//! `NmxTransferEnvelope` — 46-byte NMX wire envelope.
//!
//! Direct port of `src/MxNativeCodec/NmxTransferEnvelope.cs`. The Rust port
//! adds `reserved6_10: [u8; 4]` preservation per CLAUDE.md unknown-bytes rule
//! — the .NET reference reads only Version/InnerLength/ProtocolMarker/MessageKind
//! and discards bytes 6..10 (`NmxTransferEnvelope.cs:39-75`); the Rust codec
//! round-trips them.

// Direct byte indexing — see reference_handle.rs for rationale.
#![allow(clippy::indexing_slicing)]

use crate::error::CodecError;

/// Encoded layout per `NmxTransferEnvelope.cs:23-37`:
///
/// ```text
///  offset  size  field
///  0       2     version u16 LE          = 1
///  2       4     inner_length i32 LE     = body.len() - 46
///  6       4     reserved6_10 [u8; 4]    preserved verbatim by Rust port
/// 10       4     message_kind i32 LE     1=Metadata, 2=ItemControl, 3=Write
/// 14       4     source_galaxy_id i32 LE
/// 18       4     source_platform_id i32 LE
/// 22       4     local_engine_id i32 LE
/// 26       4     target_galaxy_id i32 LE
/// 30       4     target_platform_id i32 LE
/// 34       4     target_engine_id i32 LE
/// 38       4     protocol_marker i32 LE  = 0x0201 (bytes: 01 02 00 00)
/// 42       4     timeout_ms i32 LE       default 30000
/// 46+     body...
/// ```
pub const ENVELOPE_HEADER_LEN: usize = 46;

const VERSION: u16 = 1;
const PROTOCOL_MARKER: i32 = 0x0201;
const DEFAULT_TIMEOUT_MS: i32 = 30000;

const INNER_LENGTH_OFFSET: usize = 2;
const RESERVED_OFFSET: usize = 6;
const MESSAGE_KIND_OFFSET: usize = 10;
const SOURCE_GALAXY_OFFSET: usize = 14;
const SOURCE_PLATFORM_OFFSET: usize = 18;
const LOCAL_ENGINE_OFFSET: usize = 22;
const TARGET_GALAXY_OFFSET: usize = 26;
const TARGET_PLATFORM_OFFSET: usize = 30;
const TARGET_ENGINE_OFFSET: usize = 34;
const PROTOCOL_MARKER_OFFSET: usize = 38;
const TIMEOUT_OFFSET: usize = 42;

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
#[repr(u8)]
pub enum NmxTransferMessageKind {
    #[default]
    Unknown = 0,
    Metadata = 1,
    ItemControl = 2,
    Write = 3,
}

impl NmxTransferMessageKind {
    fn from_i32(value: i32) -> Self {
        match value {
            1 => Self::Metadata,
            2 => Self::ItemControl,
            3 => Self::Write,
            _ => Self::Unknown,
        }
    }

    fn to_i32(self) -> i32 {
        match self {
            Self::Unknown => 0,
            Self::Metadata => 1,
            Self::ItemControl => 2,
            Self::Write => 3,
        }
    }
}

/// 46-byte envelope. `reserved6_10` is preserved verbatim — the .NET reference
/// discards these bytes on parse and writes 0 on encode. The Rust port carries
/// them through so captured envelopes with non-zero values at offset 6..10
/// round-trip byte-identical.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct NmxTransferEnvelope {
    pub message_kind: NmxTransferMessageKind,
    pub source_galaxy_id: i32,
    pub source_platform_id: i32,
    pub local_engine_id: i32,
    pub target_galaxy_id: i32,
    pub target_platform_id: i32,
    pub target_engine_id: i32,
    pub timeout_ms: i32,
    /// Bytes 6..10 of the envelope. The .NET reference does not retain these;
    /// the Rust port preserves them per CLAUDE.md unknown-bytes rule.
    /// Defaults to `[0; 4]` for newly-constructed envelopes.
    pub reserved6_10: [u8; 4],
}

impl Default for NmxTransferEnvelope {
    fn default() -> Self {
        Self {
            message_kind: NmxTransferMessageKind::default(),
            source_galaxy_id: 1,
            source_platform_id: 1,
            local_engine_id: 0,
            target_galaxy_id: 0,
            target_platform_id: 0,
            target_engine_id: 0,
            timeout_ms: DEFAULT_TIMEOUT_MS,
            reserved6_10: [0; 4],
        }
    }
}

impl NmxTransferEnvelope {
    /// Header length in bytes.
    pub const HEADER_LEN: usize = ENVELOPE_HEADER_LEN;

    /// Parse a transfer body — the 46-byte header followed by the inner body.
    /// Returns the parsed envelope and the inner body length (the inner bytes
    /// themselves are accessed by the caller via `&transfer_body[46..]`).
    ///
    /// Mirrors `NmxTransferEnvelope.Parse` (`NmxTransferEnvelope.cs:39-75`).
    ///
    /// # Errors
    ///
    /// - [`CodecError::ShortRead`] if `transfer_body.len() < 46`.
    /// - [`CodecError::UnsupportedVersion`] if version != 1.
    /// - [`CodecError::InnerLengthMismatch`] if the declared `inner_length`
    ///   does not match `transfer_body.len() - 46`.
    /// - [`CodecError::UnsupportedProtocolMarker`] if the marker != 0x0201.
    pub fn parse(transfer_body: &[u8]) -> Result<Self, CodecError> {
        if transfer_body.len() < Self::HEADER_LEN {
            return Err(CodecError::ShortRead {
                expected: Self::HEADER_LEN,
                actual: transfer_body.len(),
            });
        }

        let version = read_u16_le(transfer_body, 0);
        if version != VERSION {
            return Err(CodecError::UnsupportedVersion {
                expected: VERSION,
                actual: version,
            });
        }

        let inner_length = read_i32_le(transfer_body, INNER_LENGTH_OFFSET);
        let actual_inner = transfer_body.len() - Self::HEADER_LEN;
        if inner_length != actual_inner as i32 {
            return Err(CodecError::InnerLengthMismatch {
                declared: inner_length,
                actual: actual_inner,
            });
        }

        let protocol_marker = read_i32_le(transfer_body, PROTOCOL_MARKER_OFFSET);
        if protocol_marker != PROTOCOL_MARKER {
            return Err(CodecError::UnsupportedProtocolMarker(protocol_marker));
        }

        let mut reserved6_10 = [0u8; 4];
        reserved6_10.copy_from_slice(&transfer_body[RESERVED_OFFSET..RESERVED_OFFSET + 4]);

        Ok(Self {
            message_kind: NmxTransferMessageKind::from_i32(read_i32_le(
                transfer_body,
                MESSAGE_KIND_OFFSET,
            )),
            source_galaxy_id: read_i32_le(transfer_body, SOURCE_GALAXY_OFFSET),
            source_platform_id: read_i32_le(transfer_body, SOURCE_PLATFORM_OFFSET),
            local_engine_id: read_i32_le(transfer_body, LOCAL_ENGINE_OFFSET),
            target_galaxy_id: read_i32_le(transfer_body, TARGET_GALAXY_OFFSET),
            target_platform_id: read_i32_le(transfer_body, TARGET_PLATFORM_OFFSET),
            target_engine_id: read_i32_le(transfer_body, TARGET_ENGINE_OFFSET),
            timeout_ms: read_i32_le(transfer_body, TIMEOUT_OFFSET),
            reserved6_10,
        })
    }

    /// Encode the envelope header into the front of `transfer_body`. The
    /// caller is responsible for providing a buffer of length
    /// `46 + inner_body.len()` and copying the inner body into the tail
    /// before transmission.
    ///
    /// Mirrors `NmxTransferEnvelope.Encode` (`NmxTransferEnvelope.cs:77-103`)
    /// but additionally writes `reserved6_10` (the .NET version always writes 0).
    ///
    /// # Errors
    ///
    /// Returns [`CodecError::InnerLengthMismatch`] if `transfer_body.len() < 46`.
    pub fn write_to(self, transfer_body: &mut [u8]) -> Result<(), CodecError> {
        if transfer_body.len() < Self::HEADER_LEN {
            return Err(CodecError::InnerLengthMismatch {
                declared: 0,
                actual: transfer_body.len(),
            });
        }

        let inner_len = transfer_body.len() - Self::HEADER_LEN;
        write_u16_le(transfer_body, 0, VERSION);
        write_i32_le(transfer_body, INNER_LENGTH_OFFSET, inner_len as i32);
        transfer_body[RESERVED_OFFSET..RESERVED_OFFSET + 4].copy_from_slice(&self.reserved6_10);
        write_i32_le(
            transfer_body,
            MESSAGE_KIND_OFFSET,
            self.message_kind.to_i32(),
        );
        write_i32_le(transfer_body, SOURCE_GALAXY_OFFSET, self.source_galaxy_id);
        write_i32_le(
            transfer_body,
            SOURCE_PLATFORM_OFFSET,
            self.source_platform_id,
        );
        write_i32_le(transfer_body, LOCAL_ENGINE_OFFSET, self.local_engine_id);
        write_i32_le(transfer_body, TARGET_GALAXY_OFFSET, self.target_galaxy_id);
        write_i32_le(
            transfer_body,
            TARGET_PLATFORM_OFFSET,
            self.target_platform_id,
        );
        write_i32_le(transfer_body, TARGET_ENGINE_OFFSET, self.target_engine_id);
        write_i32_le(transfer_body, PROTOCOL_MARKER_OFFSET, PROTOCOL_MARKER);
        write_i32_le(transfer_body, TIMEOUT_OFFSET, self.timeout_ms);
        Ok(())
    }

    /// Convenience encoder that allocates a buffer of `46 + inner_body.len()`,
    /// writes the header, and copies `inner_body` into the tail. Mirrors the
    /// shape of `NmxTransferEnvelope.Encode`.
    pub fn encode_with_inner(self, inner_body: &[u8]) -> Vec<u8> {
        let mut out = vec![0u8; Self::HEADER_LEN + inner_body.len()];
        // write_to validates and never errors when the buffer is large enough,
        // so this branch is unreachable in practice. We propagate the bug as
        // an empty buffer rather than panicking.
        if self.write_to(&mut out).is_err() {
            return Vec::new();
        }
        out[Self::HEADER_LEN..].copy_from_slice(inner_body);
        out
    }
}

#[inline]
fn read_u16_le(bytes: &[u8], offset: usize) -> u16 {
    u16::from_le_bytes([bytes[offset], bytes[offset + 1]])
}

#[inline]
fn read_i32_le(bytes: &[u8], offset: usize) -> i32 {
    i32::from_le_bytes([
        bytes[offset],
        bytes[offset + 1],
        bytes[offset + 2],
        bytes[offset + 3],
    ])
}

#[inline]
fn write_u16_le(bytes: &mut [u8], offset: usize, value: u16) {
    let le = value.to_le_bytes();
    bytes[offset..offset + 2].copy_from_slice(&le);
}

#[inline]
fn write_i32_le(bytes: &mut [u8], offset: usize, value: i32) {
    let le = value.to_le_bytes();
    bytes[offset..offset + 4].copy_from_slice(&le);
}

#[cfg(test)]
#[allow(clippy::unwrap_used, clippy::expect_used, clippy::indexing_slicing)]
mod tests {
    use super::*;

    fn sample_envelope() -> NmxTransferEnvelope {
        NmxTransferEnvelope {
            message_kind: NmxTransferMessageKind::Write,
            source_galaxy_id: 1,
            source_platform_id: 1,
            local_engine_id: 5,
            target_galaxy_id: 1,
            target_platform_id: 2,
            target_engine_id: 17,
            timeout_ms: 30000,
            reserved6_10: [0; 4],
        }
    }

    #[test]
    fn round_trip_default_envelope() {
        let env = sample_envelope();
        let inner = [0xab, 0xcd, 0xef];
        let encoded = env.encode_with_inner(&inner);
        assert_eq!(encoded.len(), 46 + 3);
        let parsed = NmxTransferEnvelope::parse(&encoded).unwrap();
        assert_eq!(env, parsed);
        assert_eq!(&encoded[46..], &inner);
    }

    #[test]
    fn protocol_marker_bytes_are_le() {
        let env = sample_envelope();
        let encoded = env.encode_with_inner(&[]);
        // 0x0201 LE = 01 02 00 00
        assert_eq!(encoded[38], 0x01);
        assert_eq!(encoded[39], 0x02);
        assert_eq!(encoded[40], 0x00);
        assert_eq!(encoded[41], 0x00);
    }

    #[test]
    fn version_bytes_are_le_one() {
        let env = sample_envelope();
        let encoded = env.encode_with_inner(&[]);
        assert_eq!(encoded[0], 0x01);
        assert_eq!(encoded[1], 0x00);
    }

    #[test]
    fn reserved_bytes_round_trip() {
        // Construct an envelope with non-zero reserved bytes (as if parsed
        // from a captured frame). Encode and re-parse — they must survive.
        let env = NmxTransferEnvelope {
            reserved6_10: [0xde, 0xad, 0xbe, 0xef],
            ..sample_envelope()
        };
        let encoded = env.encode_with_inner(&[]);
        assert_eq!(&encoded[6..10], &[0xde, 0xad, 0xbe, 0xef]);
        let parsed = NmxTransferEnvelope::parse(&encoded).unwrap();
        assert_eq!(parsed.reserved6_10, [0xde, 0xad, 0xbe, 0xef]);
    }

    #[test]
    fn parse_rejects_wrong_version() {
        let mut encoded = sample_envelope().encode_with_inner(&[]);
        encoded[0] = 0x02; // version = 2
        let err = NmxTransferEnvelope::parse(&encoded).unwrap_err();
        assert!(matches!(err, CodecError::UnsupportedVersion { .. }));
    }

    #[test]
    fn parse_rejects_inner_length_mismatch() {
        let mut encoded = sample_envelope().encode_with_inner(&[0; 4]);
        // Corrupt the inner_length field to claim 100 inner bytes when
        // there are only 4.
        write_i32_le(&mut encoded, INNER_LENGTH_OFFSET, 100);
        let err = NmxTransferEnvelope::parse(&encoded).unwrap_err();
        assert!(matches!(err, CodecError::InnerLengthMismatch { .. }));
    }

    #[test]
    fn parse_rejects_wrong_protocol_marker() {
        let mut encoded = sample_envelope().encode_with_inner(&[]);
        write_i32_le(&mut encoded, PROTOCOL_MARKER_OFFSET, 0x0102);
        let err = NmxTransferEnvelope::parse(&encoded).unwrap_err();
        assert!(matches!(err, CodecError::UnsupportedProtocolMarker(0x0102)));
    }

    #[test]
    fn parse_rejects_short_buffer() {
        let err = NmxTransferEnvelope::parse(&[0u8; 45]).unwrap_err();
        assert!(matches!(err, CodecError::ShortRead { .. }));
    }

    #[test]
    fn message_kind_round_trips() {
        for kind in [
            NmxTransferMessageKind::Metadata,
            NmxTransferMessageKind::ItemControl,
            NmxTransferMessageKind::Write,
        ] {
            let env = NmxTransferEnvelope {
                message_kind: kind,
                ..sample_envelope()
            };
            let encoded = env.encode_with_inner(&[]);
            let parsed = NmxTransferEnvelope::parse(&encoded).unwrap();
            assert_eq!(parsed.message_kind, kind);
        }
    }

    #[test]
    fn header_length_constant() {
        assert_eq!(NmxTransferEnvelope::HEADER_LEN, 46);
        assert_eq!(ENVELOPE_HEADER_LEN, 46);
    }
}