//! `INmxSvcCallback` request body parser + response body encoder.
//!
//! Direct port of `src/MxNativeClient/NmxSvcCallbackMessages.cs`. Decodes the
//! single `byte[] buffer` parameter the AVEVA service marshals through
//! `INmxSvcCallback::DataReceived` (opnum 3) and `StatusReceived` (opnum 4),
//! and produces the matching `HRESULT`-bearing response body the callback
//! exporter writes back.
//!
//! Per `NmxSvcCallbackMessages.cs:14-36`, the inbound body is:
//!
//! ```text
//!  offset  size                                 field
//!   0     32  OrpcThis                          (encoded length without extensions)
//!  32      4  size            i32 LE            byte-array logical length
//!  36      4  max_count       i32 LE            conformant-array max count
//!  40   size  body                              raw bytes carried inside the callback
//! ```
//!
//! `size` and `max_count` are NDR-marshalled `int` values; .NET asserts both
//! are non-negative and `max_count >= size` (`cs:24`).

#![allow(clippy::indexing_slicing)]

use crate::error::RpcError;
use crate::guid::Guid;
use crate::nmx_metadata::INMX_SVC_CALLBACK_IID;
use crate::orpc::{OrpcThat, OrpcThis};

/// Convenience re-export so callers can match the .NET `InterfaceId` static
/// (`NmxSvcCallbackMessages.cs:9`).
pub const INTERFACE_ID: Guid = INMX_SVC_CALLBACK_IID;

/// Opnum for `INmxSvcCallback::DataReceived` (`cs:11`). Same value as
/// [`crate::nmx_metadata::DATA_RECEIVED.opnum`].
pub const DATA_RECEIVED_OPNUM: u16 = 3;

/// Opnum for `INmxSvcCallback::StatusReceived` (`cs:12`). Same value as
/// [`crate::nmx_metadata::STATUS_RECEIVED.opnum`].
pub const STATUS_RECEIVED_OPNUM: u16 = 4;

/// Decoded callback request — mirrors `NmxCallbackRequest`
/// (`NmxSvcCallbackMessages.cs:5`).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct NmxCallbackRequest {
    pub orpc_this: OrpcThis,
    pub body: Vec<u8>,
}

/// Header overhead before the `body` bytes: `OrpcThis(32) + size(4) +
/// max_count(4) = 40` (`NmxSvcCallbackMessages.cs:16,29`).
pub const CALLBACK_REQUEST_HEADER_LEN: usize = OrpcThis::ENCODED_LEN + 8;

/// Parse an inbound callback request body. Mirrors `ParseCallbackRequest`
/// (`NmxSvcCallbackMessages.cs:14-36`).
///
/// # Errors
///
/// - [`RpcError::ShortRead`] when `buffer.len() < 40` (`cs:16-19`).
/// - [`RpcError::Decode`] when `size < 0` or `max_count < size`
///   (`cs:24-27`), or when the declared `size` runs past the buffer
///   (`cs:30-33`).
pub fn parse_callback_request(buffer: &[u8]) -> Result<NmxCallbackRequest, RpcError> {
    if buffer.len() < CALLBACK_REQUEST_HEADER_LEN {
        return Err(RpcError::ShortRead {
            expected: CALLBACK_REQUEST_HEADER_LEN,
            actual: buffer.len(),
        });
    }

    let orpc_this = OrpcThis::parse(&buffer[..OrpcThis::ENCODED_LEN])?;

    // size and max_count are .NET `int` (i32 LE). Negative values are
    // explicitly rejected by the .NET reference (`cs:24`).
    let size_i32 = i32::from_le_bytes([
        buffer[OrpcThis::ENCODED_LEN],
        buffer[OrpcThis::ENCODED_LEN + 1],
        buffer[OrpcThis::ENCODED_LEN + 2],
        buffer[OrpcThis::ENCODED_LEN + 3],
    ]);
    let max_count_i32 = i32::from_le_bytes([
        buffer[OrpcThis::ENCODED_LEN + 4],
        buffer[OrpcThis::ENCODED_LEN + 5],
        buffer[OrpcThis::ENCODED_LEN + 6],
        buffer[OrpcThis::ENCODED_LEN + 7],
    ]);

    if size_i32 < 0 || max_count_i32 < size_i32 {
        return Err(RpcError::Decode {
            offset: OrpcThis::ENCODED_LEN,
            reason: "callback request has invalid array size metadata",
            buffer_len: buffer.len(),
        });
    }

    let size = size_i32 as usize;
    let body_offset = CALLBACK_REQUEST_HEADER_LEN;
    if body_offset + size > buffer.len() {
        return Err(RpcError::Decode {
            offset: body_offset,
            reason: "callback request byte array is truncated",
            buffer_len: buffer.len(),
        });
    }

    Ok(NmxCallbackRequest {
        orpc_this,
        body: buffer[body_offset..body_offset + size].to_vec(),
    })
}

/// Encode the callback response body — `OrpcThat(8) + hresult(4) = 12`
/// bytes. Mirrors `EncodeCallbackResponse` (`NmxSvcCallbackMessages.cs:38-44`).
#[must_use]
pub fn encode_callback_response(hresult: i32) -> [u8; 12] {
    let mut buf = [0u8; 12];
    let orpc_that = OrpcThat {
        flags: 0,
        extensions_referent_id: 0,
    }
    .encode();
    buf[..OrpcThat::ENCODED_LEN].copy_from_slice(&orpc_that);
    buf[OrpcThat::ENCODED_LEN..].copy_from_slice(&hresult.to_le_bytes());
    buf
}

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

    fn make_request(
        body: &[u8],
        size_override: Option<i32>,
        max_count_override: Option<i32>,
    ) -> Vec<u8> {
        let mut buf = Vec::with_capacity(CALLBACK_REQUEST_HEADER_LEN + body.len());
        let orpc_this = OrpcThis::create(Guid::new([0x10; 16]), None).encode();
        buf.extend_from_slice(&orpc_this);
        let size = size_override.unwrap_or(body.len() as i32);
        let max_count = max_count_override.unwrap_or(body.len() as i32);
        buf.extend_from_slice(&size.to_le_bytes());
        buf.extend_from_slice(&max_count.to_le_bytes());
        buf.extend_from_slice(body);
        buf
    }

    #[test]
    fn opnums_match_dotnet() {
        assert_eq!(DATA_RECEIVED_OPNUM, 3);
        assert_eq!(STATUS_RECEIVED_OPNUM, 4);
    }

    #[test]
    fn interface_id_matches_callback_iid() {
        assert_eq!(INTERFACE_ID, INMX_SVC_CALLBACK_IID);
    }

    #[test]
    fn parse_round_trip_empty_body() {
        let bytes = make_request(&[], None, None);
        let parsed = parse_callback_request(&bytes).unwrap();
        assert!(parsed.body.is_empty());
    }

    #[test]
    fn parse_round_trip_carries_payload() {
        let body: &[u8] = &[0xDE, 0xAD, 0xBE, 0xEF, 0x01, 0x02];
        let bytes = make_request(body, None, None);
        let parsed = parse_callback_request(&bytes).unwrap();
        assert_eq!(parsed.body, body);
    }

    #[test]
    fn parse_short_buffer_errors() {
        let err = parse_callback_request(&[0u8; 39]).unwrap_err();
        assert!(matches!(
            err,
            RpcError::ShortRead {
                expected: 40,
                actual: 39
            }
        ));
    }

    #[test]
    fn parse_negative_size_rejected() {
        let bytes = make_request(&[], Some(-1), Some(0));
        assert!(matches!(
            parse_callback_request(&bytes),
            Err(RpcError::Decode { .. })
        ));
    }

    #[test]
    fn parse_max_count_less_than_size_rejected() {
        let bytes = make_request(&[0xAA; 8], Some(8), Some(4));
        assert!(matches!(
            parse_callback_request(&bytes),
            Err(RpcError::Decode { .. })
        ));
    }

    #[test]
    fn parse_truncated_body_rejected() {
        // Declare 16 bytes but supply only 4.
        let mut bytes = make_request(&[0xAA; 4], Some(16), Some(16));
        // Trim trailing bytes so the buffer is shorter than declared size.
        bytes.truncate(CALLBACK_REQUEST_HEADER_LEN + 4);
        assert!(matches!(
            parse_callback_request(&bytes),
            Err(RpcError::Decode { .. })
        ));
    }

    #[test]
    fn encode_response_layout() {
        // Success response — OrpcThat zeros + hresult=0.
        let r = encode_callback_response(0);
        assert_eq!(r.len(), 12);
        assert_eq!(&r[..8], &[0u8; 8]);
        assert_eq!(&r[8..], &0i32.to_le_bytes());

        // Negative hresult round-trip.
        let r = encode_callback_response(unchecked_negative());
        assert_eq!(&r[8..], &unchecked_negative().to_le_bytes());
    }

    fn unchecked_negative() -> i32 {
        // 0x80004005 = E_FAIL, the canonical generic failure HRESULT.
        // .NET would write `unchecked((int)0x80004005)`; Rust expresses
        // the same bit pattern as `i32::MIN`-aligned negative.
        0x80004005u32 as i32
    }
}