zerodds-rtps 1.0.0-rc.1

// SPDX-License-Identifier: Apache-2.0
// Copyright 2026 ZeroDDS Contributors
//! HeaderExtension Submessage (DDSI-RTPS 2.5 §8.3.3.2 / §9.4.5.2 /
//! §9.4.2.15).
//!
//! Die HeaderExtension (HE) ist eine in DDSI-RTPS 2.5 neu eingefuehrte
//! Submessage-Klasse, die einer RTPS-Nachricht optionale Header-
//! Felder anhaengt:
//!
//! - **`messageLength`**: u32 — Gesamtlaenge der RTPS-Message ab dem
//!   ersten Submessage-Header. Erlaubt Reader-Pre-Allocation und ist
//!   Pflicht-Hint fuer non-UDP-Transporte ohne implizite Datagram-
//!   Grenze.
//! - **`rtpsSendTimestamp`**: 8 Byte (Time_t) — Sender-Wallclock zum
//!   Zeitpunkt des `send()`-Calls. Speist `clockSkewDetected` im
//!   Receiver-State (§8.3.7.4).
//! - **`uextension4`**: 4 Byte vendor-spezifisch.
//! - **`wextension8`**: 8 Byte vendor-spezifisch (Spec §8.3.3.2).
//! - **`messageChecksum`**: CRC-32C (4 Byte), CRC-64-XZ (8 Byte) oder
//!   MD5-128 (16 Byte) ueber den Bereich `[RtpsHeader || alle
//!   Submessages || HE-Submessage-Header || HE-Body bis vor dem
//!   Checksum-Feld]`. C-Flag (2 Bit) selektiert die Variante.
//! - **`parameters`**: ParameterList (TLV) — bisher nur fuer
//!   Vendor-Erweiterungen reserviert; `must_understand` darin loest
//!   ganzheitlichen Message-Reject aus (§9.4.2.11.2).
//!
//! # SubmessageId und Flag-Layout (Auftrags-Spezifikation)
//!
//! - `SubmessageId = 0x80` — HE-Marker. Ausserhalb der "klassischen"
//!   Submessage-Range (≤ 0x7F) und damit forwards-kompatibel zu Pre-2.5-
//!   Implementierungen, die `octets_to_next_header` zum Skippen nutzen.
//!
//! Flag-Bits im Submessage-Header:
//!
//! | Bit | Symbol | Bedeutung |
//! |-----|--------|-----------|
//! | 0 | E | Endianness (1 = LE) |
//! | 1 | L | `messageLength` vorhanden |
//! | 2 | W | `rtpsSendTimestamp` vorhanden |
//! | 3 | U | `uextension4` vorhanden |
//! | 4 | V | `wextension8` vorhanden |
//! | 5+6 | C | Checksum-Variante (0=keine, 1=CRC-32C, 2=CRC-64, 3=MD5) |
//! | 7 | P | `parameters` (ParameterList) vorhanden |
//!
//! # Receiver-State-Update (§8.3.7.4)
//!
//! Bei Empfang einer HE aktualisiert der Receiver:
//!
//! - `Receiver.messageLength` aus dem L-Feld (falls vorhanden) →
//!   Cross-Check gegen tatsaechliche Datagram-Restlaenge. Mismatch =
//!   `WireError::ValueOutOfRange`.
//! - `Receiver.haveTimestamp = true`, `Receiver.timestamp = …` aus dem
//!   T-Feld (falls vorhanden).
//! - `Receiver.messageChecksum = …` aus dem C-Feld (falls Variante ≠ 0).
//! - `Receiver.parameters = …` aus dem P-Feld (falls vorhanden).
//! - `clockSkewDetected` Heuristik: wenn `|Receiver.timestamp - now|` >
//!   Grenzwert. (Diese Heuristik verbleibt im Receiver-Code; das
//!   Decode-Modul liefert nur die Eingabe.)
//!
//! # DoS-Cap
//!
//! `MAX_HE_LENGTH = 16 KiB`: HE-Body darf nicht laenger als 16 KiB
//! sein (deckt typische ParameterList + alle Optional-Felder).
//! Verhindert Amplification ueber boesartig grosse `parameters`.

extern crate alloc;
use alloc::vec::Vec;

use crate::error::WireError;
use crate::parameter_list::ParameterList;

/// SubmessageId fuer HeaderExtension (DDSI-RTPS 2.5).
pub const SUBMESSAGE_ID_HEADER_EXTENSION: u8 = 0x80;

/// Hard-Cap fuer HE-Body-Laenge (DoS-Schutz).
pub const MAX_HE_LENGTH: usize = 16 * 1024;

// ----- Flag-Bits -----

/// E-Flag (Bit 0) — Endianness des Bodies (1 = LE).
pub const HE_FLAG_E: u8 = 1 << 0;
/// L-Flag (Bit 1) — `messageLength` vorhanden.
pub const HE_FLAG_L: u8 = 1 << 1;
/// W-Flag (Bit 2) — `rtpsSendTimestamp` vorhanden.
pub const HE_FLAG_W: u8 = 1 << 2;
/// U-Flag (Bit 3) — `uextension4` vorhanden.
pub const HE_FLAG_U: u8 = 1 << 3;
/// V-Flag (Bit 4) — `wextension8` vorhanden.
pub const HE_FLAG_V: u8 = 1 << 4;
/// Lower-Bit der C-Maske (Bit 5).
pub const HE_FLAG_C0: u8 = 1 << 5;
/// Higher-Bit der C-Maske (Bit 6).
pub const HE_FLAG_C1: u8 = 1 << 6;
/// Maske der beiden Checksum-Selector-Bits (Bits 5+6).
pub const HE_FLAG_C_MASK: u8 = HE_FLAG_C0 | HE_FLAG_C1;
/// P-Flag (Bit 7) — `parameters` (ParameterList) vorhanden.
pub const HE_FLAG_P: u8 = 1 << 7;

// =====================================================================
// Wire-Typen
// =====================================================================

/// Spec §9.4.2.15.2 — Auswahl der `messageChecksum`-Variante. Wert
/// liegt in den C-Bits (5+6) des Flag-Bytes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ChecksumKind {
    /// `C = 00` — keine Checksum.
    None,
    /// `C = 01` — CRC-32C (RFC 4960 App. B), 4 Byte.
    Crc32c,
    /// `C = 10` — CRC-64-XZ (ECMA-182, XZ utils), 8 Byte.
    Crc64,
    /// `C = 11` — MD5-128 (RFC 1321), 16 Byte.
    Md5,
}

impl ChecksumKind {
    /// Wire-Size in Bytes (0/4/8/16).
    #[must_use]
    pub fn wire_size(self) -> usize {
        match self {
            Self::None => 0,
            Self::Crc32c => 4,
            Self::Crc64 => 8,
            Self::Md5 => 16,
        }
    }

    /// Aus den C-Bits eines Flag-Bytes extrahieren.
    #[must_use]
    pub fn from_flags(flags: u8) -> Self {
        // Maskierung garantiert Werte 0..=3.
        match (flags & HE_FLAG_C_MASK) >> 5 {
            1 => Self::Crc32c,
            2 => Self::Crc64,
            3 => Self::Md5,
            // 0 oder unmoegliche oberen Bits: keine Checksum.
            _ => Self::None,
        }
    }

    /// In die C-Bits eines Flag-Bytes encodieren.
    #[must_use]
    pub fn to_flag_bits(self) -> u8 {
        let raw = match self {
            Self::None => 0u8,
            Self::Crc32c => 1,
            Self::Crc64 => 2,
            Self::Md5 => 3,
        };
        (raw << 5) & HE_FLAG_C_MASK
    }
}

/// Inhalt der `messageChecksum` (variabel nach [`ChecksumKind`]).
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub enum ChecksumValue {
    /// Kein Checksum-Feld geschrieben.
    #[default]
    None,
    /// CRC-32C u32.
    Crc32c(u32),
    /// CRC-64-XZ u64.
    Crc64(u64),
    /// MD5-128 16-Byte-Hash.
    Md5([u8; 16]),
}

impl ChecksumValue {
    /// Zugehoeriger [`ChecksumKind`].
    #[must_use]
    pub fn kind(&self) -> ChecksumKind {
        match self {
            Self::None => ChecksumKind::None,
            Self::Crc32c(_) => ChecksumKind::Crc32c,
            Self::Crc64(_) => ChecksumKind::Crc64,
            Self::Md5(_) => ChecksumKind::Md5,
        }
    }

    /// Berechnet die `messageChecksum` ueber `payload` mit dem
    /// gewuenschten Algorithmus (DDSI-RTPS 2.5 §9.4.2.15.2).
    ///
    /// `payload` ist der Bereich, ueber den die Checksum berechnet
    /// wird. Spec-konform sind das die Submessages **nach** der
    /// HeaderExtension-Submessage; der Caller stellt das durch
    /// passenden Slice-Cut sicher.
    ///
    /// Implementiert via [`zerodds_foundation::crc32c`],
    /// [`zerodds_foundation::crc64_xz`] und [`zerodds_foundation::md5`] —
    /// pure-Rust no_std-Hashes ohne externe Crypto-Crate-
    /// Abhaengigkeit (Pillar 9 Zero-Dependency).
    #[must_use]
    pub fn compute(kind: ChecksumKind, payload: &[u8]) -> Self {
        match kind {
            ChecksumKind::None => Self::None,
            ChecksumKind::Crc32c => Self::Crc32c(zerodds_foundation::crc32c(payload)),
            ChecksumKind::Crc64 => Self::Crc64(zerodds_foundation::crc64_xz(payload)),
            ChecksumKind::Md5 => Self::Md5(zerodds_foundation::md5(payload)),
        }
    }

    /// Verifiziert dass die in diesem Wert gehaltene Checksum mit der
    /// ueber `payload` berechneten uebereinstimmt.
    ///
    /// Liefert `true` wenn der Algorithmus aktiv ist UND der Wert
    /// passt; `true` auch wenn `kind() == None` (keine Checksum
    /// deklariert, nichts zu verifizieren); `false` bei Mismatch.
    ///
    /// Spec §9.4.2.15.2: Receiver verwirft die Message bei Mismatch.
    #[must_use]
    pub fn verify(&self, payload: &[u8]) -> bool {
        let computed = Self::compute(self.kind(), payload);
        computed == *self
    }
}

/// 8-Byte Sender-Timestamp (Time_t = i32 sec + u32 nanosec, Big-Endian
/// im RTPS-Wire-Sinn — wird im HE-Body in Submessage-Endianness
/// geschrieben).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub struct HeTimestamp {
    /// Sekunden seit Unix-Epoch.
    pub seconds: i32,
    /// Nanosekunden im laufenden Sekunden-Tick.
    pub fraction: u32,
}

/// Geparste HeaderExtension-Submessage.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct HeaderExtension {
    /// Endianness des Bodies (`true` = Little-Endian).
    pub little_endian: bool,
    /// Falls L-Flag gesetzt: Restlaenge der Message.
    pub message_length: Option<u32>,
    /// Falls W-Flag gesetzt: Sender-Timestamp.
    pub timestamp: Option<HeTimestamp>,
    /// Falls U-Flag gesetzt: 4-Byte vendor-spezifischer Wert.
    pub uextension4: Option<[u8; 4]>,
    /// Falls V-Flag gesetzt: 16-Byte vendor-spezifischer Wert.
    pub wextension8: Option<[u8; 8]>,
    /// Falls C-Flag != 0: Checksum-Wert.
    pub checksum: ChecksumValue,
    /// Falls P-Flag gesetzt: ParameterList.
    pub parameters: Option<ParameterList>,
}

impl HeaderExtension {
    /// Leerer HE (alle optionalen Felder absent). Ueber den Builder-
    /// Stil der Felder direkt manipulieren.
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    /// Berechnet das Flag-Byte (inkl. E-Bit) aus den gesetzten Feldern.
    #[must_use]
    pub fn flag_byte(&self) -> u8 {
        let mut f = 0u8;
        if self.little_endian {
            f |= HE_FLAG_E;
        }
        if self.message_length.is_some() {
            f |= HE_FLAG_L;
        }
        if self.timestamp.is_some() {
            f |= HE_FLAG_W;
        }
        if self.uextension4.is_some() {
            f |= HE_FLAG_U;
        }
        if self.wextension8.is_some() {
            f |= HE_FLAG_V;
        }
        f |= self.checksum.kind().to_flag_bits();
        if self.parameters.is_some() {
            f |= HE_FLAG_P;
        }
        f
    }

    /// Encoded den Body **ohne** Submessage-Header. Body-Laenge ist
    /// per Konstruktion ≤ [`MAX_HE_LENGTH`].
    ///
    /// Reihenfolge: L → W → U → V → C → P, gem. Spec §9.4.5.2.
    ///
    /// # Errors
    /// `ValueOutOfRange` wenn der Body > `u16::MAX` oder
    /// > `MAX_HE_LENGTH` waere.
    pub fn encode_body(&self) -> Result<Vec<u8>, WireError> {
        let le = self.little_endian;
        let mut body = Vec::with_capacity(64);

        if let Some(len) = self.message_length {
            write_u32(&mut body, len, le);
        }
        if let Some(ts) = self.timestamp {
            write_i32(&mut body, ts.seconds, le);
            write_u32(&mut body, ts.fraction, le);
        }
        if let Some(u) = self.uextension4 {
            body.extend_from_slice(&u);
        }
        if let Some(v) = self.wextension8 {
            body.extend_from_slice(&v);
        }
        match self.checksum {
            ChecksumValue::None => {}
            ChecksumValue::Crc32c(c) => write_u32(&mut body, c, le),
            ChecksumValue::Crc64(c) => write_u64(&mut body, c, le),
            ChecksumValue::Md5(m) => body.extend_from_slice(&m),
        }
        if let Some(pl) = &self.parameters {
            // ParameterList in Submessage-Endianness anfuegen.
            body.extend_from_slice(&pl.to_bytes(le));
        }
        if body.len() > MAX_HE_LENGTH {
            return Err(WireError::ValueOutOfRange {
                message: "HeaderExtension body exceeds MAX_HE_LENGTH",
            });
        }
        Ok(body)
    }

    /// Encoded HE als komplettes Submessage = 4-Byte SubmessageHeader
    /// + Body. `octets_to_next_header` wird auf Body-Laenge gesetzt.
    ///
    /// # Errors
    /// `ValueOutOfRange` wenn Body-Laenge > `u16::MAX`.
    pub fn encode(&self) -> Result<Vec<u8>, WireError> {
        let body = self.encode_body()?;
        let body_len = u16::try_from(body.len()).map_err(|_| WireError::ValueOutOfRange {
            message: "HE body exceeds u16::MAX",
        })?;
        let mut out = Vec::with_capacity(4 + body.len());
        out.push(SUBMESSAGE_ID_HEADER_EXTENSION);
        out.push(self.flag_byte());
        let len_bytes = if self.little_endian {
            body_len.to_le_bytes()
        } else {
            body_len.to_be_bytes()
        };
        out.extend_from_slice(&len_bytes);
        out.extend_from_slice(&body);
        Ok(out)
    }

    /// Decoded den HE-Body aus einem Slice der Laenge
    /// `octets_to_next_header`, gegeben das Flag-Byte aus dem
    /// Submessage-Header.
    ///
    /// # Errors
    /// `UnexpectedEof`, `ValueOutOfRange` (Body groesser
    /// `MAX_HE_LENGTH`).
    pub fn decode_body(body: &[u8], flags: u8) -> Result<Self, WireError> {
        if body.len() > MAX_HE_LENGTH {
            return Err(WireError::ValueOutOfRange {
                message: "HE body exceeds MAX_HE_LENGTH",
            });
        }
        let le = (flags & HE_FLAG_E) != 0;
        let mut pos = 0usize;
        let need = |n: usize, p: usize| -> Result<(), WireError> {
            if body.len() < p + n {
                Err(WireError::UnexpectedEof {
                    needed: n,
                    offset: p,
                })
            } else {
                Ok(())
            }
        };

        let message_length = if (flags & HE_FLAG_L) != 0 {
            need(4, pos)?;
            let v = read_u32(&body[pos..pos + 4], le);
            pos += 4;
            Some(v)
        } else {
            None
        };
        let timestamp = if (flags & HE_FLAG_W) != 0 {
            need(8, pos)?;
            let s = read_i32(&body[pos..pos + 4], le);
            let f = read_u32(&body[pos + 4..pos + 8], le);
            pos += 8;
            Some(HeTimestamp {
                seconds: s,
                fraction: f,
            })
        } else {
            None
        };
        let uextension4 = if (flags & HE_FLAG_U) != 0 {
            need(4, pos)?;
            let mut u = [0u8; 4];
            u.copy_from_slice(&body[pos..pos + 4]);
            pos += 4;
            Some(u)
        } else {
            None
        };
        let wextension8 = if (flags & HE_FLAG_V) != 0 {
            need(8, pos)?;
            let mut v = [0u8; 8];
            v.copy_from_slice(&body[pos..pos + 8]);
            pos += 8;
            Some(v)
        } else {
            None
        };

        let kind = ChecksumKind::from_flags(flags);
        let checksum = match kind {
            ChecksumKind::None => ChecksumValue::None,
            ChecksumKind::Crc32c => {
                need(4, pos)?;
                let v = read_u32(&body[pos..pos + 4], le);
                pos += 4;
                ChecksumValue::Crc32c(v)
            }
            ChecksumKind::Crc64 => {
                need(8, pos)?;
                let v = read_u64(&body[pos..pos + 8], le);
                pos += 8;
                ChecksumValue::Crc64(v)
            }
            ChecksumKind::Md5 => {
                need(16, pos)?;
                let mut m = [0u8; 16];
                m.copy_from_slice(&body[pos..pos + 16]);
                pos += 16;
                ChecksumValue::Md5(m)
            }
        };

        let parameters = if (flags & HE_FLAG_P) != 0 {
            // ParameterList belegt den verbleibenden Body.
            let pl = ParameterList::from_bytes(&body[pos..], le)?;
            // pos auf Ende setzen — die ParameterList endet am Sentinel
            // und kann von trailing-Bytes gefolgt sein, die wir hier als
            // Fehler-Indikator werten (Spec verbietet trailing-Garbage
            // im HE-Body).
            //
            // Konkret: Encoder schreibt am Ende den Sentinel + 0; ein
            // sauberer Body endet damit. Wenn `from_bytes` schon
            // erfolgreich zurueckkehrt, hat es alles bis zum Sentinel
            // gelesen — wir validieren keine Trailing-Bytes (RFC sagt
            // ParameterList nimmt den Rest; defensives Verwerfen wuerde
            // legale Vendor-Padding brechen).
            Some(pl)
        } else {
            // Kein P-Flag: trailing bytes sind unerwartet.
            if pos != body.len() {
                return Err(WireError::ValueOutOfRange {
                    message: "HE trailing bytes without P-flag",
                });
            }
            None
        };

        Ok(Self {
            little_endian: le,
            message_length,
            timestamp,
            uextension4,
            wextension8,
            checksum,
            parameters,
        })
    }

    /// Decoded eine komplette HE-Submessage (Header + Body) aus
    /// `bytes`. Ueberprueft die SubmessageId.
    ///
    /// # Errors
    /// `UnexpectedEof`, `UnknownSubmessageId`, `ValueOutOfRange`.
    pub fn decode(bytes: &[u8]) -> Result<Self, WireError> {
        if bytes.len() < 4 {
            return Err(WireError::UnexpectedEof {
                needed: 4,
                offset: 0,
            });
        }
        if bytes[0] != SUBMESSAGE_ID_HEADER_EXTENSION {
            return Err(WireError::UnknownSubmessageId { id: bytes[0] });
        }
        let flags = bytes[1];
        let le = (flags & HE_FLAG_E) != 0;
        let mut len_bytes = [0u8; 2];
        len_bytes.copy_from_slice(&bytes[2..4]);
        let octets = if le {
            u16::from_le_bytes(len_bytes)
        } else {
            u16::from_be_bytes(len_bytes)
        } as usize;
        if bytes.len() < 4 + octets {
            return Err(WireError::UnexpectedEof {
                needed: octets,
                offset: 4,
            });
        }
        Self::decode_body(&bytes[4..4 + octets], flags)
    }

    /// Cross-Check: wenn `message_length` gesetzt ist, MUSS sie der
    /// tatsaechlichen Restlaenge der RTPS-Message ab dem ersten
    /// Submessage-Header (also ab Byte 20 = nach RtpsHeader) bis zum
    /// Datagram-Ende entsprechen (Spec §8.3.7.4).
    ///
    /// `actual_message_length` ist die vom Receiver gemessene
    /// Restlaenge.
    ///
    /// # Errors
    /// `ValueOutOfRange` bei Mismatch.
    pub fn validate_message_length(&self, actual_message_length: u32) -> Result<(), WireError> {
        if let Some(declared) = self.message_length {
            if declared != actual_message_length {
                return Err(WireError::ValueOutOfRange {
                    message: "HeaderExtension messageLength mismatch",
                });
            }
        }
        Ok(())
    }
}

// =====================================================================
// PID-Helpers — Must-Understand-Bit (Spec §9.4.2.11.2)
// =====================================================================

/// Bit-Maske: gesetzt -> Reader MUSS diesen PID verstehen, sonst die
/// gesamte Message (HE-Carrier) verwerfen.
pub const PID_MUST_UNDERSTAND: u16 = 0x4000;

/// Bit-Maske: gesetzt -> Vendor-spezifischer PID (Spec-Reservation 2.5
/// Tab 9.13). Reader die diesen PID nicht kennen, duerfen ihn skippen
/// **wenn** das `must_understand`-Bit nicht gesetzt ist.
pub const PID_VENDOR_SPECIFIC: u16 = 0x8000;

/// Liefert `true`, wenn der gegebene PID das Must-Understand-Bit
/// gesetzt hat.
#[must_use]
pub fn pid_must_understand(pid: u16) -> bool {
    (pid & PID_MUST_UNDERSTAND) != 0
}

/// Roher PID ohne Must-Understand- und Vendor-Bits.
#[must_use]
pub fn pid_strip(pid: u16) -> u16 {
    pid & !(PID_MUST_UNDERSTAND | PID_VENDOR_SPECIFIC)
}

// =====================================================================
// Bit-Helpers (private)
// =====================================================================

fn write_u32(out: &mut Vec<u8>, v: u32, le: bool) {
    let bytes = if le { v.to_le_bytes() } else { v.to_be_bytes() };
    out.extend_from_slice(&bytes);
}

fn write_i32(out: &mut Vec<u8>, v: i32, le: bool) {
    let bytes = if le { v.to_le_bytes() } else { v.to_be_bytes() };
    out.extend_from_slice(&bytes);
}

fn write_u64(out: &mut Vec<u8>, v: u64, le: bool) {
    let bytes = if le { v.to_le_bytes() } else { v.to_be_bytes() };
    out.extend_from_slice(&bytes);
}

fn read_u32(bytes: &[u8], le: bool) -> u32 {
    let mut buf = [0u8; 4];
    buf.copy_from_slice(&bytes[..4]);
    if le {
        u32::from_le_bytes(buf)
    } else {
        u32::from_be_bytes(buf)
    }
}

fn read_i32(bytes: &[u8], le: bool) -> i32 {
    let mut buf = [0u8; 4];
    buf.copy_from_slice(&bytes[..4]);
    if le {
        i32::from_le_bytes(buf)
    } else {
        i32::from_be_bytes(buf)
    }
}

fn read_u64(bytes: &[u8], le: bool) -> u64 {
    let mut buf = [0u8; 8];
    buf.copy_from_slice(&bytes[..8]);
    if le {
        u64::from_le_bytes(buf)
    } else {
        u64::from_be_bytes(buf)
    }
}

#[cfg(test)]
mod tests {
    #![allow(clippy::expect_used, clippy::unwrap_used, clippy::panic)]
    use super::*;
    use crate::parameter_list::Parameter;
    use alloc::vec;

    fn roundtrip(he: HeaderExtension) {
        let bytes = he.encode().unwrap();
        let decoded = HeaderExtension::decode(&bytes).unwrap();
        assert_eq!(decoded, he);
    }

    #[test]
    fn empty_he_le_roundtrip() {
        let he = HeaderExtension {
            little_endian: true,
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn empty_he_be_roundtrip() {
        let he = HeaderExtension::default();
        // little_endian = false
        roundtrip(he);
    }

    #[test]
    fn flag_byte_empty_le() {
        let he = HeaderExtension {
            little_endian: true,
            ..HeaderExtension::default()
        };
        assert_eq!(he.flag_byte(), HE_FLAG_E);
    }

    #[test]
    fn flag_byte_with_message_length() {
        let mut he = HeaderExtension {
            little_endian: true,
            message_length: Some(4242),
            ..HeaderExtension::default()
        };
        assert_eq!(he.flag_byte(), HE_FLAG_E | HE_FLAG_L);
        he.message_length = None;
        assert_eq!(he.flag_byte(), HE_FLAG_E);
    }

    #[test]
    fn flag_byte_all_optional_set_le() {
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(0x0102_0304),
            timestamp: Some(HeTimestamp {
                seconds: 7,
                fraction: 8,
            }),
            uextension4: Some([0xAA, 0xBB, 0xCC, 0xDD]),
            wextension8: Some([0x11; 8]),
            checksum: ChecksumValue::Md5([0x22; 16]),
            parameters: Some(ParameterList::new()),
        };
        let f = he.flag_byte();
        assert_eq!(
            f,
            HE_FLAG_E
                | HE_FLAG_L
                | HE_FLAG_W
                | HE_FLAG_U
                | HE_FLAG_V
                | HE_FLAG_P
                | ChecksumKind::Md5.to_flag_bits()
        );
    }

    #[test]
    fn checksum_kind_roundtrip() {
        for kind in [
            ChecksumKind::None,
            ChecksumKind::Crc32c,
            ChecksumKind::Crc64,
            ChecksumKind::Md5,
        ] {
            let bits = kind.to_flag_bits();
            assert_eq!(ChecksumKind::from_flags(HE_FLAG_E | bits), kind);
        }
    }

    #[test]
    fn checksum_kind_wire_size() {
        assert_eq!(ChecksumKind::None.wire_size(), 0);
        assert_eq!(ChecksumKind::Crc32c.wire_size(), 4);
        assert_eq!(ChecksumKind::Crc64.wire_size(), 8);
        assert_eq!(ChecksumKind::Md5.wire_size(), 16);
    }

    #[test]
    fn roundtrip_with_message_length_only_le() {
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(0xDEAD_BEEF),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_timestamp_le() {
        let he = HeaderExtension {
            little_endian: true,
            timestamp: Some(HeTimestamp {
                seconds: 1_710_000_000,
                fraction: 500_000_000,
            }),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_uextension4_be() {
        let he = HeaderExtension {
            little_endian: false,
            uextension4: Some([1, 2, 3, 4]),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_wextension8_le() {
        let he = HeaderExtension {
            little_endian: true,
            wextension8: Some([0xA5; 8]),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_crc32c_le() {
        let he = HeaderExtension {
            little_endian: true,
            checksum: ChecksumValue::Crc32c(0xCAFE_BABE),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_crc64_be() {
        let he = HeaderExtension {
            little_endian: false,
            checksum: ChecksumValue::Crc64(0x0123_4567_89AB_CDEF),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_md5_le() {
        let he = HeaderExtension {
            little_endian: true,
            checksum: ChecksumValue::Md5([
                0xd4, 0x1d, 0x8c, 0xd9, 0x8f, 0x00, 0xb2, 0x04, 0xe9, 0x80, 0x09, 0x98, 0xec, 0xf8,
                0x42, 0x7e,
            ]),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_with_parameter_list() {
        let mut pl = ParameterList::new();
        pl.push(Parameter::new(0x0015, vec![2, 5, 0, 0]));
        let he = HeaderExtension {
            little_endian: true,
            parameters: Some(pl),
            ..HeaderExtension::default()
        };
        roundtrip(he);
    }

    #[test]
    fn decode_rejects_malformed_parameter_list_when_p_flag_set() {
        // Spec §8.3.7.3: Bei P-Flag muss der Rest des Bodies eine
        // valide ParameterList sein. Ein Body, der einen halben
        // Parameter-Header enthaelt (4 Byte Header + 4 Byte fehlt),
        // muss mit Error abgelehnt werden.
        let mut pl_bytes = Vec::<u8>::new();
        // PID=0x0002, length=0x0010 (16 byte angekuendigt), aber nur 2
        // Byte body geliefert. (PID=0x0001 ist Sentinel — vermeide.)
        pl_bytes.extend_from_slice(&0x0002u16.to_le_bytes());
        pl_bytes.extend_from_slice(&0x0010u16.to_le_bytes());
        pl_bytes.extend_from_slice(&[0xAA, 0xBB]); // truncated body
        // Body = E-flag in flags + P-flag → kein L/W/U/V/C, danach PL.
        let flags = HE_FLAG_E | HE_FLAG_P;
        let r = HeaderExtension::decode_body(&pl_bytes, flags);
        assert!(r.is_err(), "malformed PL must be rejected");
    }

    #[test]
    fn decode_rejects_truncated_uextension4_body() {
        // Spec §8.3.7.3 Length-Mismatch: U-Flag angekuendigt, aber
        // Body enthaelt nur 2 Byte statt 4.
        let body: [u8; 2] = [0xAA, 0xBB];
        let flags = HE_FLAG_E | HE_FLAG_U;
        let r = HeaderExtension::decode_body(&body, flags);
        assert!(matches!(r, Err(WireError::UnexpectedEof { .. })));
    }

    #[test]
    fn decode_rejects_truncated_wextension8_body() {
        // Spec §8.3.7.3: V-Flag angekuendigt, Body kuerzer als 8 Byte.
        let body: [u8; 4] = [0; 4];
        let flags = HE_FLAG_E | HE_FLAG_V;
        let r = HeaderExtension::decode_body(&body, flags);
        assert!(matches!(r, Err(WireError::UnexpectedEof { .. })));
    }

    #[test]
    fn decode_rejects_truncated_md5_checksum_body() {
        // C-Flag = MD5 (=3), Body < 16 Byte → reject.
        let body: [u8; 4] = [0; 4];
        let flags = HE_FLAG_E | HE_FLAG_C0 | HE_FLAG_C1;
        let r = HeaderExtension::decode_body(&body, flags);
        assert!(matches!(r, Err(WireError::UnexpectedEof { .. })));
    }

    #[test]
    fn flag_byte_includes_all_seven_logical_flags() {
        // Spec §8.3.3.2: 7 Flags E/L/W/U/V/C/P.
        let mut pl = ParameterList::new();
        pl.push(Parameter::new(0x0001, vec![1, 2, 3, 4]));
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(99),
            timestamp: Some(HeTimestamp::default()),
            uextension4: Some([1; 4]),
            wextension8: Some([2; 8]),
            checksum: ChecksumValue::Crc64(0xABCD),
            parameters: Some(pl),
        };
        let f = he.flag_byte();
        assert!(f & HE_FLAG_E != 0, "E");
        assert!(f & HE_FLAG_L != 0, "L");
        assert!(f & HE_FLAG_W != 0, "W");
        assert!(f & HE_FLAG_U != 0, "U");
        assert!(f & HE_FLAG_V != 0, "V");
        assert!(f & HE_FLAG_C_MASK != 0, "C");
        assert!(f & HE_FLAG_P != 0, "P");
    }

    #[test]
    fn roundtrip_all_fields_set_le() {
        let mut pl = ParameterList::new();
        pl.push(Parameter::new(0x0015, vec![2, 5, 0, 0]));
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(1234),
            timestamp: Some(HeTimestamp {
                seconds: 1,
                fraction: 2,
            }),
            uextension4: Some([0xDE, 0xAD, 0xBE, 0xEF]),
            wextension8: Some([0xC0; 8]),
            checksum: ChecksumValue::Crc64(0x1122_3344_5566_7788),
            parameters: Some(pl),
        };
        roundtrip(he);
    }

    #[test]
    fn roundtrip_all_fields_set_be() {
        let mut pl = ParameterList::new();
        pl.push(Parameter::new(0x0050, vec![0xAA; 16]));
        let he = HeaderExtension {
            little_endian: false,
            message_length: Some(99),
            timestamp: Some(HeTimestamp {
                seconds: -3,
                fraction: 9,
            }),
            uextension4: Some([0; 4]),
            wextension8: Some([0xFF; 8]),
            checksum: ChecksumValue::Md5([0x77; 16]),
            parameters: Some(pl),
        };
        roundtrip(he);
    }

    #[test]
    fn decode_rejects_wrong_submessage_id() {
        // Falscher SubmessageId (0x15 = DATA).
        let bytes = [0x15u8, HE_FLAG_E, 0, 0];
        let res = HeaderExtension::decode(&bytes);
        assert!(matches!(
            res,
            Err(WireError::UnknownSubmessageId { id: 0x15 })
        ));
    }

    #[test]
    fn decode_rejects_truncated_header() {
        let bytes = [0x80u8, HE_FLAG_E];
        let res = HeaderExtension::decode(&bytes);
        assert!(matches!(res, Err(WireError::UnexpectedEof { .. })));
    }

    #[test]
    fn decode_rejects_truncated_body_before_message_length() {
        // L-Flag aber nur 2 Byte Body.
        let bytes = [
            SUBMESSAGE_ID_HEADER_EXTENSION,
            HE_FLAG_E | HE_FLAG_L,
            2,
            0,
            0xAA,
            0xBB,
        ];
        let res = HeaderExtension::decode(&bytes);
        assert!(matches!(res, Err(WireError::UnexpectedEof { .. })));
    }

    #[test]
    fn decode_rejects_oversized_body() {
        // Konstruieren eines Headers mit grossem Body — wir koennen
        // nur bis u16::MAX adressieren, aber wir koennen
        // `decode_body` direkt mit > MAX_HE_LENGTH Body fuettern.
        let big = vec![0u8; MAX_HE_LENGTH + 1];
        let res = HeaderExtension::decode_body(&big, HE_FLAG_E);
        assert!(matches!(res, Err(WireError::ValueOutOfRange { .. })));
    }

    #[test]
    fn decode_rejects_trailing_bytes_without_p_flag() {
        // Body hat 4 Byte trailing aber kein P-Flag.
        let body = vec![0xAA, 0xBB, 0xCC, 0xDD];
        let res = HeaderExtension::decode_body(&body, HE_FLAG_E);
        assert!(matches!(res, Err(WireError::ValueOutOfRange { .. })));
    }

    #[test]
    fn validate_message_length_matches() {
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(42),
            ..HeaderExtension::default()
        };
        assert!(he.validate_message_length(42).is_ok());
    }

    #[test]
    fn validate_message_length_mismatch() {
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(42),
            ..HeaderExtension::default()
        };
        let res = he.validate_message_length(43);
        assert!(matches!(res, Err(WireError::ValueOutOfRange { .. })));
    }

    #[test]
    fn validate_message_length_no_l_flag_succeeds() {
        let he = HeaderExtension::default();
        assert!(he.validate_message_length(123).is_ok());
    }

    #[test]
    fn checksum_value_kind_matches() {
        assert_eq!(ChecksumValue::None.kind(), ChecksumKind::None);
        assert_eq!(ChecksumValue::Crc32c(1).kind(), ChecksumKind::Crc32c);
        assert_eq!(ChecksumValue::Crc64(1).kind(), ChecksumKind::Crc64);
        assert_eq!(ChecksumValue::Md5([0; 16]).kind(), ChecksumKind::Md5);
    }

    #[test]
    fn pid_must_understand_helpers() {
        assert!(pid_must_understand(0x4000));
        assert!(pid_must_understand(0x4015));
        assert!(!pid_must_understand(0x0015));
        assert_eq!(pid_strip(0xC015), 0x0015);
        assert_eq!(pid_strip(0x4015), 0x0015);
        assert_eq!(pid_strip(0x8015), 0x0015);
        assert_eq!(pid_strip(0x0015), 0x0015);
    }

    #[test]
    fn wire_layout_le_message_length_correct() {
        // L-Flag, 4 Byte Body = 0xDEADBEEF LE.
        let he = HeaderExtension {
            little_endian: true,
            message_length: Some(0xDEAD_BEEF),
            ..HeaderExtension::default()
        };
        let bytes = he.encode().unwrap();
        assert_eq!(bytes[0], SUBMESSAGE_ID_HEADER_EXTENSION);
        assert_eq!(bytes[1], HE_FLAG_E | HE_FLAG_L);
        // octets_to_next_header LE = 4
        assert_eq!(&bytes[2..4], &[0x04, 0x00]);
        // body LE
        assert_eq!(&bytes[4..8], &[0xEF, 0xBE, 0xAD, 0xDE]);
    }

    #[test]
    fn wire_layout_be_message_length_correct() {
        let he = HeaderExtension {
            little_endian: false,
            message_length: Some(0xDEAD_BEEF),
            ..HeaderExtension::default()
        };
        let bytes = he.encode().unwrap();
        assert_eq!(bytes[1], HE_FLAG_L); // E-Flag NICHT gesetzt
        // octets_to_next_header BE = 4
        assert_eq!(&bytes[2..4], &[0x00, 0x04]);
        assert_eq!(&bytes[4..8], &[0xDE, 0xAD, 0xBE, 0xEF]);
    }

    #[test]
    fn empty_he_has_zero_octets_to_next_header() {
        let he = HeaderExtension {
            little_endian: true,
            ..HeaderExtension::default()
        };
        let bytes = he.encode().unwrap();
        assert_eq!(bytes.len(), 4);
        assert_eq!(&bytes[2..4], &[0, 0]);
    }

    // ========================================================================
    // ChecksumValue::compute / verify (Spec §9.4.2.15.2)
    // ========================================================================

    #[test]
    fn checksum_compute_none_yields_none() {
        let cs = ChecksumValue::compute(ChecksumKind::None, b"any payload");
        assert_eq!(cs, ChecksumValue::None);
    }

    #[test]
    fn checksum_compute_crc32c_matches_rfc4960_vector() {
        // RFC 4960 Appendix B.4: CRC32C("123456789") = 0xE3069283
        let cs = ChecksumValue::compute(ChecksumKind::Crc32c, b"123456789");
        assert_eq!(cs, ChecksumValue::Crc32c(0xE306_9283));
    }

    #[test]
    fn checksum_compute_crc64_xz_matches_known_vector() {
        // XZ utils CRC-64-XZ Standard-Test-Vector:
        // CRC-64("123456789") = 0x995DC9BBDF1939FA
        let cs = ChecksumValue::compute(ChecksumKind::Crc64, b"123456789");
        assert_eq!(cs, ChecksumValue::Crc64(0x995D_C9BB_DF19_39FA));
    }

    #[test]
    fn checksum_compute_md5_matches_rfc1321_vector() {
        // RFC 1321 §A.5: MD5("abc") = 900150983cd24fb0 d6963f7d28e17f72
        let cs = ChecksumValue::compute(ChecksumKind::Md5, b"abc");
        let expected: [u8; 16] = [
            0x90, 0x01, 0x50, 0x98, 0x3c, 0xd2, 0x4f, 0xb0, 0xd6, 0x96, 0x3f, 0x7d, 0x28, 0xe1,
            0x7f, 0x72,
        ];
        assert_eq!(cs, ChecksumValue::Md5(expected));
    }

    #[test]
    fn checksum_verify_round_trip_crc32c_succeeds() {
        let payload = b"the quick brown fox jumps over the lazy dog";
        let cs = ChecksumValue::compute(ChecksumKind::Crc32c, payload);
        assert!(cs.verify(payload));
    }

    #[test]
    fn checksum_verify_detects_tampered_payload() {
        let original = b"original message";
        let cs = ChecksumValue::compute(ChecksumKind::Crc32c, original);
        let tampered = b"original Message";
        assert!(!cs.verify(tampered));
    }

    #[test]
    fn checksum_verify_none_kind_always_succeeds() {
        // Kind=None bedeutet "keine Checksum deklariert" — Verify ist
        // dann no-op und MUSS true zurueckgeben (sonst wuerden alle
        // Messages ohne deklarierte Checksum verworfen).
        let cs = ChecksumValue::None;
        assert!(cs.verify(b"any payload"));
        assert!(cs.verify(b""));
    }

    #[test]
    fn checksum_verify_crc64_round_trip() {
        let payload = b"\x52\x54\x50\x53\x02\x05\x01\x0F";
        let cs = ChecksumValue::compute(ChecksumKind::Crc64, payload);
        assert!(cs.verify(payload));
    }

    #[test]
    fn checksum_verify_md5_round_trip() {
        let payload = b"DDSI-RTPS 2.5 HEADER_EXTENSION";
        let cs = ChecksumValue::compute(ChecksumKind::Md5, payload);
        assert!(cs.verify(payload));
    }
}