//! `NmxSubscriptionMessage` — `0x32` SubscriptionStatus / `0x33` DataUpdate
//! callback decoder.
//!
//! Direct port of `src/MxNativeCodec/NmxSubscriptionMessage.cs`. This module
//! decodes the inner-body of the NMX subscription callback delivered through
//! the NMX `TransferData` envelope (callers should call
//! [`NmxTransferEnvelope::parse`](crate::NmxTransferEnvelope::parse) first to
//! peel the 46-byte transfer header, then hand the remaining bytes to
//! [`NmxSubscriptionMessage::parse_inner`]).
//!
//! ## Wire layout summary
//!
//! Both message kinds share the 23-byte preamble
//! (`NmxSubscriptionMessage.cs:52-55`):
//!
//! ```text
//!  offset  size  field
//!  0       1     command          (0x32 SubscriptionStatus, 0x33 DataUpdate)
//!  1       2     version          u16 LE
//!  3       4     record_count     i32 LE
//!  7       16    operation_id     GUID (.NET layout)
//! ```
//!
//! `0x32` SubscriptionStatus extends to 39 bytes by appending a 16-byte
//! `item_correlation_id` GUID at offset 23 (`NmxSubscriptionMessage.cs:98-99`).
//! `0x33` DataUpdate has **no** correlation id — its records start at offset 23
//! (`NmxSubscriptionMessage.cs:76-77`).
//!
//! ## Record layout
//!
//! - SubscriptionStatus record: `status i32 + detail_status i32 + quality u16
//!   + timestamp_filetime i64 + wire_kind u8 + value` (`hasDetailStatus=true`,
//!   `NmxSubscriptionMessage.cs:117-155`).
//! - DataUpdate record: `quality u16 + timestamp_filetime i64 + wire_kind u8
//!   + value` (`hasDetailStatus=false`).
//!
//! ## Hard-error: DataUpdate multi-record
//!
//! The .NET reference rejects DataUpdate bodies with `record_count != 1`
//! (`NmxSubscriptionMessage.cs:71-74`). The Rust codec mirrors that hard error
//! via [`CodecError::Decode`] — see `design/70-risks-and-open-questions.md` R13
//! for the soft-error path that the higher-level session layer may add later.
//!
//! ## Encoder/decoder asymmetry: array element width
//!
//! On the wire, the array header is `count u16 LE` at body+4 followed by
//! `element_width` at body+6. The decoder reads `element_width` as **`i32`
//! LE** (`NmxSubscriptionMessage.cs:264-265`); the encoder side (`write_message.rs`,
//! NOT this module) writes `u16/u16`. This asymmetry is real and intentional —
//! the decoder must accept whatever the native NMX service emits.
//!
//! ## Wire-kind table
//!
//! Scalar: `0x01` Boolean, `0x02` Int32, `0x03` Float32, `0x04` Float64,
//! `0x05` String, `0x06` DateTime, `0x07` ElapsedTime
//! (`NmxSubscriptionMessage.cs:165-176`).
//!
//! Array: `0x41` BoolArray, `0x42` Int32Array, `0x43` Float32Array,
//! `0x44` Float64Array, `0x45` StringArray, `0x46` DateTimeArray
//! (`NmxSubscriptionMessage.cs:268-277`). Note the encoder collapses
//! StringArray/DateTimeArray to `0x45`; the decoder keeps `0x46` as
//! DateTimeArray.

// Direct byte indexing — see reference_handle.rs for rationale (every byte
// access is preceded by an explicit length check; matches the .NET source's
// `BinaryPrimitives` calls 1:1 and is far more readable than `.get(n)?`).
#![allow(clippy::indexing_slicing)]

use crate::error::CodecError;
use crate::{MxValue, MxValueKind};

/// `0x32` — SubscriptionStatus. Per `NmxSubscriptionMessage.cs:36`.
pub const SUBSCRIPTION_STATUS_COMMAND: u8 = 0x32;

/// `0x33` — DataUpdate. Per `NmxSubscriptionMessage.cs:37`.
pub const DATA_UPDATE_COMMAND: u8 = 0x33;

/// 16-byte GUID with the .NET on-the-wire layout used by `new Guid(span)` /
/// `Guid.WriteToSpan` (data1 LE, data2 LE, data3 LE, then 8 raw bytes).
/// Mirrors `NmxSubscriptionMessage.cs:55,98` (the .NET `Guid(ReadOnlySpan<byte>)`
/// constructor consumes exactly 16 bytes in this layout).
///
/// Stored as the raw 16 bytes verbatim — the codec preserves whatever the
/// service emits and surfaces it to consumers as a stable identifier; no
/// interpretation is needed at the codec layer.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
pub struct NmxGuid(pub [u8; 16]);

impl NmxGuid {
    /// Encoded GUID size on the wire.
    pub const ENCODED_LEN: usize = 16;

    /// Construct from a 16-byte slice.
    ///
    /// # Errors
    ///
    /// Returns [`CodecError::ShortRead`] if `bytes.len() != 16`.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, CodecError> {
        if bytes.len() != Self::ENCODED_LEN {
            return Err(CodecError::ShortRead {
                expected: Self::ENCODED_LEN,
                actual: bytes.len(),
            });
        }
        let mut buf = [0u8; 16];
        buf.copy_from_slice(bytes);
        Ok(Self(buf))
    }
}

/// One record from a [`NmxSubscriptionMessage`]. SubscriptionStatus records
/// carry `status`/`detail_status`; DataUpdate records leave both as `None`.
///
/// Per `NmxSubscriptionMessage.cs:12-26`.
#[derive(Debug, Clone, PartialEq)]
pub struct NmxSubscriptionRecord {
    /// Status code — `i32` always present for both DataUpdate and
    /// SubscriptionStatus records (`NmxSubscriptionMessage.cs:126-127` reads
    /// it unconditionally).
    pub status: i32,
    /// Detail-status code — present for SubscriptionStatus (`0x32`) only;
    /// `None` for DataUpdate (`NmxSubscriptionMessage.cs:129-134`).
    pub detail_status: Option<i32>,
    /// Quality bitfield (`NmxSubscriptionMessage.cs:136-137`).
    pub quality: u16,
    /// Windows FILETIME ticks (100ns units since 1601-01-01 UTC). Mirrors the
    /// raw `i64` that the .NET reference passes to `DateTime.FromFileTimeUtc`
    /// (`NmxSubscriptionMessage.cs:139-150`).
    pub timestamp_filetime: i64,
    /// Wire-kind tag from the body (`NmxSubscriptionMessage.cs:142`).
    pub wire_kind: u8,
    /// Decoded value, when the wire kind is recognized and the body is
    /// well-formed. Malformed/unknown payloads (negative lengths, bad
    /// element widths, unknown wire kinds) yield `None` — mirrors the .NET
    /// `NmxCallbackValue.Value = null` paths (e.g. `NmxSubscriptionMessage.cs:182,199`).
    pub value: Option<MxValue>,
    /// Offset into the inner buffer where this record began.
    pub offset: usize,
    /// Total bytes this record consumed from the inner buffer.
    pub length: usize,
}

/// Parsed `0x32`/`0x33` subscription callback message.
/// Per `NmxSubscriptionMessage.cs:28-34`.
#[derive(Debug, Clone, PartialEq)]
pub struct NmxSubscriptionMessage {
    /// `0x32` SubscriptionStatus or `0x33` DataUpdate.
    pub command: u8,
    /// `version` field (`NmxSubscriptionMessage.cs:53`).
    pub version: u16,
    /// `record_count` field (`NmxSubscriptionMessage.cs:54`).
    pub record_count: i32,
    /// `operation_id` GUID (`NmxSubscriptionMessage.cs:55`).
    pub operation_id: NmxGuid,
    /// `item_correlation_id` GUID — present only on `0x32` SubscriptionStatus
    /// (`NmxSubscriptionMessage.cs:98`); always `None` on `0x33` DataUpdate.
    pub item_correlation_id: Option<NmxGuid>,
    /// Decoded records.
    pub records: Vec<NmxSubscriptionRecord>,
}

impl NmxSubscriptionMessage {
    /// Length of the shared 23-byte preamble (`NmxSubscriptionMessage.cs:47`).
    pub const PREAMBLE_LEN: usize = 23;

    /// Length of the SubscriptionStatus header — preamble + 16-byte
    /// correlation id (`NmxSubscriptionMessage.cs:93,99`).
    pub const SUBSCRIPTION_STATUS_HEADER_LEN: usize = 39;

    /// Parse the inner body (post-46-byte-envelope) of an NMX subscription
    /// callback. Mirrors `NmxSubscriptionMessage.ParseInner`
    /// (`NmxSubscriptionMessage.cs:45-63`).
    ///
    /// # Errors
    ///
    /// - [`CodecError::ShortRead`] if `inner.len() < 23`.
    /// - [`CodecError::UnexpectedOpcode`] if the command byte is neither
    ///   `0x32` nor `0x33`.
    /// - [`CodecError::Decode`] for protocol violations (multi-record
    ///   DataUpdate, truncated records, etc.).
    pub fn parse_inner(inner: &[u8]) -> Result<Self, CodecError> {
        if inner.len() < Self::PREAMBLE_LEN {
            return Err(CodecError::ShortRead {
                expected: Self::PREAMBLE_LEN,
                actual: inner.len(),
            });
        }

        let command = inner[0];
        let version = read_u16_le(inner, 1);
        let record_count = read_i32_le(inner, 3);
        let operation_id = NmxGuid::from_bytes(&inner[7..23])?;

        match command {
            SUBSCRIPTION_STATUS_COMMAND => {
                parse_subscription_status(inner, version, record_count, operation_id)
            }
            DATA_UPDATE_COMMAND => parse_data_update(inner, version, record_count, operation_id),
            _ => Err(CodecError::UnexpectedOpcode(command)),
        }
    }
}

/// `0x33` DataUpdate. Mirrors `NmxSubscriptionMessage.ParseDataUpdate`
/// (`NmxSubscriptionMessage.cs:65-85`).
fn parse_data_update(
    inner: &[u8],
    version: u16,
    record_count: i32,
    operation_id: NmxGuid,
) -> Result<NmxSubscriptionMessage, CodecError> {
    // .NET hard-throws when `record_count != 1` (`NmxSubscriptionMessage.cs:71-74`).
    // Mirror that here — the soft-error path is owned by the higher session
    // layer (R13 in `design/70-risks-and-open-questions.md`).
    if record_count != 1 {
        return Err(CodecError::Decode {
            offset: 3,
            reason: "DataUpdate multi-record bodies are not yet supported",
            buffer_len: inner.len(),
        });
    }

    // Records start immediately after the 23-byte preamble — DataUpdate has
    // no correlation id (`NmxSubscriptionMessage.cs:76-77`).
    let record = parse_record(inner, NmxSubscriptionMessage::PREAMBLE_LEN, false)?;
    Ok(NmxSubscriptionMessage {
        command: DATA_UPDATE_COMMAND,
        version,
        record_count,
        operation_id,
        item_correlation_id: None,
        records: vec![record],
    })
}

/// `0x32` SubscriptionStatus. Mirrors
/// `NmxSubscriptionMessage.ParseSubscriptionStatus`
/// (`NmxSubscriptionMessage.cs:87-115`).
fn parse_subscription_status(
    inner: &[u8],
    version: u16,
    record_count: i32,
    operation_id: NmxGuid,
) -> Result<NmxSubscriptionMessage, CodecError> {
    if inner.len() < NmxSubscriptionMessage::SUBSCRIPTION_STATUS_HEADER_LEN {
        return Err(CodecError::ShortRead {
            expected: NmxSubscriptionMessage::SUBSCRIPTION_STATUS_HEADER_LEN,
            actual: inner.len(),
        });
    }

    let item_correlation_id = NmxGuid::from_bytes(&inner[23..39])?;
    let mut offset = NmxSubscriptionMessage::SUBSCRIPTION_STATUS_HEADER_LEN;
    // `record_count` is `i32` on the wire; clamp negatives to zero. The .NET
    // for-loop `for (int i = 0; i < recordCount; i++)` also yields zero
    // iterations for negative counts (`NmxSubscriptionMessage.cs:101`).
    let count = if record_count < 0 {
        0usize
    } else {
        record_count as usize
    };
    let mut records = Vec::with_capacity(count);
    for _ in 0..count {
        let record = parse_record(inner, offset, true)?;
        offset += record.length;
        records.push(record);
    }

    Ok(NmxSubscriptionMessage {
        command: SUBSCRIPTION_STATUS_COMMAND,
        version,
        record_count,
        operation_id,
        item_correlation_id: Some(item_correlation_id),
        records,
    })
}

/// Parse a single record. Mirrors `NmxSubscriptionMessage.ParseRecord`
/// (`NmxSubscriptionMessage.cs:117-155`). When `has_detail_status` is true the
/// record begins with `status i32 + detail_status i32`; otherwise neither is
/// present.
fn parse_record(
    body: &[u8],
    offset: usize,
    has_detail_status: bool,
) -> Result<NmxSubscriptionRecord, CodecError> {
    // Minimum length is 19 with detail-status (status + detail_status +
    // quality + timestamp + wire_kind = 4+4+2+8+1) and 15 without
    // (`NmxSubscriptionMessage.cs:119`). We additionally require the
    // wire-kind byte itself to be present (`body[offset++]` at line 142).
    let minimum_length = if has_detail_status { 19 } else { 15 };
    if offset + minimum_length > body.len() {
        return Err(CodecError::Decode {
            offset,
            reason: "subscription record truncated before fixed header",
            buffer_len: body.len(),
        });
    }

    let start = offset;
    let mut cursor = offset;

    // `status: i32` is read unconditionally for both DataUpdate and
    // SubscriptionStatus records (`NmxSubscriptionMessage.cs:126-127`).
    //
    // FOLLOW-UP (M1 wave-1 audit): An earlier port draft conditionally read
    // `status` only when `has_detail_status=true`, then required min length 15
    // for DataUpdate without consuming the leading 4 bytes — leaving them to
    // be misread as `quality`. Verified fixed here; if any other codec agent
    // applied the same `hasDetailStatus`-gated conditional read pattern,
    // re-audit. Min lengths are 15 (DataUpdate, status+quality+filetime+kind)
    // and 19 (SubscriptionStatus, +detail_status). See
    // `design/70-risks-and-open-questions.md` "M1 hasDetailStatus audit"
    // follow-up entry.
    let status = read_i32_le(body, cursor);
    cursor += 4;

    let detail_status = if has_detail_status {
        let d = read_i32_le(body, cursor);
        cursor += 4;
        Some(d)
    } else {
        None
    };

    let quality = read_u16_le(body, cursor);
    cursor += 2;

    let timestamp_filetime = read_i64_le(body, cursor);
    cursor += 8;

    let wire_kind = body[cursor];
    cursor += 1;

    let (value, encoded_len) = decode_value(wire_kind, &body[cursor..]);
    cursor += encoded_len;

    Ok(NmxSubscriptionRecord {
        status,
        detail_status,
        quality,
        timestamp_filetime,
        wire_kind,
        value,
        offset: start,
        length: cursor - start,
    })
}

/// Decode a value following a wire-kind byte. Returns `(decoded, bytes_consumed)`.
/// Mirrors `NmxSubscriptionMessage.DecodeValue` (`NmxSubscriptionMessage.cs:157-176`)
/// and the per-kind helpers below it.
///
/// On any short / malformed payload returns `(None, 0)` — matching the .NET
/// behaviour where `NmxCallbackValue.Value` is null and `EncodedLength` is 0.
/// (Subsequent records following a malformed value are unrecoverable; the
/// .NET reference exhibits the same property.)
fn decode_value(wire_kind: u8, body: &[u8]) -> (Option<MxValue>, usize) {
    if body.is_empty() {
        return (None, 0);
    }

    match wire_kind {
        // 0x01 Boolean — single byte, non-zero is true (`NmxSubscriptionMessage.cs:166`).
        0x01 if !body.is_empty() => (Some(MxValue::Boolean(body[0] != 0)), 1),
        // 0x02 Int32 (`NmxSubscriptionMessage.cs:167`).
        0x02 if body.len() >= 4 => (Some(MxValue::Int32(read_i32_le(body, 0))), 4),
        // 0x03 Float32 — bit-cast i32->f32 mirrors `Int32BitsToSingle`
        // (`NmxSubscriptionMessage.cs:168`).
        0x03 if body.len() >= 4 => {
            let bits = read_i32_le(body, 0);
            (Some(MxValue::Float32(f32::from_bits(bits as u32))), 4)
        }
        // 0x04 Float64 — bit-cast i64->f64 (`NmxSubscriptionMessage.cs:169`).
        0x04 if body.len() >= 8 => {
            let bits = read_i64_le(body, 0);
            (Some(MxValue::Float64(f64::from_bits(bits as u64))), 8)
        }
        // 0x05 String (`NmxSubscriptionMessage.cs:170`).
        0x05 => decode_string_value(body),
        // 0x06 DateTime (`NmxSubscriptionMessage.cs:171`).
        0x06 => decode_datetime_value(body),
        // 0x07 ElapsedTime (`NmxSubscriptionMessage.cs:172`).
        0x07 => decode_elapsed_time_value(body),
        // Arrays 0x41..0x46 (`NmxSubscriptionMessage.cs:173`).
        0x41..=0x46 => decode_array_value(wire_kind, body),
        // Unknown / malformed: matches the `_ => new NmxCallbackValue(...
        // EncodedLength=0)` arms in the .NET source.
        _ => (None, 0),
    }
}

/// Decode a string body. Mirrors `DecodeStringValue`
/// (`NmxSubscriptionMessage.cs:178-210`).
///
/// Layout: `record_length i32 + text_byte_length i32 + utf16le bytes`. The
/// degenerate `record_length == 4` case represents an empty string and
/// consumes exactly 4 bytes (`NmxSubscriptionMessage.cs:186-189`). Otherwise
/// the trailing two-byte UTF-16 NUL is stripped if present
/// (`NmxSubscriptionMessage.cs:203-206`).
fn decode_string_value(body: &[u8]) -> (Option<MxValue>, usize) {
    if body.len() < 4 {
        return (None, 0);
    }
    let record_length = read_i32_le(body, 0);
    if record_length == 4 {
        return (Some(MxValue::String(String::new())), 4);
    }
    if body.len() < 8 {
        return (None, 0);
    }
    let text_byte_length = read_i32_le(body, 4);
    // .NET checks: `recordLength < 8 || textByteLength < 0 ||
    // recordLength != textByteLength + 4 || body.Length < 8 + textByteLength`
    // (`NmxSubscriptionMessage.cs:197`).
    if record_length < 8 || text_byte_length < 0 {
        return (None, 0);
    }
    let text_byte_length_us = text_byte_length as usize;
    if record_length as usize != text_byte_length_us + 4 || body.len() < 8 + text_byte_length_us {
        return (None, 0);
    }

    let mut text_bytes = &body[8..8 + text_byte_length_us];
    // Strip optional UTF-16LE NUL terminator.
    if text_bytes.len() >= 2
        && text_bytes[text_bytes.len() - 2] == 0
        && text_bytes[text_bytes.len() - 1] == 0
    {
        text_bytes = &text_bytes[..text_bytes.len() - 2];
    }
    let value = decode_utf16_le_lossy(text_bytes);
    (Some(MxValue::String(value)), 8 + text_byte_length_us)
}

/// Decode a DateTime body. Mirrors `DecodeDateTimeValue`
/// (`NmxSubscriptionMessage.cs:212-243`).
///
/// Two shapes exist on the wire:
/// 1. `record_length i32 + filetime i64 (+ trailer)` — used when
///    `body.len() >= 14` (`record_length >= 10`); consumes
///    `4 + record_length` bytes.
/// 2. Bare `filetime i64` — fallback when the framed shape doesn't fit;
///    consumes 8 bytes.
fn decode_datetime_value(body: &[u8]) -> (Option<MxValue>, usize) {
    if body.len() >= 14 {
        let record_length = read_i32_le(body, 0);
        if record_length >= 10 && body.len() >= 4 + record_length as usize {
            let file_time = read_i64_le(body, 4);
            // The .NET reference returns `Value = null` when the FILETIME
            // is out of range (`NmxSubscriptionMessage.cs:229`) but still
            // consumes `4 + record_length` bytes. We carry the raw FILETIME
            // verbatim — the codec preserves the wire value and lets the
            // higher layer judge validity.
            return (
                Some(MxValue::DateTime(file_time)),
                4 + record_length as usize,
            );
        }
    }
    if body.len() >= 8 {
        let file_time = read_i64_le(body, 0);
        return (Some(MxValue::DateTime(file_time)), 8);
    }
    (None, 0)
}

/// Decode an ElapsedTime body. Mirrors `DecodeElapsedTimeValue`
/// (`NmxSubscriptionMessage.cs:245-254`).
///
/// **Wire is signed i32 milliseconds** (`NmxSubscriptionMessage.cs:252` reads
/// `BinaryPrimitives.ReadInt32LittleEndian`). Negative values are valid and
/// must round-trip — Rust `Duration` is unsigned so we widen to `i64` ms in
/// `MxValue::ElapsedTime` (lib.rs:73-74).
fn decode_elapsed_time_value(body: &[u8]) -> (Option<MxValue>, usize) {
    if body.len() < 4 {
        return (None, 0);
    }
    let milliseconds = read_i32_le(body, 0);
    (Some(MxValue::ElapsedTime(milliseconds as i64)), 4)
}

/// Decode an array body. Mirrors `DecodeArrayValue`
/// (`NmxSubscriptionMessage.cs:256-278`).
///
/// Header layout (per the **decoder**, see module-level note about asymmetry):
/// `unknown 4 bytes + count u16 LE @+4 + element_width i32 LE @+6 + values`.
/// Total header = 10 bytes (`NmxSubscriptionMessage.cs:258`). The first 4
/// bytes appear to be a record-length / record-kind framing field; the .NET
/// reference does not interpret them and neither do we — they pass through
/// the consumed-byte accounting via the fixed `arrayHeaderLength = 10`.
fn decode_array_value(wire_kind: u8, body: &[u8]) -> (Option<MxValue>, usize) {
    const ARRAY_HEADER_LEN: usize = 10;
    if body.len() < ARRAY_HEADER_LEN {
        return (None, 0);
    }
    let count = read_u16_le(body, 4) as usize;
    // Decoder reads element_width as i32 LE — the encoder writes u16/u16 but
    // the wire emitted by NmxSvc puts an i32 here. (`NmxSubscriptionMessage.cs:265`.)
    let element_width = read_i32_le(body, 6);
    let values = &body[ARRAY_HEADER_LEN..];

    match wire_kind {
        0x41 => decode_bool_array(count, element_width, values),
        0x42 => decode_int32_array(count, element_width, values),
        0x43 => decode_float32_array(count, element_width, values),
        0x44 => decode_float64_array(count, element_width, values),
        0x45 => decode_string_array(count, values),
        0x46 => decode_datetime_array(count, element_width, values),
        // Unreachable given the guard in `decode_value`, but keep it total.
        _ => (None, 0),
    }
}

/// Bool array decoder — element width must be `sizeof(short) == 2`, elements
/// are `i16` LE where any non-zero value is true. Per the .NET reference the
/// wire encoding is `-1`/`0` (`NmxSubscriptionMessage.cs:280-294`).
fn decode_bool_array(count: usize, element_width: i32, values: &[u8]) -> (Option<MxValue>, usize) {
    if element_width != 2 {
        return (None, 0);
    }
    let needed = count.saturating_mul(2);
    if values.len() < needed {
        return (None, 0);
    }
    let mut out = Vec::with_capacity(count);
    for i in 0..count {
        // Boolean elements are i16 (`-1`/`0` in practice); `!= 0` covers both.
        let raw = read_i16_le(values, i * 2);
        out.push(raw != 0);
    }
    (Some(MxValue::BoolArray(out)), 10 + count * 2)
}

/// Int32 array decoder. Per `NmxSubscriptionMessage.cs:296-310`.
fn decode_int32_array(count: usize, element_width: i32, values: &[u8]) -> (Option<MxValue>, usize) {
    if element_width != 4 {
        return (None, 0);
    }
    let needed = count.saturating_mul(4);
    if values.len() < needed {
        return (None, 0);
    }
    let mut out = Vec::with_capacity(count);
    for i in 0..count {
        out.push(read_i32_le(values, i * 4));
    }
    (Some(MxValue::Int32Array(out)), 10 + count * 4)
}

/// Float32 array decoder. Per `NmxSubscriptionMessage.cs:312-326`.
fn decode_float32_array(
    count: usize,
    element_width: i32,
    values: &[u8],
) -> (Option<MxValue>, usize) {
    if element_width != 4 {
        return (None, 0);
    }
    let needed = count.saturating_mul(4);
    if values.len() < needed {
        return (None, 0);
    }
    let mut out = Vec::with_capacity(count);
    for i in 0..count {
        let bits = read_i32_le(values, i * 4);
        out.push(f32::from_bits(bits as u32));
    }
    (Some(MxValue::Float32Array(out)), 10 + count * 4)
}

/// Float64 array decoder. Per `NmxSubscriptionMessage.cs:328-342`.
fn decode_float64_array(
    count: usize,
    element_width: i32,
    values: &[u8],
) -> (Option<MxValue>, usize) {
    if element_width != 8 {
        return (None, 0);
    }
    let needed = count.saturating_mul(8);
    if values.len() < needed {
        return (None, 0);
    }
    let mut out = Vec::with_capacity(count);
    for i in 0..count {
        let bits = read_i64_le(values, i * 8);
        out.push(f64::from_bits(bits as u64));
    }
    (Some(MxValue::Float64Array(out)), 10 + count * 8)
}

/// DateTime array decoder. Per `NmxSubscriptionMessage.cs:344-359`.
/// Element width is **12** on the wire (FILETIME i64 + 4 bytes of padding /
/// trailer); we read the leading 8 bytes as the FILETIME and skip the rest.
fn decode_datetime_array(
    count: usize,
    element_width: i32,
    values: &[u8],
) -> (Option<MxValue>, usize) {
    if element_width != 12 {
        return (None, 0);
    }
    let needed = count.saturating_mul(12);
    if values.len() < needed {
        return (None, 0);
    }
    let mut out = Vec::with_capacity(count);
    for i in 0..count {
        // Only the leading 8 bytes are interpreted; the trailing 4 bytes
        // are not consumed by the .NET reference either (it calls
        // `BinaryPrimitives.ReadInt64LittleEndian` on the slice's first
        // 8 bytes, `NmxSubscriptionMessage.cs:354`).
        let file_time = read_i64_le(values, i * 12);
        out.push(file_time);
    }
    (Some(MxValue::DateTimeArray(out)), 10 + count * 12)
}

/// String array decoder. Per `NmxSubscriptionMessage.cs:361-392`.
///
/// Each element is `record_length i32 + element_kind u8 (must be 0x05) +
/// text_record_length i32 + text_byte_length i32 + utf16le bytes`. The
/// element header is 13 bytes. The whole array consumes
/// `10 + sum(13 + text_byte_length)` bytes.
fn decode_string_array(count: usize, values: &[u8]) -> (Option<MxValue>, usize) {
    let mut out = Vec::with_capacity(count);
    let mut offset = 0usize;
    for _ in 0..count {
        if offset + 13 > values.len() {
            return (None, 0);
        }
        let record_length = read_i32_le(values, offset);
        let element_kind = values[offset + 4];
        let text_record_length = read_i32_le(values, offset + 5);
        let text_byte_length = read_i32_le(values, offset + 9);

        // .NET checks: `recordLength < 9 || elementKind != 0x05 ||
        // textRecordLength != textByteLength + sizeof(int) ||
        // recordLength != 1 + sizeof(int) + sizeof(int) + textByteLength ||
        // offset + 13 + textByteLength > values.Length`
        // (`NmxSubscriptionMessage.cs:376`).
        if record_length < 9
            || element_kind != 0x05
            || text_byte_length < 0
            || text_record_length != text_byte_length + 4
            || record_length != 1 + 4 + 4 + text_byte_length
        {
            return (None, 0);
        }
        let text_byte_length_us = text_byte_length as usize;
        if offset + 13 + text_byte_length_us > values.len() {
            return (None, 0);
        }

        let mut text_bytes = &values[offset + 13..offset + 13 + text_byte_length_us];
        if text_bytes.len() >= 2
            && text_bytes[text_bytes.len() - 2] == 0
            && text_bytes[text_bytes.len() - 1] == 0
        {
            text_bytes = &text_bytes[..text_bytes.len() - 2];
        }
        out.push(decode_utf16_le_lossy(text_bytes));
        offset += 13 + text_byte_length_us;
    }
    (Some(MxValue::StringArray(out)), 10 + offset)
}

/// Map a wire-kind byte to its [`MxValueKind`] without decoding the payload.
/// Mirrors `ToValueKindOrNull` (`NmxSubscriptionMessage.cs:394-413`).
pub fn wire_kind_to_value_kind(wire_kind: u8) -> Option<MxValueKind> {
    Some(match wire_kind {
        0x01 => MxValueKind::Boolean,
        0x02 => MxValueKind::Int32,
        0x03 => MxValueKind::Float32,
        0x04 => MxValueKind::Float64,
        0x05 => MxValueKind::String,
        0x06 => MxValueKind::DateTime,
        0x07 => MxValueKind::ElapsedTime,
        0x41 => MxValueKind::BoolArray,
        0x42 => MxValueKind::Int32Array,
        0x43 => MxValueKind::Float32Array,
        0x44 => MxValueKind::Float64Array,
        0x45 => MxValueKind::StringArray,
        0x46 => MxValueKind::DateTimeArray,
        _ => return None,
    })
}

/// Decode UTF-16LE bytes lossily — invalid sequences become U+FFFD. Mirrors
/// `Encoding.Unicode.GetString` (`NmxSubscriptionMessage.cs:208,387`) which
/// is also lossy on malformed surrogates.
fn decode_utf16_le_lossy(bytes: &[u8]) -> String {
    let units: Vec<u16> = bytes
        .chunks_exact(2)
        .map(|c| u16::from_le_bytes([c[0], c[1]]))
        .collect();
    String::from_utf16_lossy(&units)
}

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

#[inline]
fn read_i16_le(bytes: &[u8], offset: usize) -> i16 {
    i16::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 read_i64_le(bytes: &[u8], offset: usize) -> i64 {
    i64::from_le_bytes([
        bytes[offset],
        bytes[offset + 1],
        bytes[offset + 2],
        bytes[offset + 3],
        bytes[offset + 4],
        bytes[offset + 5],
        bytes[offset + 6],
        bytes[offset + 7],
    ])
}

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

    /// Sample 16-byte GUID for hand-crafted bodies.
    const OPERATION_ID_BYTES: [u8; 16] = [
        0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
        0x10,
    ];
    const CORRELATION_ID_BYTES: [u8; 16] = [
        0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
        0x20,
    ];

    /// Build a DataUpdate (`0x33`) body with the given record bytes appended.
    fn data_update_body(record_count: i32, record: &[u8]) -> Vec<u8> {
        let mut out = Vec::with_capacity(23 + record.len());
        out.push(DATA_UPDATE_COMMAND);
        out.extend_from_slice(&1u16.to_le_bytes()); // version
        out.extend_from_slice(&record_count.to_le_bytes());
        out.extend_from_slice(&OPERATION_ID_BYTES);
        out.extend_from_slice(record);
        out
    }

    /// Build a SubscriptionStatus (`0x32`) body.
    fn subscription_status_body(record_count: i32, records: &[u8]) -> Vec<u8> {
        let mut out = Vec::with_capacity(39 + records.len());
        out.push(SUBSCRIPTION_STATUS_COMMAND);
        out.extend_from_slice(&1u16.to_le_bytes());
        out.extend_from_slice(&record_count.to_le_bytes());
        out.extend_from_slice(&OPERATION_ID_BYTES);
        out.extend_from_slice(&CORRELATION_ID_BYTES);
        out.extend_from_slice(records);
        out
    }

    /// DataUpdate record: `status(4) + quality(2) + filetime(8) + wire_kind(1)
    /// + value` — 15-byte fixed prefix per `NmxSubscriptionMessage.cs:119,126-143`
    ///   (status is read unconditionally; detail_status is the only field
    ///   gated on hasDetailStatus).
    fn data_record(quality: u16, filetime: i64, wire_kind: u8, value: &[u8]) -> Vec<u8> {
        data_record_with_status(0, quality, filetime, wire_kind, value)
    }

    fn data_record_with_status(
        status: i32,
        quality: u16,
        filetime: i64,
        wire_kind: u8,
        value: &[u8],
    ) -> Vec<u8> {
        let mut out = Vec::with_capacity(15 + value.len());
        out.extend_from_slice(&status.to_le_bytes());
        out.extend_from_slice(&quality.to_le_bytes());
        out.extend_from_slice(&filetime.to_le_bytes());
        out.push(wire_kind);
        out.extend_from_slice(value);
        out
    }

    /// SubscriptionStatus record: `status(4) + detail_status(4) + quality(2) +
    /// filetime(8) + wire_kind(1) + value`.
    fn status_record(
        status: i32,
        detail_status: i32,
        quality: u16,
        filetime: i64,
        wire_kind: u8,
        value: &[u8],
    ) -> Vec<u8> {
        let mut out = Vec::with_capacity(19 + value.len());
        out.extend_from_slice(&status.to_le_bytes());
        out.extend_from_slice(&detail_status.to_le_bytes());
        out.extend_from_slice(&quality.to_le_bytes());
        out.extend_from_slice(&filetime.to_le_bytes());
        out.push(wire_kind);
        out.extend_from_slice(value);
        out
    }

    #[test]
    fn data_update_boolean_round_trip() {
        let rec = data_record(0x00C0, 132_000_000_000, 0x01, &[0x01]);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.command, DATA_UPDATE_COMMAND);
        assert_eq!(msg.record_count, 1);
        assert!(msg.item_correlation_id.is_none());
        assert_eq!(msg.operation_id.0, OPERATION_ID_BYTES);
        assert_eq!(msg.records.len(), 1);
        let r = &msg.records[0];
        assert_eq!(r.status, 0);
        assert_eq!(r.detail_status, None);
        assert_eq!(r.quality, 0x00C0);
        assert_eq!(r.timestamp_filetime, 132_000_000_000);
        assert_eq!(r.wire_kind, 0x01);
        assert_eq!(r.value, Some(MxValue::Boolean(true)));
    }

    #[test]
    fn data_update_int32() {
        let mut payload = Vec::new();
        payload.extend_from_slice(&42i32.to_le_bytes());
        let rec = data_record(0x00C0, 0, 0x02, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::Int32(42)));
    }

    #[test]
    fn data_update_float32() {
        let mut payload = Vec::new();
        payload.extend_from_slice(&1.5f32.to_le_bytes());
        let rec = data_record(0x00C0, 0, 0x03, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::Float32(1.5)));
    }

    #[test]
    fn data_update_float64() {
        let mut payload = Vec::new();
        payload.extend_from_slice(&3.25f64.to_le_bytes());
        let rec = data_record(0x00C0, 0, 0x04, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::Float64(3.25)));
    }

    #[test]
    fn data_update_string() {
        // "Hi" UTF-16LE = [0x48, 0x00, 0x69, 0x00] then NUL [0x00, 0x00] = 6 bytes.
        let utf16 = [0x48, 0x00, 0x69, 0x00, 0x00, 0x00];
        let text_byte_length: i32 = utf16.len() as i32;
        let record_length: i32 = text_byte_length + 4;
        let mut payload = Vec::new();
        payload.extend_from_slice(&record_length.to_le_bytes());
        payload.extend_from_slice(&text_byte_length.to_le_bytes());
        payload.extend_from_slice(&utf16);
        let rec = data_record(0x00C0, 0, 0x05, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(
            msg.records[0].value,
            Some(MxValue::String("Hi".to_string()))
        );
    }

    #[test]
    fn data_update_string_empty() {
        // record_length == 4 indicates empty string; only 4 bytes consumed.
        let mut payload = Vec::new();
        payload.extend_from_slice(&4i32.to_le_bytes());
        let rec = data_record(0x00C0, 0, 0x05, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::String(String::new())));
    }

    #[test]
    fn data_update_datetime_framed() {
        // Framed: record_length(4) + filetime(8) + 2 trailer bytes => 14 byte body.
        let file_time: i64 = 132_500_000_000;
        let record_length: i32 = 10;
        let mut payload = Vec::new();
        payload.extend_from_slice(&record_length.to_le_bytes());
        payload.extend_from_slice(&file_time.to_le_bytes());
        payload.extend_from_slice(&[0x00, 0x00]); // trailer
        let rec = data_record(0x00C0, 0, 0x06, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::DateTime(file_time)));
    }

    #[test]
    fn data_update_elapsed_time_negative() {
        // Encode -500ms; expect a signed-preserving round-trip.
        let mut payload = Vec::new();
        payload.extend_from_slice(&(-500i32).to_le_bytes());
        let rec = data_record(0x00C0, 0, 0x07, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.records[0].value, Some(MxValue::ElapsedTime(-500)));
    }

    #[test]
    fn subscription_status_int32_round_trip() {
        let mut payload = Vec::new();
        payload.extend_from_slice(&7i32.to_le_bytes());
        let rec = status_record(-1, -2, 0x00C0, 0, 0x02, &payload);
        let body = subscription_status_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.command, SUBSCRIPTION_STATUS_COMMAND);
        assert_eq!(msg.records.len(), 1);
        assert_eq!(msg.item_correlation_id.unwrap().0, CORRELATION_ID_BYTES);
        let r = &msg.records[0];
        assert_eq!(r.status, -1);
        assert_eq!(r.detail_status, Some(-2));
        assert_eq!(r.quality, 0x00C0);
        assert_eq!(r.value, Some(MxValue::Int32(7)));
    }

    #[test]
    fn data_update_record_count_not_one_hard_errors() {
        // recordCount = 2 must hard-error per NmxSubscriptionMessage.cs:71-74.
        let body = data_update_body(2, &[]);
        let err = NmxSubscriptionMessage::parse_inner(&body).unwrap_err();
        match err {
            CodecError::Decode { offset, reason, .. } => {
                assert_eq!(offset, 3);
                assert!(
                    reason.contains("multi-record"),
                    "unexpected reason: {reason}"
                );
            }
            other => panic!("expected CodecError::Decode, got {other:?}"),
        }

        // record_count = 0 also rejected.
        let body0 = data_update_body(0, &[]);
        assert!(matches!(
            NmxSubscriptionMessage::parse_inner(&body0).unwrap_err(),
            CodecError::Decode { .. }
        ));
    }

    #[test]
    fn data_update_has_no_correlation_id() {
        // DataUpdate records start at offset 23 — there is no correlation id
        // gap. Verify by feeding a body that *would* be malformed if 16 extra
        // bytes were consumed before the record.
        let rec = data_record(0x00C0, 0, 0x01, &[0x01]);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert!(msg.item_correlation_id.is_none());
        // First record begins at offset 23, not 39.
        assert_eq!(msg.records[0].offset, 23);
    }

    #[test]
    fn subscription_status_reads_correlation_id() {
        let mut payload = Vec::new();
        payload.extend_from_slice(&0i32.to_le_bytes());
        let rec = status_record(0, 0, 0x00C0, 0, 0x02, &payload);
        let body = subscription_status_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(msg.item_correlation_id.unwrap().0, CORRELATION_ID_BYTES);
        // First record begins at offset 39 (preamble + correlation id).
        assert_eq!(msg.records[0].offset, 39);
    }

    #[test]
    fn boolean_array_minus_one_is_true() {
        // Array header: 4 unknown bytes + count u16 + element_width i32 (=2)
        // + values (count * 2 bytes).
        let count: u16 = 2;
        let element_width: i32 = 2;
        let mut payload = Vec::new();
        payload.extend_from_slice(&[0u8; 4]); // unknown header bytes
        payload.extend_from_slice(&count.to_le_bytes());
        payload.extend_from_slice(&element_width.to_le_bytes());
        // -1 (true) and 0 (false) as i16 LE
        payload.extend_from_slice(&(-1i16).to_le_bytes()); // 0xff 0xff
        payload.extend_from_slice(&0i16.to_le_bytes()); // 0x00 0x00
        let rec = data_record(0x00C0, 0, 0x41, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(
            msg.records[0].value,
            Some(MxValue::BoolArray(vec![true, false]))
        );
    }

    #[test]
    fn boolean_array_byte_pattern_ff_ff_is_true_00_00_is_false() {
        // Sanity: `[0xff, 0xff]` as i16 LE = -1 (true); `[0x00, 0x00]` = 0 (false).
        let count: u16 = 2;
        let element_width: i32 = 2;
        let mut payload = Vec::new();
        payload.extend_from_slice(&[0u8; 4]);
        payload.extend_from_slice(&count.to_le_bytes());
        payload.extend_from_slice(&element_width.to_le_bytes());
        payload.extend_from_slice(&[0xff, 0xff, 0x00, 0x00]);
        let rec = data_record(0x00C0, 0, 0x41, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(
            msg.records[0].value,
            Some(MxValue::BoolArray(vec![true, false]))
        );
    }

    #[test]
    fn datetime_array_decodes_0x46() {
        // Element width is 12: 8-byte filetime + 4 bytes of trailing padding.
        let count: u16 = 2;
        let element_width: i32 = 12;
        let mut payload = Vec::new();
        payload.extend_from_slice(&[0u8; 4]);
        payload.extend_from_slice(&count.to_le_bytes());
        payload.extend_from_slice(&element_width.to_le_bytes());
        // Two FILETIMEs plus 4 bytes of trailing padding each.
        payload.extend_from_slice(&132_000_000_000i64.to_le_bytes());
        payload.extend_from_slice(&[0u8; 4]);
        payload.extend_from_slice(&132_500_000_000i64.to_le_bytes());
        payload.extend_from_slice(&[0u8; 4]);
        let rec = data_record(0x00C0, 0, 0x46, &payload);
        let body = data_update_body(1, &rec);
        let msg = NmxSubscriptionMessage::parse_inner(&body).unwrap();
        assert_eq!(
            msg.records[0].value,
            Some(MxValue::DateTimeArray(vec![
                132_000_000_000,
                132_500_000_000
            ]))
        );
    }

    #[test]
    fn unknown_command_is_unexpected_opcode() {
        let mut body = data_update_body(1, &[]);
        body[0] = 0x99;
        let err = NmxSubscriptionMessage::parse_inner(&body).unwrap_err();
        assert!(matches!(err, CodecError::UnexpectedOpcode(0x99)));
    }

    #[test]
    fn short_inner_is_short_read() {
        let err = NmxSubscriptionMessage::parse_inner(&[0u8; 22]).unwrap_err();
        assert!(matches!(err, CodecError::ShortRead { .. }));
    }

    #[test]
    fn subscription_status_short_header_is_short_read() {
        // 23 bytes is the preamble length, but SubscriptionStatus needs 39.
        let mut body = Vec::with_capacity(23);
        body.push(SUBSCRIPTION_STATUS_COMMAND);
        body.extend_from_slice(&1u16.to_le_bytes());
        body.extend_from_slice(&0i32.to_le_bytes());
        body.extend_from_slice(&OPERATION_ID_BYTES);
        let err = NmxSubscriptionMessage::parse_inner(&body).unwrap_err();
        assert!(matches!(err, CodecError::ShortRead { .. }));
    }

    #[test]
    fn wire_kind_to_value_kind_table() {
        assert_eq!(wire_kind_to_value_kind(0x01), Some(MxValueKind::Boolean));
        assert_eq!(
            wire_kind_to_value_kind(0x07),
            Some(MxValueKind::ElapsedTime)
        );
        assert_eq!(
            wire_kind_to_value_kind(0x46),
            Some(MxValueKind::DateTimeArray)
        );
        assert_eq!(wire_kind_to_value_kind(0x99), None);
    }

    #[test]
    fn command_constants_match_dotnet() {
        // NmxSubscriptionMessage.cs:36-37
        assert_eq!(SUBSCRIPTION_STATUS_COMMAND, 0x32);
        assert_eq!(DATA_UPDATE_COMMAND, 0x33);
    }
}