dvb-ci 0.12.0

//! Transport Protocol Data Unit (TPDU) framing — ETSI EN 50221 Annex A §A.4.1,
//! Tables A.1-A.16 (PDF pp. 63-70).
//!
//! The transport layer is a command-response protocol: the host sends a Command
//! TPDU (C_TPDU), the module replies with a Response TPDU (R_TPDU) that always
//! ends in a Status Byte. The `length_field` uses the Table 1 coding (see
//! [`crate::length`]). This module covers the wire framing only — the PC-Card
//! hardware transport itself is out of scope.

use crate::error::{Error, Result};
use crate::length::{self, LengthForm};
use alloc::vec::Vec;
use broadcast_common::len::{need_in, need_out};
use broadcast_common::{Parse, Serialize};

/// `tpdu_tag` values — Table A.16 (p. 70). One byte each.
pub mod tags {
    /// `TSB` (status byte) = `80`.
    pub const SB: u8 = 0x80;
    /// `TRCV` (receive data) = `81`.
    pub const RCV: u8 = 0x81;
    /// `Tcreate_t_c` = `82`.
    pub const CREATE_T_C: u8 = 0x82;
    /// `Tc_t_c_reply` = `83`.
    pub const C_T_C_REPLY: u8 = 0x83;
    /// `Tdelete_t_c` = `84`.
    pub const DELETE_T_C: u8 = 0x84;
    /// `Td_t_c_reply` = `85`.
    pub const D_T_C_REPLY: u8 = 0x85;
    /// `Trequest_t_c` = `86`.
    pub const REQUEST_T_C: u8 = 0x86;
    /// `Tnew_t_c` = `87`.
    pub const NEW_T_C: u8 = 0x87;
    /// `Tt_c_error` = `88`.
    pub const T_C_ERROR: u8 = 0x88;
    /// `Tdata_last` (last data block) = `A0`.
    pub const DATA_LAST: u8 = 0xA0;
    /// `Tdata_more` (more data follows) = `A1`.
    pub const DATA_MORE: u8 = 0xA1;
}

/// Status Byte value (`SB_value`, Figure A.6 + Table A.3, p. 64). The single
/// 1-bit DA (Data Available) flag is bit 8; the remaining bits are reserved
/// (shall be zero) and preserved here for fidelity.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct SbValue(pub u8);

impl SbValue {
    /// DA (Data Available): true when the module has a message for the host.
    #[must_use]
    pub const fn data_available(self) -> bool {
        self.0 & 0x80 != 0
    }
    /// Build an `SB_value` with the DA bit set/clear and reserved bits zero.
    #[must_use]
    pub const fn new(data_available: bool) -> Self {
        Self(if data_available { 0x80 } else { 0x00 })
    }
}

/// `c_TPDU_tag` for the data-carrying C_TPDU (chaining): last vs. more (§A.4.1).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
#[non_exhaustive]
pub enum DataBlock {
    /// `Tdata_last` (`A0`): the final (or only) data block.
    Last,
    /// `Tdata_more` (`A1`): more blocks follow.
    More,
}

impl DataBlock {
    fn from_tag(tag: u8) -> Option<Self> {
        match tag {
            tags::DATA_LAST => Some(Self::Last),
            tags::DATA_MORE => Some(Self::More),
            _ => None,
        }
    }
    /// The `tpdu_tag` byte for this data block (`A0` last / `A1` more).
    #[must_use]
    pub fn to_tag(self) -> u8 {
        match self {
            Self::Last => tags::DATA_LAST,
            Self::More => tags::DATA_MORE,
        }
    }
    /// Spec token.
    #[must_use]
    pub fn name(&self) -> &'static str {
        match self {
            Self::Last => "data_last",
            Self::More => "data_more",
        }
    }
}
broadcast_common::impl_spec_display!(DataBlock);

// --- single-field connection-management objects (tag + length + t_c_id) ---

/// A connection-management object carrying just `t_c_id`: `Create_T_C`
/// (Table A.4), `C_T_C_Reply`, `Delete_T_C` (A.6), `D_T_C_Reply`, `Request_T_C`
/// (A.8). The `tag` distinguishes which (`length_field = 1`).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct TcObject {
    /// The `tpdu_tag` (one of `CREATE_T_C`/`C_T_C_REPLY`/`DELETE_T_C`/
    /// `D_T_C_REPLY`/`REQUEST_T_C`).
    pub tag: u8,
    /// `t_c_id` — the transport connection identifier.
    pub t_c_id: u8,
    /// How the `length_field` was coded on the wire (non-minimal long forms
    /// such as `82 00 09` are preserved so the object re-serializes
    /// byte-for-byte; [`LengthForm::Minimal`] for a hand-built value).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub length_form: LengthForm,
}

impl TcObject {
    /// `true` when `tag` is one of the five single-`t_c_id` connection
    /// objects this type models (Table A.16): `Tcreate_t_c`, `Tc_t_c_reply`,
    /// `Tdelete_t_c`, `Td_t_c_reply`, `Trequest_t_c`. Any other tag belongs to
    /// a different object (`SB`, `RCV`, `New_T_C`, `T_C_Error`, data) with a
    /// different body layout (audit r10-O-3).
    #[must_use]
    pub const fn is_tc_tag(tag: u8) -> bool {
        matches!(
            tag,
            tags::CREATE_T_C
                | tags::C_T_C_REPLY
                | tags::DELETE_T_C
                | tags::D_T_C_REPLY
                | tags::REQUEST_T_C
        )
    }
}

impl<'a> Parse<'a> for TcObject {
    type Error = Error;
    fn parse(bytes: &'a [u8]) -> Result<Self> {
        need_in(bytes, 1, "TcObject")?;
        let tag = bytes[0];
        if !Self::is_tc_tag(tag) {
            return Err(Error::InvalidObject {
                what: "TcObject",
                reason: "tpdu_tag is not a single-t_c_id connection object",
            });
        }
        let (len, hdr, length_form) = length::decode_form(&bytes[1..])?;
        if len != 1 {
            return Err(Error::InvalidObject {
                what: "TcObject",
                reason: "length_field must be 1",
            });
        }
        let t_c_id = *bytes.get(1 + hdr).ok_or(Error::BufferTooShort(
            broadcast_common::error::BufferTooShort {
                need: 1 + hdr + 1,
                have: bytes.len(),
                what: "TcObject t_c_id",
            },
        ))?;
        Ok(Self {
            tag,
            t_c_id,
            length_form,
        })
    }
}
impl Serialize for TcObject {
    type Error = Error;
    fn serialized_len(&self) -> usize {
        // tag + length_field(value 1) + t_c_id
        1 + length::encoded_len_with(1, self.length_form) + 1
    }
    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
        let total = self.serialized_len();
        need_out(buf, total)?;
        if !Self::is_tc_tag(self.tag) {
            return Err(Error::InvalidObject {
                what: "TcObject",
                reason: "tpdu_tag is not a single-t_c_id connection object",
            });
        }
        buf[0] = self.tag;
        let pos = 1 + length::encode_with(1, self.length_form, &mut buf[1..])?;
        buf[pos] = self.t_c_id;
        Ok(pos + 1)
    }
}

/// `New_T_C` (Table A.9, `length=2`): a new connection identifier to establish.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct NewTc {
    /// `t_c_id` of the connection issuing this.
    pub t_c_id: u8,
    /// `new_t_c_id` — the identifier for the new connection.
    pub new_t_c_id: u8,
    /// How the `length_field` was coded on the wire (non-minimal long forms
    /// such as `82 00 09` are preserved so the object re-serializes
    /// byte-for-byte; [`LengthForm::Minimal`] for a hand-built value).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub length_form: LengthForm,
}

impl<'a> Parse<'a> for NewTc {
    type Error = Error;
    fn parse(bytes: &'a [u8]) -> Result<Self> {
        let (length_form, body) = parse_fixed(bytes, tags::NEW_T_C, 2, "New_T_C")?;
        Ok(Self {
            t_c_id: body[0],
            new_t_c_id: body[1],
            length_form,
        })
    }
}
impl Serialize for NewTc {
    type Error = Error;
    fn serialized_len(&self) -> usize {
        1 + length::encoded_len_with(2, self.length_form) + 2
    }
    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
        write_fixed(
            tags::NEW_T_C,
            self.length_form,
            &[self.t_c_id, self.new_t_c_id],
            buf,
        )
    }
}

/// `T_C_Error` (Table A.10, `length=2`): an error on a transport connection.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct TcError {
    /// `t_c_id`.
    pub t_c_id: u8,
    /// `error_code` (Table A.11: `1` = no transport connections available).
    pub error_code: u8,
    /// How the `length_field` was coded on the wire (non-minimal long forms
    /// such as `82 00 09` are preserved so the object re-serializes
    /// byte-for-byte; [`LengthForm::Minimal`] for a hand-built value).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub length_form: LengthForm,
}

impl<'a> Parse<'a> for TcError {
    type Error = Error;
    fn parse(bytes: &'a [u8]) -> Result<Self> {
        let (length_form, body) = parse_fixed(bytes, tags::T_C_ERROR, 2, "T_C_Error")?;
        Ok(Self {
            t_c_id: body[0],
            error_code: body[1],
            length_form,
        })
    }
}
impl Serialize for TcError {
    type Error = Error;
    fn serialized_len(&self) -> usize {
        1 + length::encoded_len_with(2, self.length_form) + 2
    }
    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
        write_fixed(
            tags::T_C_ERROR,
            self.length_form,
            &[self.t_c_id, self.error_code],
            buf,
        )
    }
}

/// `C_TPDU` (Table A.1): a Command TPDU, host → module. `length_field` covers
/// `t_c_id` + data.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct CommandTpdu<'a> {
    /// The `c_tpdu_tag` (e.g. `RCV`, or `DATA_LAST`/`DATA_MORE` for Send Data).
    pub tag: u8,
    /// `t_c_id`.
    pub t_c_id: u8,
    /// Data field (`length_value - 1` bytes).
    #[cfg_attr(feature = "serde", serde(borrow, with = "crate::objects::bytes_serde"))]
    pub data: &'a [u8],
    /// How the `length_field` was coded on the wire (non-minimal long forms
    /// such as `82 00 09` are preserved so the object re-serializes
    /// byte-for-byte; [`LengthForm::Minimal`] for a hand-built value).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub length_form: LengthForm,
}

impl<'a> Parse<'a> for CommandTpdu<'a> {
    type Error = Error;
    fn parse(bytes: &'a [u8]) -> Result<Self> {
        need_in(bytes, 1, "C_TPDU")?;
        let tag = bytes[0];
        let (len, hdr, length_form) = length::decode_form(&bytes[1..])?;
        if len == 0 {
            return Err(Error::InvalidObject {
                what: "C_TPDU",
                reason: "length_field must include t_c_id (>=1)",
            });
        }
        let start = 1 + hdr;
        let end = start + len;
        if bytes.len() < end {
            return Err(Error::LengthMismatch(
                broadcast_common::error::LengthMismatch {
                    what: "C_TPDU",
                    declared: len,
                    actual: bytes.len().saturating_sub(start),
                },
            ));
        }
        Ok(Self {
            tag,
            t_c_id: bytes[start],
            data: &bytes[start + 1..end],
            length_form,
        })
    }
}
impl Serialize for CommandTpdu<'_> {
    type Error = Error;
    fn serialized_len(&self) -> usize {
        let len_value = 1 + self.data.len();
        1 + length::encoded_len_with(len_value, self.length_form) + len_value
    }
    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
        let len_value = 1 + self.data.len();
        let total = self.serialized_len();
        need_out(buf, total)?;
        buf[0] = self.tag;
        let mut pos = 1 + length::encode_with(len_value, self.length_form, &mut buf[1..])?;
        buf[pos] = self.t_c_id;
        pos += 1;
        buf[pos..pos + self.data.len()].copy_from_slice(self.data);
        Ok(pos + self.data.len())
    }
}

/// `R_TPDU` (Table A.2): a Response TPDU, module → host. The mandatory trailing
/// Status (SB) is NOT included in the `length_field` and is modelled separately.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
pub struct ResponseTpdu<'a> {
    /// The `r_tpdu_tag` (e.g. `DATA_LAST`/`DATA_MORE`).
    pub tag: u8,
    /// `t_c_id`.
    pub t_c_id: u8,
    /// Data field (`length_value - 1` bytes).
    #[cfg_attr(feature = "serde", serde(borrow, with = "crate::objects::bytes_serde"))]
    pub data: &'a [u8],
    /// `SB_value` from the mandatory trailing Status.
    pub sb_value: SbValue,
    /// Whether the data block was the last (`Tdata_last`) — convenience view of
    /// `tag` for chaining, `None` if `tag` is not a data block tag.
    pub block: Option<DataBlock>,
    /// How the `length_field` was coded on the wire (non-minimal long forms
    /// such as `82 00 09` are preserved so the object re-serializes
    /// byte-for-byte; [`LengthForm::Minimal`] for a hand-built value).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub length_form: LengthForm,
    /// How the trailing Status (SB) object's own `length_field` (value 2) was
    /// coded; see [`length_form`](Self::length_form).
    #[cfg_attr(feature = "serde", serde(skip))]
    pub status_length_form: LengthForm,
}

impl<'a> Parse<'a> for ResponseTpdu<'a> {
    type Error = Error;
    fn parse(bytes: &'a [u8]) -> Result<Self> {
        need_in(bytes, 1, "R_TPDU")?;
        let tag = bytes[0];
        let (len, hdr, length_form) = length::decode_form(&bytes[1..])?;
        if len == 0 {
            return Err(Error::InvalidObject {
                what: "R_TPDU",
                reason: "length_field must include t_c_id (>=1)",
            });
        }
        let start = 1 + hdr;
        let data_end = start + len;
        // Status trailer: SB_tag + length_field(value 2) + t_c_id + SB_value.
        // Need at least the tag and one length byte to learn the field's width.
        need_in(bytes, data_end + 2, "R_TPDU status")?;
        if bytes[data_end] != tags::SB {
            return Err(Error::UnexpectedTpduTag {
                got: bytes[data_end],
                expected: tags::SB,
                what: "R_TPDU SB_tag",
            });
        }
        let (sb_len, sb_hdr, status_length_form) = length::decode_form(&bytes[data_end + 1..])?;
        if sb_len != 2 {
            return Err(Error::InvalidObject {
                what: "R_TPDU status",
                reason: "SB length_field must be 2",
            });
        }
        let sb_body = data_end + 1 + sb_hdr;
        let status_end = sb_body + 2;
        need_in(bytes, status_end, "R_TPDU status")?;
        // bytes[sb_body] = t_c_id (status), bytes[sb_body+1] = SB_value.
        // The status trailer names the same transport connection as the
        // TPDU body's own leading t_c_id (bytes[start]) — a mismatch means
        // this status is not actually reporting on this TPDU.
        if bytes[sb_body] != bytes[start] {
            return Err(Error::InvalidObject {
                what: "R_TPDU status",
                reason: "status t_c_id does not match the TPDU's t_c_id",
            });
        }
        Ok(Self {
            tag,
            t_c_id: bytes[start],
            data: &bytes[start + 1..data_end],
            sb_value: SbValue(bytes[sb_body + 1]),
            block: DataBlock::from_tag(tag),
            length_form,
            status_length_form,
        })
    }
}
impl Serialize for ResponseTpdu<'_> {
    type Error = Error;
    fn serialized_len(&self) -> usize {
        let len_value = 1 + self.data.len();
        1 + length::encoded_len_with(len_value, self.length_form)
            + len_value
            + 1
            + length::encoded_len_with(2, self.status_length_form)
            + 2
    }
    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
        let total = self.serialized_len();
        need_out(buf, total)?;
        let len_value = 1 + self.data.len();
        buf[0] = self.tag;
        let mut pos = 1 + length::encode_with(len_value, self.length_form, &mut buf[1..])?;
        buf[pos] = self.t_c_id;
        pos += 1;
        buf[pos..pos + self.data.len()].copy_from_slice(self.data);
        pos += self.data.len();
        // Status trailer.
        buf[pos] = tags::SB;
        pos += 1;
        pos += length::encode_with(2, self.status_length_form, &mut buf[pos..])?;
        buf[pos] = self.t_c_id;
        buf[pos + 1] = self.sb_value.0;
        Ok(pos + 2)
    }
}

// --- helpers for the fixed-length tag+length+body objects ---

fn parse_fixed<'a>(
    bytes: &'a [u8],
    expected: u8,
    body_len: usize,
    what: &'static str,
) -> Result<(LengthForm, &'a [u8])> {
    if bytes.is_empty() {
        return Err(Error::BufferTooShort(
            broadcast_common::error::BufferTooShort {
                need: 1,
                have: 0,
                what,
            },
        ));
    }
    if bytes[0] != expected {
        return Err(Error::UnexpectedTpduTag {
            got: bytes[0],
            expected,
            what,
        });
    }
    let (len, hdr, form) = length::decode_form(&bytes[1..])?;
    if len != body_len {
        return Err(Error::InvalidObject {
            what,
            reason: "unexpected length_field",
        });
    }
    let start = 1 + hdr;
    let end = start + body_len;
    if bytes.len() < end {
        return Err(Error::BufferTooShort(
            broadcast_common::error::BufferTooShort {
                need: end,
                have: bytes.len(),
                what,
            },
        ));
    }
    Ok((form, &bytes[start..end]))
}

fn write_fixed(tag: u8, form: LengthForm, body: &[u8], buf: &mut [u8]) -> Result<usize> {
    let total = 1 + length::encoded_len_with(body.len(), form) + body.len();
    need_out(buf, total)?;
    buf[0] = tag;
    let pos = 1 + length::encode_with(body.len(), form, &mut buf[1..])?;
    buf[pos..pos + body.len()].copy_from_slice(body);
    Ok(total)
}

/// Build a `Create_T_C` object (`tag = CREATE_T_C`, `t_c_id`).
#[must_use]
pub fn create_t_c(t_c_id: u8) -> TcObject {
    TcObject {
        tag: tags::CREATE_T_C,
        t_c_id,
        length_form: LengthForm::Minimal,
    }
}

/// Collect a `Vec` of the wire bytes for a [`TcObject`] (convenience).
///
/// Fails with [`Error::InvalidObject`] when `o.tag` is not a connection
/// object tag (audit r10-O-3 / T15: this used to panic through `to_bytes`).
pub fn tc_object_bytes(o: &TcObject) -> Result<Vec<u8>> {
    o.try_to_bytes()
}

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

    #[test]
    fn tc_object_rejects_foreign_tags_both_ways() {
        // 0x80 is TSB (status byte), 0x87 New_T_C: valid TPDU tags, wrong
        // object. Previously any tag byte was accepted and re-emitted.
        for tag in [
            tags::SB,
            tags::RCV,
            tags::NEW_T_C,
            tags::T_C_ERROR,
            0x00,
            0xFF,
        ] {
            let wire = [tag, 0x01, 0x05];
            assert!(
                matches!(TcObject::parse(&wire), Err(Error::InvalidObject { .. })),
                "tag {tag:#04X} must be rejected on parse"
            );
            let o = TcObject {
                length_form: crate::length::LengthForm::Minimal,
                tag,
                t_c_id: 5,
            };
            assert!(matches!(
                tc_object_bytes(&o),
                Err(Error::InvalidObject { .. })
            ));
        }
        for tag in [
            tags::CREATE_T_C,
            tags::C_T_C_REPLY,
            tags::DELETE_T_C,
            tags::D_T_C_REPLY,
            tags::REQUEST_T_C,
        ] {
            let o = TcObject::parse(&[tag, 0x01, 0x05]).unwrap();
            assert_eq!(
                o,
                TcObject {
                    length_form: crate::length::LengthForm::Minimal,
                    tag,
                    t_c_id: 5
                }
            );
            assert_eq!(tc_object_bytes(&o).unwrap(), [tag, 0x01, 0x05]);
        }
    }

    #[test]
    fn tc_object_round_trip() {
        let o = create_t_c(0x01);
        let bytes = o.try_to_bytes().unwrap();
        assert_eq!(bytes, [0x82, 0x01, 0x01]);
        assert_eq!(TcObject::parse(&bytes).unwrap(), o);
    }

    #[test]
    fn new_tc_round_trip() {
        let n = NewTc {
            length_form: crate::length::LengthForm::Minimal,
            t_c_id: 1,
            new_t_c_id: 2,
        };
        let bytes = n.try_to_bytes().unwrap();
        assert_eq!(bytes, [0x87, 0x02, 0x01, 0x02]);
        assert_eq!(NewTc::parse(&bytes).unwrap(), n);
    }

    #[test]
    fn tc_error_round_trip() {
        let e = TcError {
            length_form: crate::length::LengthForm::Minimal,
            t_c_id: 3,
            error_code: 1,
        };
        let bytes = e.try_to_bytes().unwrap();
        assert_eq!(bytes, [0x88, 0x02, 0x03, 0x01]);
        assert_eq!(TcError::parse(&bytes).unwrap(), e);
    }

    #[test]
    fn command_tpdu_round_trip() {
        let c = CommandTpdu {
            length_form: crate::length::LengthForm::Minimal,
            tag: tags::DATA_LAST,
            t_c_id: 1,
            data: &[0xAA, 0xBB, 0xCC],
        };
        let bytes = c.try_to_bytes().unwrap();
        // tag A0, length 4 (t_c_id + 3 data), t_c_id, data.
        assert_eq!(bytes, [0xA0, 0x04, 0x01, 0xAA, 0xBB, 0xCC]);
        assert_eq!(CommandTpdu::parse(&bytes).unwrap(), c);
    }

    #[test]
    fn receive_data_command_no_payload() {
        let c = CommandTpdu {
            length_form: crate::length::LengthForm::Minimal,
            tag: tags::RCV,
            t_c_id: 1,
            data: &[],
        };
        let bytes = c.try_to_bytes().unwrap();
        assert_eq!(bytes, [0x81, 0x01, 0x01]);
        assert_eq!(CommandTpdu::parse(&bytes).unwrap(), c);
    }

    #[test]
    fn response_tpdu_round_trip_with_status() {
        let r = ResponseTpdu {
            length_form: crate::length::LengthForm::Minimal,
            status_length_form: crate::length::LengthForm::Minimal,
            tag: tags::DATA_LAST,
            t_c_id: 1,
            data: &[0x9F, 0x80, 0x30, 0x00],
            sb_value: SbValue::new(true),
            block: Some(DataBlock::Last),
        };
        let bytes = r.try_to_bytes().unwrap();
        // header: A0 05 01 + 4 data ; status: 80 02 01 80.
        assert_eq!(
            bytes,
            [
                0xA0, 0x05, 0x01, 0x9F, 0x80, 0x30, 0x00, 0x80, 0x02, 0x01, 0x80
            ]
        );
        let parsed = ResponseTpdu::parse(&bytes).unwrap();
        assert_eq!(parsed, r);
        assert!(parsed.sb_value.data_available());
        assert_eq!(parsed.block, Some(DataBlock::Last));
    }

    /// r10-W-8: the status trailer's own `t_c_id` (`bytes[data_end+2]`) must
    /// match the TPDU body's `t_c_id`, not merely be read past and dropped.
    /// A status reporting on a different transport connection than the one
    /// this TPDU's body names is not a well-formed R_TPDU.
    #[test]
    fn response_tpdu_rejects_mismatched_status_t_c_id() {
        let r = ResponseTpdu {
            length_form: crate::length::LengthForm::Minimal,
            status_length_form: crate::length::LengthForm::Minimal,
            tag: tags::DATA_LAST,
            t_c_id: 1,
            data: &[0x9F, 0x80, 0x30, 0x00],
            sb_value: SbValue::new(true),
            block: Some(DataBlock::Last),
        };
        let mut bytes = r.try_to_bytes().unwrap();
        let status_t_c_id_idx = bytes.len() - 2;
        assert_eq!(bytes[status_t_c_id_idx], 1);
        bytes[status_t_c_id_idx] = 2; // status now claims a different t_c_id
        assert!(matches!(
            ResponseTpdu::parse(&bytes),
            Err(Error::InvalidObject { .. })
        ));
    }

    #[test]
    fn mutating_data_changes_bytes() {
        let c = CommandTpdu {
            length_form: crate::length::LengthForm::Minimal,
            tag: tags::DATA_LAST,
            t_c_id: 1,
            data: &[0xAA],
        };
        let a = c.try_to_bytes().unwrap();
        let b = CommandTpdu {
            length_form: crate::length::LengthForm::Minimal,
            tag: tags::DATA_LAST,
            t_c_id: 1,
            data: &[0xBB],
        }
        .try_to_bytes()
        .unwrap();
        assert_ne!(a, b);
    }

    #[test]
    fn sb_value_da() {
        assert!(SbValue::new(true).data_available());
        assert!(!SbValue::new(false).data_available());
    }

    /// Rewrite the `length_field` of a TPDU wire (1-byte tag + minimal
    /// length) into the long form with `octets` length octets.
    fn widen(wire: &[u8], octets: usize) -> alloc::vec::Vec<u8> {
        let (value, hdr) = length::decode(&wire[1..]).unwrap();
        let mut out = alloc::vec![wire[0], 0x80 | octets as u8];
        for i in (0..octets).rev() {
            out.push((value >> (8 * i)) as u8);
        }
        out.extend_from_slice(&wire[1 + hdr..]);
        out
    }

    /// The module's `profile_reply` frame, captured raw on hardware
    /// (dvb-ci-runtime #337): the R_TPDU `length_field` is the long form
    /// `82 00 09` for the value 9. It must parse and re-serialize to exactly
    /// these 17 bytes, not the minimal `A0 09 ..`.
    #[test]
    fn captured_long_form_r_tpdu_round_trips_byte_exactly() {
        let frame: [u8; 17] = [
            0xA0, 0x82, 0x00, 0x09, 0x01, 0x90, 0x02, 0x00, 0x01, 0x9F, 0x80, 0x11, 0x00, 0x80,
            0x02, 0x01, 0x00,
        ];
        let r = ResponseTpdu::parse(&frame).unwrap();
        assert_eq!(r.length_form, LengthForm::Long { octets: 2 });
        assert_eq!(r.status_length_form, LengthForm::Minimal);
        assert_eq!(r.t_c_id, 1);
        assert_eq!(r.data, &frame[5..13]);
        assert_eq!(r.try_to_bytes().unwrap(), frame);
    }

    /// r10-W-4: each TPDU object round-trips every length form byte-for-byte,
    /// including the R_TPDU's own trailing Status `length_field`.
    #[test]
    fn every_tpdu_round_trips_each_length_form_byte_exactly() {
        macro_rules! forms {
            ($ty:ty, $wire:expr) => {{
                let wire: &[u8] = &$wire;
                assert_eq!(<$ty>::parse(wire).unwrap().try_to_bytes().unwrap(), wire);
                for octets in 1..=4 {
                    let wide = widen(wire, octets);
                    let parsed = <$ty>::parse(&wide).unwrap();
                    assert_eq!(
                        parsed.try_to_bytes().unwrap(),
                        wide,
                        "{} {octets}",
                        stringify!($ty)
                    );
                    assert_ne!(parsed, <$ty>::parse(wire).unwrap());
                }
            }};
        }
        forms!(TcObject, [tags::CREATE_T_C, 0x01, 0x05]);
        forms!(NewTc, [tags::NEW_T_C, 0x02, 0x01, 0x02]);
        forms!(TcError, [tags::T_C_ERROR, 0x02, 0x01, 0x01]);
        forms!(CommandTpdu, [tags::DATA_LAST, 0x03, 0x01, 0xAA, 0xBB]);
        forms!(
            ResponseTpdu,
            [
                tags::DATA_LAST,
                0x03,
                0x01,
                0xAA,
                0xBB,
                0x80,
                0x02,
                0x01,
                0x80
            ]
        );
        // The Status trailer's own length_field, widened independently.
        let wire = [
            tags::DATA_LAST,
            0x03,
            0x01,
            0xAA,
            0xBB,
            0x80,
            0x82,
            0x00,
            0x02,
            0x01,
            0x80,
        ];
        let r = ResponseTpdu::parse(&wire).unwrap();
        assert_eq!(r.status_length_form, LengthForm::Long { octets: 2 });
        assert_eq!(r.try_to_bytes().unwrap(), wire);
    }
}