from_slice(&header.encode());
    datagram.extend_from_slice(payload);
    Ok(datagram)
}

pub fn validate_datagram_budget(
    datagram_len: usize,
    max_datagram_size: usize,
) -> Result<(), ProtoError> {
    if datagram_len > max_datagram_size {
        return Err(ProtoError::DatagramExceedsBudget {
            len: datagram_len,
            max: max_datagram_size,
        });
    }

    Ok(())
}

pub fn decode_datagram(bytes: &[u8]) -> Result<OverlayPacket<'_>, ProtoError> {
    let header = OverlayHeader::decode(bytes)?;
    let payload = &bytes[OVERLAY_HEADER_LEN..];

    OverlayPacket::new(header, payload)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn encodes_and_decodes_datagrams() {
        let payload = [1, 2, 3, 4];
        let datagram = encode_datagram(
            FrameType::Ethernet,
            0x0102_0304_0506_0708,
            0x0a0b_0c0d,
            OVERLAY_FLAGS_NONE,
            &payload,
        )
        .unwrap();

        assert_eq!(datagram.len(), OVERLAY_HEADER_LEN + payload.len());
        assert_eq!(&datagram[0..4], &OVERLAY_MAGIC.to_be_bytes());
        assert_eq!(datagram[4], OVERLAY_VERSION);
        assert_eq!(datagram[5], FrameType::Ethernet.as_u8());

        let packet = decode_datagram(&datagram).unwrap();
        let header = packet.header();
        assert_eq!(header.frame_type(), FrameType::Ethernet);
        assert_eq!(header.room_id(), 0x0102_0304_0506_0708);
        assert_eq!(header.peer_id(), 0x0a0b_0c0d);
        assert_eq!(header.flags(), OVERLAY_FLAGS_NONE);
        assert_eq!(header.payload_len(), 4);
        assert_eq!(packet.payload(), payload);
    }

    #[test]
    fn rejects_payload_length_mismatch() {
        let mut datagram = encode_datagram(FrameType::Keepalive, 1, 2, 0, &[1, 2, 3]).unwrap();
        datagram.pop();

        let error = decode_datagram(&datagram).unwrap_err();

        assert!(matches!(
            error,
            ProtoError::PayloadLengthMismatch {
                declared: 3,
                actual: 2
            }
        ));
    }

    #[test]
    fn rejects_bad_magic_and_version() {
        let mut datagram = encode_datagram(FrameType::Control, 1, 2, 0, &[]).unwrap();
        datagram[0] = 0;
        assert!(matches!(
            decode_datagram(&datagram).unwrap_err(),
            ProtoError::BadMagic { .. }
        ));

        let mut datagram = encode_datagram(FrameType::Control, 1, 2, 0, &[]).unwrap();
        datagram[4] = 99;
        assert!(matches!(
            decode_datagram(&datagram).unwrap_err(),
            ProtoError::UnsupportedVersion { actual: 99 }
        ));
    }

    #[test]
    fn rejects_unknown_frame_type() {
        let mut datagram = encode_datagram(FrameType::Control, 1, 2, 0, &[]).unwrap();
        datagram[5] = 99;

        assert_eq!(
            decode_datagram(&datagram).unwrap_err(),
            ProtoError::UnknownFrameType(99)
        );
    }

    #[test]
    fn rejects_reserved_overlay_flags() {
        assert_eq!(
            OverlayHeader::new(FrameType::Ethernet, 1, 2, 1, 0).unwrap_err(),
            ProtoError::UnsupportedFlags { actual: 1 }
        );

        let mut datagram = encode_datagram(FrameType::Ethernet, 1, 2, 0, &[]).unwrap();
        datagram[18..20].copy_from_slice(&0x8000_u16.to_be_bytes());

        assert_eq!(
            decode_datagram(&datagram).unwrap_err(),
            ProtoError::UnsupportedFlags { actual: 0x8000 }
        );
    }

    #[test]
    fn rejects_payloads_too_large_for_header() {
        let payload = vec![0; usize::from(u16::MAX) + 1];

        assert!(matches!(
            encode_datagram(FrameType::Ethernet, 1, 2, 0, &payload).unwrap_err(),
            ProtoError::PayloadTooLarge { .. }
        ));
    }

    #[test]
    fn rejects_datagrams_over_negotiated_budget() {
        let datagram = encode_datagram(FrameType::Ethernet, 1, 2, 0, &[1, 2, 3]).unwrap();

        assert!(validate_datagram_budget(datagram.len(), datagram.len()).is_ok());
        assert_eq!(
            validate_datagram_budget(datagram.len(), datagram.len() - 1).unwrap_err(),
            ProtoError::DatagramExceedsBudget {
                len: datagram.len(),
                max: datagram.len() - 1
            }
        );
    }
}