rns-core 0.7.0

use alloc::sync::Arc;
use alloc::vec::Vec;
use core::fmt;

use crate::constants;
use crate::transport::types::PacketBytes;

/// Storage for a packet's wire bytes that can hand them over as shared
/// [`PacketBytes`]: shared bytes are cloned, anything else is copied.
pub trait PacketStorage: AsRef<[u8]> {
    fn to_shared(&self) -> PacketBytes;
}

impl PacketStorage for Vec<u8> {
    fn to_shared(&self) -> PacketBytes {
        PacketBytes::from(self.as_slice())
    }
}

impl PacketStorage for &[u8] {
    fn to_shared(&self) -> PacketBytes {
        PacketBytes::from(*self)
    }
}

impl PacketStorage for Arc<[u8]> {
    fn to_shared(&self) -> PacketBytes {
        self.clone()
    }
}

#[derive(Debug)]
pub enum PacketError {
    TooShort,
    ExceedsMtu,
    MissingTransportId,
    InvalidHeaderType,
    ZeroLengthData,
    /// Hop counts at or above `PATHFINDER_M` are malformed, as upstream
    /// `Packet.unpack` (Packet.py:250) rejects them before the transport
    /// hop limiter is consulted; the receiving interface counts a protocol
    /// violation (Transport.py:2008).
    InvalidHopCount(u8),
}

impl fmt::Display for PacketError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PacketError::TooShort => write!(f, "Packet too short"),
            PacketError::ExceedsMtu => write!(f, "Packet exceeds MTU"),
            PacketError::MissingTransportId => write!(f, "HEADER_2 requires transport_id"),
            PacketError::InvalidHeaderType => write!(f, "Invalid header type"),
            PacketError::ZeroLengthData => write!(f, "Zero-length data field"),
            PacketError::InvalidHopCount(hops) => write!(f, "Invalid hop count: {}", hops),
        }
    }
}

// =============================================================================
// PacketFlags: packs 5 fields into one byte
// =============================================================================

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PacketFlags {
    pub header_type: u8,
    pub context_flag: u8,
    pub transport_type: u8,
    pub destination_type: u8,
    pub packet_type: u8,
}

impl PacketFlags {
    /// Pack fields into a single flags byte.
    ///
    /// Bit layout:
    /// ```text
    /// Bit 6:     header_type (1 bit)
    /// Bit 5:     context_flag (1 bit)
    /// Bit 4:     transport_type (1 bit)
    /// Bits 3-2:  destination_type (2 bits)
    /// Bits 1-0:  packet_type (2 bits)
    /// ```
    pub fn pack(&self) -> u8 {
        (self.header_type << 6)
            | (self.context_flag << 5)
            | (self.transport_type << 4)
            | (self.destination_type << 2)
            | self.packet_type
    }

    /// Unpack a flags byte into fields.
    pub fn unpack(byte: u8) -> Self {
        PacketFlags {
            header_type: (byte & 0b01000000) >> 6,
            context_flag: (byte & 0b00100000) >> 5,
            transport_type: (byte & 0b00010000) >> 4,
            destination_type: (byte & 0b00001100) >> 2,
            packet_type: byte & 0b00000011,
        }
    }
}

// =============================================================================
// RawPacket: wire-level packet representation
// =============================================================================

/// A packet's header fields and its wire bytes `raw`, owned (`Vec<u8>`, the
/// default) or in other storage: a borrowed view (`&[u8]`) reads a packet
/// without copying it, and shared bytes (`PacketBytes`) are cloned, not copied.
#[derive(Debug, Clone)]
pub struct RawPacket<B = Vec<u8>> {
    pub flags: PacketFlags,
    pub hops: u8,
    pub transport_id: Option<[u8; 16]>,
    pub destination_hash: [u8; 16],
    pub context: u8,
    pub raw: B,
    pub packet_hash: [u8; 32],
    pub rssi: Option<i16>,
    pub snr: Option<f32>,
}

impl RawPacket {
    /// Offset of the payload in the wire bytes of a packet with this header type.
    #[inline]
    pub const fn data_offset(header_type: u8) -> usize {
        if header_type == constants::HEADER_2 {
            2 + 2 * (constants::TRUNCATED_HASHLENGTH / 8) + 1
        } else {
            2 + constants::TRUNCATED_HASHLENGTH / 8 + 1
        }
    }

    /// [`RawPacket::is_unhashed_link_resource`] for wire bytes that are not
    /// unpacked; `false` when they are too short to carry a context byte.
    pub fn is_unhashed_link_resource_wire(raw: &[u8]) -> bool {
        let Some(&flags) = raw.first() else {
            return false;
        };
        let flags = PacketFlags::unpack(flags);
        raw.get(Self::data_offset(flags.header_type) - 1)
            .is_some_and(|&context| Self::is_unhashed_link_resource_header(&flags, context))
    }

    #[inline]
    fn is_unhashed_link_resource_header(flags: &PacketFlags, context: u8) -> bool {
        flags.destination_type == constants::DESTINATION_LINK
            && match context {
                constants::CONTEXT_RESOURCE | constants::CONTEXT_RESOURCE_REQ => {
                    flags.packet_type == constants::PACKET_TYPE_DATA
                }
                constants::CONTEXT_RESOURCE_PRF => {
                    flags.packet_type == constants::PACKET_TYPE_PROOF
                }
                _ => false,
            }
    }

    /// Pack fields into raw bytes.
    pub fn pack(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
    ) -> Result<Self, PacketError> {
        Self::pack_with_max_mtu(
            flags,
            hops,
            destination_hash,
            transport_id,
            context,
            data,
            constants::MTU,
        )
    }

    /// Pack fields into raw bytes and packet hash without constructing a full RawPacket.
    pub fn pack_raw_with_hash(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
    ) -> Result<(Vec<u8>, [u8; 32]), PacketError> {
        Self::pack_raw_with_hash_with_max_mtu(
            flags,
            hops,
            destination_hash,
            transport_id,
            context,
            data,
            constants::MTU,
        )
    }

    /// Pack fields into raw bytes with a caller-provided MTU limit.
    pub fn pack_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
        max_mtu: usize,
    ) -> Result<Self, PacketError> {
        let (raw, packet_hash) = Self::pack_raw_with_hash_with_max_mtu(
            flags,
            hops,
            destination_hash,
            transport_id,
            context,
            data,
            max_mtu,
        )?;

        Ok(RawPacket {
            flags,
            hops,
            transport_id: transport_id.copied(),
            destination_hash: *destination_hash,
            context,
            raw,
            packet_hash,
            rssi: None,
            snr: None,
        })
    }

    /// Pack fields into raw bytes and packet hash with a caller-provided MTU limit.
    pub fn pack_raw_with_hash_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
        max_mtu: usize,
    ) -> Result<(Vec<u8>, [u8; 32]), PacketError> {
        let raw = Self::pack_raw_with_max_mtu(
            flags,
            hops,
            destination_hash,
            transport_id,
            context,
            data,
            max_mtu,
        )?;
        let packet_hash = Self::compute_packet_hash(flags.header_type, &raw);
        Ok((raw, packet_hash))
    }

    /// Pack only wire bytes when the caller does not need a packet hash.
    /// Validation and encoding match the hash-producing packing methods.
    /// [`RawPacket::pack_raw_with_hash_with_max_mtu`] into shared bytes.
    #[allow(clippy::too_many_arguments)]
    pub fn pack_raw_shared_with_hash_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
        max_mtu: usize,
    ) -> Result<(PacketBytes, [u8; 32]), PacketError> {
        let raw = Self::pack_raw_shared_with_max_mtu(
            flags,
            hops,
            destination_hash,
            transport_id,
            context,
            data,
            max_mtu,
        )?;
        let packet_hash = Self::compute_packet_hash(flags.header_type, &raw);
        Ok((raw, packet_hash))
    }

    /// The packet hash of packed wire bytes with the given header type.
    pub fn packet_hash_of(header_type: u8, raw: &[u8]) -> [u8; 32] {
        Self::compute_packet_hash(header_type, raw)
    }

    /// [`RawPacket::pack_raw_shared_with_max_mtu`] with a payload of
    /// `payload_len` bytes that `fill` writes in place, so a payload produced
    /// for this packet (a ciphertext, say) needs no buffer of its own. `fill`
    /// receives the payload zeroed.
    #[allow(clippy::too_many_arguments)]
    pub fn pack_raw_shared_filled_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        payload_len: usize,
        max_mtu: usize,
        fill: impl FnOnce(&mut [u8]),
    ) -> Result<PacketBytes, PacketError> {
        if flags.header_type == constants::HEADER_2 && transport_id.is_none() {
            return Err(PacketError::MissingTransportId);
        }
        let transport_id = transport_id.filter(|_| flags.header_type == constants::HEADER_2);
        let header_len = 2 + transport_id.map_or(0, |id| id.len()) + destination_hash.len() + 1;
        let len = header_len + payload_len;
        if len > max_mtu {
            return Err(PacketError::ExceedsMtu);
        }
        let mut shared = Arc::<[u8]>::new_uninit_slice(len);
        let out = Arc::get_mut(&mut shared).expect("a new Arc is unique");
        // SAFETY: `out` is `len` writable bytes; zeroing initialises them all,
        // and u8 and MaybeUninit<u8> share a layout.
        let out: &mut [u8] = unsafe {
            core::ptr::write_bytes(out.as_mut_ptr(), 0, len);
            core::slice::from_raw_parts_mut(out.as_mut_ptr().cast::<u8>(), len)
        };
        out[0] = flags.pack();
        out[1] = hops;
        let mut at = 2;
        if let Some(transport_id) = transport_id {
            out[at..at + 16].copy_from_slice(transport_id);
            at += 16;
        }
        out[at..at + 16].copy_from_slice(destination_hash);
        at += 16;
        out[at] = context;
        fill(&mut out[header_len..]);
        // SAFETY: every byte was initialised by the zeroing above.
        Ok(unsafe { shared.assume_init() })
    }

    /// [`RawPacket::pack_raw_with_max_mtu`] into shared bytes, allocated once
    /// at their final size, for packets that are routed and sent as they are.
    pub fn pack_raw_shared_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
        max_mtu: usize,
    ) -> Result<PacketBytes, PacketError> {
        if flags.header_type == constants::HEADER_2 && transport_id.is_none() {
            return Err(PacketError::MissingTransportId);
        }
        let transport_id = transport_id.filter(|_| flags.header_type == constants::HEADER_2);
        let len =
            2 + transport_id.map_or(0, |id| id.len()) + destination_hash.len() + 1 + data.len();
        if len > max_mtu {
            return Err(PacketError::ExceedsMtu);
        }
        let mut shared = Arc::<[u8]>::new_uninit_slice(len);
        let out = Arc::get_mut(&mut shared).expect("a new Arc is unique");
        let mut at = 0;
        let mut put = |bytes: &[u8]| {
            let slots = &mut out[at..at + bytes.len()];
            // SAFETY: `slots` is in bounds and exactly `bytes.len()` long; u8
            // and MaybeUninit<u8> share a layout.
            unsafe {
                core::ptr::copy_nonoverlapping(
                    bytes.as_ptr(),
                    slots.as_mut_ptr().cast::<u8>(),
                    bytes.len(),
                )
            };
            at += bytes.len();
        };
        put(&[flags.pack(), hops]);
        if let Some(transport_id) = transport_id {
            put(transport_id);
        }
        put(destination_hash);
        put(&[context]);
        put(data);
        assert_eq!(at, len);
        // SAFETY: every one of the `len` bytes was written above.
        Ok(unsafe { shared.assume_init() })
    }

    pub fn pack_raw_with_max_mtu(
        flags: PacketFlags,
        hops: u8,
        destination_hash: &[u8; 16],
        transport_id: Option<&[u8; 16]>,
        context: u8,
        data: &[u8],
        max_mtu: usize,
    ) -> Result<Vec<u8>, PacketError> {
        if flags.header_type == constants::HEADER_2 && transport_id.is_none() {
            return Err(PacketError::MissingTransportId);
        }

        let transport_id = transport_id.filter(|_| flags.header_type == constants::HEADER_2);
        // flags, hops, [transport id], destination hash, context, data
        let len =
            2 + transport_id.map_or(0, |id| id.len()) + destination_hash.len() + 1 + data.len();
        if len > max_mtu {
            return Err(PacketError::ExceedsMtu);
        }

        // Sized once: growing from empty cost an allocation and several
        // reallocations per packet.
        let mut raw = Vec::with_capacity(len);
        raw.push(flags.pack());
        raw.push(hops);
        if let Some(transport_id) = transport_id {
            raw.extend_from_slice(transport_id);
        }
        raw.extend_from_slice(destination_hash);
        raw.push(context);
        raw.extend_from_slice(data);
        debug_assert_eq!(raw.len(), len);

        Ok(raw)
    }

    /// Unpack raw bytes into fields.
    pub fn unpack(raw: &[u8]) -> Result<Self, PacketError> {
        RawPacket::<&[u8]>::parse::<true>(raw).map(|packet| packet.map_raw(<[u8]>::to_vec))
    }

    /// Unpack wire bytes the caller owns, keeping them as `raw` instead of
    /// copying them.
    pub fn unpack_owned(raw: Vec<u8>) -> Result<Self, PacketError> {
        Self::parse::<true>(raw)
    }

    /// Unpack owned wire bytes into fields, leaving `packet_hash` zeroed.
    /// Callers must hash the packet with [`RawPacket::hash_in_place`] before
    /// anything reads `packet_hash`.
    pub(crate) fn unpack_unhashed(raw: Vec<u8>) -> Result<Self, PacketError> {
        Self::parse::<false>(raw)
    }

    /// Unpack owned wire bytes of a packet this node sends. A link resource
    /// part, part request or proof is left unhashed, its `packet_hash`
    /// zeroed, because nothing reads it (see
    /// [`RawPacket::is_unhashed_link_resource`]).
    pub fn unpack_outbound(raw: Vec<u8>) -> Result<Self, PacketError> {
        let mut packet = Self::unpack_unhashed(raw)?;
        if !packet.is_unhashed_link_resource() {
            packet.hash_in_place();
        }
        Ok(packet)
    }

    /// [`RawPacket::unpack_owned`] for shared bytes, kept as they are.
    pub fn unpack_shared(raw: PacketBytes) -> Result<RawPacket<PacketBytes>, PacketError> {
        RawPacket::<PacketBytes>::parse::<true>(raw)
    }

    /// [`RawPacket::unpack`] for shared bytes, leaving `packet_hash` zeroed
    /// (see [`RawPacket::unpack_unhashed`]).
    pub(crate) fn unpack_shared_unhashed(
        raw: PacketBytes,
    ) -> Result<RawPacket<PacketBytes>, PacketError> {
        RawPacket::<PacketBytes>::parse::<false>(raw)
    }

    /// [`RawPacket::unpack_outbound`] for shared bytes, kept as they are.
    pub fn unpack_outbound_shared(raw: PacketBytes) -> Result<RawPacket<PacketBytes>, PacketError> {
        let mut packet = RawPacket::<PacketBytes>::parse::<false>(raw)?;
        if !packet.is_unhashed_link_resource() {
            packet.hash_in_place();
        }
        Ok(packet)
    }

    /// Unpack wire bytes whose packet hash the caller already computed from
    /// these exact bytes (for example a delivered packet's hash), instead of
    /// hashing them again. The hash is trusted, not checked.
    pub fn unpack_prehashed(raw: &[u8], packet_hash: [u8; 32]) -> Result<Self, PacketError> {
        RawPacket::view_prehashed(raw, packet_hash).map(|packet| packet.map_raw(<[u8]>::to_vec))
    }

    fn hashable_tail(header_type: u8, raw: &[u8]) -> &[u8] {
        if header_type == constants::HEADER_2 {
            // Skip transport_id: raw[2..18] is transport_id (16 bytes)
            &raw[(constants::TRUNCATED_HASHLENGTH / 8 + 2)..]
        } else {
            &raw[2..]
        }
    }

    fn compute_hashable_part(header_type: u8, raw: &[u8]) -> Vec<u8> {
        let tail = Self::hashable_tail(header_type, raw);
        let mut hashable = Vec::with_capacity(1 + tail.len());
        hashable.push(raw[0] & 0b00001111);
        hashable.extend_from_slice(tail);
        hashable
    }

    /// SHA-256 of the hashable part, fed from the wire bytes without building
    /// the concatenated buffer that [`RawPacket::get_hashable_part`] returns.
    fn compute_packet_hash(header_type: u8, raw: &[u8]) -> [u8; 32] {
        let mut hasher = rns_crypto::sha256::Sha256::new();
        hasher.update(&[raw[0] & 0b00001111]);
        hasher.update(Self::hashable_tail(header_type, raw));
        hasher.digest()
    }
}

impl<B: AsRef<[u8]>> RawPacket<B> {
    /// The payload: the wire bytes after the header.
    #[inline]
    pub fn data(&self) -> &[u8] {
        &self.raw.as_ref()[<RawPacket>::data_offset(self.flags.header_type)..]
    }

    /// Whether this packet is link resource traffic whose hash nothing
    /// reads: a part, a part request, or a resource proof. The packet filter
    /// admits these contexts before deduplication (`Transport.packet_filter`),
    /// no proof or receipt covers them, and their handlers read only the
    /// payload (the receiver locates a part by its map hash). Their hashes
    /// cannot mark any other packet as a duplicate either, since the context
    /// byte is hashed, so they are neither computed nor remembered.
    #[inline]
    pub fn is_unhashed_link_resource(&self) -> bool {
        <RawPacket>::is_unhashed_link_resource_header(&self.flags, self.context)
    }

    /// Whether a link peer never proves this packet: link data in a context
    /// with its own protocol handling (requests and responses, resource
    /// control and parts, keepalives, identification, RTT and close). Only
    /// plain link data and channel messages are proven, so a sent packet in
    /// one of these contexts has no receipt to wait for.
    pub fn is_unproven_link_data(&self) -> bool {
        self.flags.packet_type == constants::PACKET_TYPE_DATA
            && self.flags.destination_type == constants::DESTINATION_LINK
            && matches!(
                self.context,
                constants::CONTEXT_REQUEST
                    | constants::CONTEXT_RESPONSE
                    | constants::CONTEXT_RESOURCE
                    | constants::CONTEXT_RESOURCE_ADV
                    | constants::CONTEXT_RESOURCE_REQ
                    | constants::CONTEXT_RESOURCE_HMU
                    | constants::CONTEXT_RESOURCE_ICL
                    | constants::CONTEXT_RESOURCE_RCL
                    | constants::CONTEXT_KEEPALIVE
                    | constants::CONTEXT_LINKIDENTIFY
                    | constants::CONTEXT_LRRTT
                    | constants::CONTEXT_LINKCLOSE
            )
    }

    /// Get the hashable part of the packet.
    ///
    /// From Python Packet.py:354-361:
    /// - Take raw[0] & 0x0F (mask out upper 4 bits of flags)
    /// - For HEADER_1: append raw[2:]
    /// - For HEADER_2: skip transport_id: append raw[18:]
    pub fn get_hashable_part(&self) -> Vec<u8> {
        <RawPacket>::compute_hashable_part(self.flags.header_type, self.raw.as_ref())
    }

    /// Compute `packet_hash` from the packet's wire bytes.
    pub(crate) fn hash_in_place(&mut self) {
        self.packet_hash =
            <RawPacket>::compute_packet_hash(self.flags.header_type, self.raw.as_ref());
    }

    /// Full SHA-256 hash of the hashable part.
    pub fn get_hash(&self) -> [u8; 32] {
        self.packet_hash
    }

    /// Truncated hash (first 16 bytes) of the cached packet hash.
    ///
    /// Packet construction and unpacking compute `packet_hash` once; routing
    /// paths use this prefix instead of hashing the raw packet again.
    pub fn get_truncated_hash(&self) -> [u8; 16] {
        let mut result = [0u8; 16];
        result.copy_from_slice(&self.packet_hash[..16]);
        result
    }
    /// A view of this packet borrowing its bytes.
    pub fn as_view(&self) -> RawPacket<&[u8]> {
        RawPacket {
            flags: self.flags,
            hops: self.hops,
            transport_id: self.transport_id,
            destination_hash: self.destination_hash,
            context: self.context,
            raw: self.raw.as_ref(),
            packet_hash: self.packet_hash,
            rssi: self.rssi,
            snr: self.snr,
        }
    }

    /// This packet with its bytes copied into an owned buffer.
    pub fn to_owned_packet(&self) -> RawPacket {
        self.as_view().map_raw(<[u8]>::to_vec)
    }

    /// This packet with its bytes converted by `f`, fields unchanged.
    pub fn map_raw<C>(self, f: impl FnOnce(B) -> C) -> RawPacket<C> {
        RawPacket {
            flags: self.flags,
            hops: self.hops,
            transport_id: self.transport_id,
            destination_hash: self.destination_hash,
            context: self.context,
            raw: f(self.raw),
            packet_hash: self.packet_hash,
            rssi: self.rssi,
            snr: self.snr,
        }
    }

    /// Parse `input`, keeping it as `raw`.
    fn parse<const HASH: bool>(input: B) -> Result<Self, PacketError> {
        let raw = input.as_ref();
        if raw.len() < constants::HEADER_MINSIZE {
            return Err(PacketError::TooShort);
        }

        let flags = PacketFlags::unpack(raw[0]);
        let hops = raw[1];
        if hops >= constants::PATHFINDER_M {
            return Err(PacketError::InvalidHopCount(hops));
        }

        let dst_len = constants::TRUNCATED_HASHLENGTH / 8; // 16

        if flags.header_type == constants::HEADER_2 {
            // HEADER_2: [flags:1][hops:1][transport_id:16][dest_hash:16][context:1][data:*]
            let min_len = 2 + dst_len * 2 + 1;
            if raw.len() < min_len {
                return Err(PacketError::TooShort);
            }

            let mut transport_id = [0u8; 16];
            transport_id.copy_from_slice(&raw[2..2 + dst_len]);

            let mut destination_hash = [0u8; 16];
            destination_hash.copy_from_slice(&raw[2 + dst_len..2 + 2 * dst_len]);

            let context = raw[2 + 2 * dst_len];
            if raw.len() == 2 + 2 * dst_len + 1 {
                return Err(PacketError::ZeroLengthData);
            }

            let packet_hash = if HASH {
                <RawPacket>::compute_packet_hash(flags.header_type, raw)
            } else {
                [0; 32]
            };

            Ok(RawPacket {
                flags,
                hops,
                transport_id: Some(transport_id),
                destination_hash,
                context,
                raw: input,
                packet_hash,
                rssi: None,
                snr: None,
            })
        } else if flags.header_type == constants::HEADER_1 {
            // HEADER_1: [flags:1][hops:1][dest_hash:16][context:1][data:*]
            let min_len = 2 + dst_len + 1;
            if raw.len() < min_len {
                return Err(PacketError::TooShort);
            }

            let mut destination_hash = [0u8; 16];
            destination_hash.copy_from_slice(&raw[2..2 + dst_len]);

            let context = raw[2 + dst_len];
            if raw.len() == 2 + dst_len + 1 {
                return Err(PacketError::ZeroLengthData);
            }

            let packet_hash = if HASH {
                <RawPacket>::compute_packet_hash(flags.header_type, raw)
            } else {
                [0; 32]
            };

            Ok(RawPacket {
                flags,
                hops,
                transport_id: None,
                destination_hash,
                context,
                raw: input,
                packet_hash,
                rssi: None,
                snr: None,
            })
        } else {
            Err(PacketError::InvalidHeaderType)
        }
    }
}

impl<'a> RawPacket<&'a [u8]> {
    /// [`RawPacket::unpack_prehashed`] as a view of `raw`, without copying it.
    pub fn view_prehashed(raw: &'a [u8], packet_hash: [u8; 32]) -> Result<Self, PacketError> {
        let mut packet = Self::parse::<false>(raw)?;
        packet.packet_hash = packet_hash;
        Ok(packet)
    }
}

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

    #[test]
    fn unpack_prehashed_matches_unpack_but_keeps_the_given_hash() {
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_LINK,
            packet_type: constants::PACKET_TYPE_DATA,
        };
        let packet = RawPacket::pack(
            flags,
            3,
            &[0x42; 16],
            None,
            constants::CONTEXT_RESOURCE,
            &[9; 300],
        )
        .unwrap();
        let full = RawPacket::unpack(&packet.raw).unwrap();
        let known = RawPacket::unpack_prehashed(&packet.raw, full.packet_hash).unwrap();
        assert_eq!(known.raw, full.raw);
        assert_eq!(known.packet_hash, full.packet_hash);
        assert_eq!(known.destination_hash, full.destination_hash);
        assert_eq!((known.hops, known.context), (full.hops, full.context));
        assert_eq!(known.data(), full.data());
        // The hash is trusted, not recomputed.
        let other = RawPacket::unpack_prehashed(&packet.raw, [7; 32]).unwrap();
        assert_eq!(other.packet_hash, [7; 32]);
        // Structural errors are the same.
        assert_eq!(
            format!(
                "{:?}",
                RawPacket::unpack_prehashed(&packet.raw[..10], [0; 32]).unwrap_err()
            ),
            format!("{:?}", RawPacket::unpack(&packet.raw[..10]).unwrap_err())
        );
    }

    #[test]
    fn only_link_resource_transfers_are_sent_unhashed() {
        let cases = [
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_RESOURCE,
                true,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_CHANNEL,
                false,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_RESOURCE_REQ,
                true,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_PROOF,
                constants::CONTEXT_RESOURCE_PRF,
                true,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_RESOURCE_PRF,
                false,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_RESOURCE_HMU,
                false,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_PROOF,
                constants::CONTEXT_RESOURCE,
                false,
            ),
            (
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_PROOF,
                constants::CONTEXT_NONE,
                false,
            ),
            (
                constants::DESTINATION_SINGLE,
                constants::PACKET_TYPE_DATA,
                constants::CONTEXT_RESOURCE,
                false,
            ),
        ];
        for header_type in [constants::HEADER_1, constants::HEADER_2] {
            for (destination_type, packet_type, context, part) in cases {
                let flags = PacketFlags {
                    header_type,
                    context_flag: constants::FLAG_UNSET,
                    transport_type: constants::TRANSPORT_BROADCAST,
                    destination_type,
                    packet_type,
                };
                let transport_id = (header_type == constants::HEADER_2).then_some([0x11; 16]);
                let packet = RawPacket::pack(
                    flags,
                    0,
                    &[0x42; 16],
                    transport_id.as_ref(),
                    context,
                    &[9; 300],
                )
                .unwrap();
                assert_eq!(packet.is_unhashed_link_resource(), part);
                assert_eq!(RawPacket::is_unhashed_link_resource_wire(&packet.raw), part);
                let sent = RawPacket::unpack_outbound(packet.raw.clone()).unwrap();
                assert_eq!(sent.raw, packet.raw);
                let expected = if part { [0; 32] } else { packet.packet_hash };
                assert_eq!(sent.packet_hash, expected);
            }
        }
        // Bytes too short to carry a context byte are not a part.
        let part = RawPacket::pack(
            PacketFlags {
                header_type: constants::HEADER_1,
                context_flag: constants::FLAG_UNSET,
                transport_type: constants::TRANSPORT_BROADCAST,
                destination_type: constants::DESTINATION_LINK,
                packet_type: constants::PACKET_TYPE_DATA,
            },
            0,
            &[0x42; 16],
            None,
            constants::CONTEXT_RESOURCE,
            &[9; 4],
        )
        .unwrap();
        let context_at = RawPacket::data_offset(constants::HEADER_1) - 1;
        assert!(RawPacket::is_unhashed_link_resource_wire(
            &part.raw[..=context_at]
        ));
        assert!(!RawPacket::is_unhashed_link_resource_wire(
            &part.raw[..context_at]
        ));
        assert!(!RawPacket::is_unhashed_link_resource_wire(&[]));
    }

    #[test]
    fn only_plain_link_data_and_channel_messages_await_proofs() {
        let packet = |destination_type, packet_type, context| {
            RawPacket::pack(
                PacketFlags {
                    header_type: constants::HEADER_1,
                    context_flag: constants::FLAG_UNSET,
                    transport_type: constants::TRANSPORT_BROADCAST,
                    destination_type,
                    packet_type,
                },
                0,
                &[0x42; 16],
                None,
                context,
                b"payload",
            )
            .unwrap()
        };
        let link_data = |context| {
            packet(
                constants::DESTINATION_LINK,
                constants::PACKET_TYPE_DATA,
                context,
            )
        };
        for context in [constants::CONTEXT_NONE, constants::CONTEXT_CHANNEL] {
            assert!(
                !link_data(context).is_unproven_link_data(),
                "context {context}"
            );
        }
        for context in [
            constants::CONTEXT_REQUEST,
            constants::CONTEXT_RESPONSE,
            constants::CONTEXT_RESOURCE,
            constants::CONTEXT_RESOURCE_ADV,
            constants::CONTEXT_RESOURCE_REQ,
            constants::CONTEXT_RESOURCE_HMU,
            constants::CONTEXT_RESOURCE_ICL,
            constants::CONTEXT_RESOURCE_RCL,
            constants::CONTEXT_KEEPALIVE,
            constants::CONTEXT_LINKIDENTIFY,
            constants::CONTEXT_LRRTT,
            constants::CONTEXT_LINKCLOSE,
        ] {
            assert!(
                link_data(context).is_unproven_link_data(),
                "context {context}"
            );
        }
        // Only link data qualifies.
        let request = constants::CONTEXT_REQUEST;
        assert!(!packet(
            constants::DESTINATION_SINGLE,
            constants::PACKET_TYPE_DATA,
            request
        )
        .is_unproven_link_data());
        assert!(!packet(
            constants::DESTINATION_LINK,
            constants::PACKET_TYPE_PROOF,
            request
        )
        .is_unproven_link_data());
    }

    #[test]
    fn shared_packing_matches_owned_packing_and_unpacks_alike() {
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_LINK,
            packet_type: constants::PACKET_TYPE_DATA,
        };
        let header2 = PacketFlags {
            header_type: constants::HEADER_2,
            ..flags
        };
        let dest = [0x11; 16];
        let tid = [0x22; 16];
        for (flags, tid) in [(flags, None), (flags, Some(&tid)), (header2, Some(&tid))] {
            for len in [1usize, 2, 15, 16, 400, 465] {
                let data: Vec<u8> = (0..len).map(|i| i as u8).collect();
                for mtu in [len + 18, len + 35, 500] {
                    let owned =
                        RawPacket::pack_raw_with_max_mtu(flags, 3, &dest, tid, 0x07, &data, mtu);
                    let shared = RawPacket::pack_raw_shared_with_max_mtu(
                        flags, 3, &dest, tid, 0x07, &data, mtu,
                    );
                    let filled = RawPacket::pack_raw_shared_filled_with_max_mtu(
                        flags,
                        3,
                        &dest,
                        tid,
                        0x07,
                        data.len(),
                        mtu,
                        |payload| payload.copy_from_slice(&data),
                    );
                    assert_eq!(format!("{shared:?}"), format!("{filled:?}"));
                    match (owned, shared) {
                        (Ok(owned), Ok(shared)) => {
                            assert_eq!(&owned[..], &shared[..]);
                            let a = RawPacket::unpack_outbound(owned).unwrap();
                            let b = RawPacket::unpack_outbound_shared(shared).unwrap();
                            assert_eq!(a.packet_hash, b.packet_hash);
                            assert_eq!(a.data(), b.data());
                        }
                        (Err(PacketError::ExceedsMtu), Err(PacketError::ExceedsMtu)) => {}
                        other => panic!("diverged: {other:?}"),
                    }
                }
            }
        }
        assert!(matches!(
            RawPacket::pack_raw_shared_with_max_mtu(header2, 0, &dest, None, 0, &[1], 500),
            Err(PacketError::MissingTransportId)
        ));
        assert!(matches!(
            RawPacket::pack_raw_shared_filled_with_max_mtu(
                header2,
                0,
                &dest,
                None,
                0,
                1,
                500,
                |_| {}
            ),
            Err(PacketError::MissingTransportId)
        ));
    }

    #[test]
    fn raw_only_packing_preserves_wire_layout_and_limits() {
        for header in [constants::HEADER_1, constants::HEADER_2] {
            for transport in [None, Some([0xBB; 16])] {
                for len in [0, 1, 464, 16348] {
                    let flags = PacketFlags {
                        header_type: header,
                        context_flag: constants::FLAG_UNSET,
                        transport_type: constants::TRANSPORT_BROADCAST,
                        destination_type: constants::DESTINATION_LINK,
                        packet_type: constants::PACKET_TYPE_DATA,
                    };
                    let data = vec![0xCC; len];
                    let mut expected = vec![flags.pack(), 7];
                    if header == constants::HEADER_2 {
                        expected.extend_from_slice(&[0xBB; 16]);
                    }
                    expected.extend_from_slice(&[0xAA; 16]);
                    expected.push(constants::CONTEXT_RESOURCE);
                    expected.extend_from_slice(&data);
                    for limit in [0, expected.len() - 1, expected.len(), expected.len() + 1] {
                        let result = RawPacket::pack_raw_with_max_mtu(
                            flags,
                            7,
                            &[0xAA; 16],
                            transport.as_ref(),
                            constants::CONTEXT_RESOURCE,
                            &data,
                            limit,
                        );
                        if header == constants::HEADER_2 && transport.is_none() {
                            assert!(matches!(result, Err(PacketError::MissingTransportId)));
                        } else if limit < expected.len() {
                            assert!(matches!(result, Err(PacketError::ExceedsMtu)));
                        } else {
                            assert_eq!(result.unwrap(), expected);
                            let (raw, hash) = RawPacket::pack_raw_with_hash_with_max_mtu(
                                flags,
                                7,
                                &[0xAA; 16],
                                transport.as_ref(),
                                constants::CONTEXT_RESOURCE,
                                &data,
                                limit,
                            )
                            .unwrap();
                            assert_eq!(raw, expected);
                            if len == 0 {
                                assert!(matches!(
                                    RawPacket::unpack(&raw),
                                    Err(PacketError::ZeroLengthData)
                                ));
                            } else {
                                assert_eq!(RawPacket::unpack(&raw).unwrap().packet_hash, hash);
                            }
                        }
                    }
                }
            }
        }
    }

    #[test]
    fn test_flags_pack_header1_data_single_broadcast() {
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };
        assert_eq!(flags.pack(), 0x00);
    }

    #[test]
    fn test_flags_pack_header2_announce_single_transport() {
        let flags = PacketFlags {
            header_type: constants::HEADER_2,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_TRANSPORT,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_ANNOUNCE,
        };
        // 0b01010001 = 0x51
        assert_eq!(flags.pack(), 0x51);
    }

    #[test]
    fn test_flags_roundtrip() {
        for byte in 0..=0x7Fu8 {
            let flags = PacketFlags::unpack(byte);
            assert_eq!(flags.pack(), byte);
        }
    }

    #[test]
    fn test_pack_header1() {
        let dest_hash = [0xAA; 16];
        let data = b"hello";
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };

        let pkt =
            RawPacket::pack(flags, 0, &dest_hash, None, constants::CONTEXT_NONE, data).unwrap();

        // Verify layout: [flags:1][hops:1][dest:16][context:1][data:5] = 24 bytes
        assert_eq!(pkt.raw.len(), 24);
        assert_eq!(pkt.raw[0], 0x00); // flags
        assert_eq!(pkt.raw[1], 0x00); // hops
        assert_eq!(&pkt.raw[2..18], &dest_hash); // dest hash
        assert_eq!(pkt.raw[18], 0x00); // context
        assert_eq!(&pkt.raw[19..], b"hello"); // data
    }

    #[test]
    fn test_pack_header2() {
        let dest_hash = [0xAA; 16];
        let transport_id = [0xBB; 16];
        let data = b"world";
        let flags = PacketFlags {
            header_type: constants::HEADER_2,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_TRANSPORT,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_ANNOUNCE,
        };

        let pkt = RawPacket::pack(
            flags,
            3,
            &dest_hash,
            Some(&transport_id),
            constants::CONTEXT_NONE,
            data,
        )
        .unwrap();

        // Layout: [flags:1][hops:1][transport:16][dest:16][context:1][data:5] = 40 bytes
        assert_eq!(pkt.raw.len(), 40);
        assert_eq!(pkt.raw[0], flags.pack());
        assert_eq!(pkt.raw[1], 3);
        assert_eq!(&pkt.raw[2..18], &transport_id);
        assert_eq!(&pkt.raw[18..34], &dest_hash);
        assert_eq!(pkt.raw[34], 0x00);
        assert_eq!(&pkt.raw[35..], b"world");
    }

    #[test]
    fn test_unpack_roundtrip_header1() {
        let dest_hash = [0x11; 16];
        let data = b"test data";
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };

        let pkt = RawPacket::pack(
            flags,
            5,
            &dest_hash,
            None,
            constants::CONTEXT_RESOURCE,
            data,
        )
        .unwrap();
        let unpacked = RawPacket::unpack(&pkt.raw).unwrap();

        assert_eq!(unpacked.flags, flags);
        assert_eq!(unpacked.hops, 5);
        assert!(unpacked.transport_id.is_none());
        assert_eq!(unpacked.destination_hash, dest_hash);
        assert_eq!(unpacked.context, constants::CONTEXT_RESOURCE);
        assert_eq!(unpacked.data(), data);
        assert_eq!(unpacked.packet_hash, pkt.packet_hash);
    }

    #[test]
    fn test_unpack_roundtrip_header2() {
        let dest_hash = [0x22; 16];
        let transport_id = [0x33; 16];
        let data = b"transported";
        let flags = PacketFlags {
            header_type: constants::HEADER_2,
            context_flag: constants::FLAG_SET,
            transport_type: constants::TRANSPORT_TRANSPORT,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_ANNOUNCE,
        };

        let pkt = RawPacket::pack(
            flags,
            2,
            &dest_hash,
            Some(&transport_id),
            constants::CONTEXT_NONE,
            data,
        )
        .unwrap();
        let unpacked = RawPacket::unpack(&pkt.raw).unwrap();

        assert_eq!(unpacked.flags, flags);
        assert_eq!(unpacked.hops, 2);
        assert_eq!(unpacked.transport_id.unwrap(), transport_id);
        assert_eq!(unpacked.destination_hash, dest_hash);
        assert_eq!(unpacked.context, constants::CONTEXT_NONE);
        assert_eq!(unpacked.data(), data);
        assert_eq!(unpacked.packet_hash, pkt.packet_hash);
    }

    #[test]
    fn truncated_hash_is_derived_from_cached_packet_hash() {
        let mut packet = RawPacket::pack(
            PacketFlags {
                header_type: constants::HEADER_1,
                context_flag: constants::FLAG_UNSET,
                transport_type: constants::TRANSPORT_BROADCAST,
                destination_type: constants::DESTINATION_SINGLE,
                packet_type: constants::PACKET_TYPE_DATA,
            },
            1,
            &[0x42; 16],
            None,
            constants::CONTEXT_NONE,
            b"cached hash",
        )
        .unwrap();
        let expected: [u8; 16] = packet.packet_hash[..16].try_into().unwrap();

        // Changing the retained wire buffer demonstrates that this accessor is
        // a prefix operation, not a second hash computation.
        let last = packet.raw.len() - 1;
        packet.raw[last] ^= 0xff;

        assert_eq!(packet.get_truncated_hash(), expected);
    }

    #[test]
    fn test_unpack_too_short() {
        assert!(RawPacket::unpack(&[0x00; 5]).is_err());
    }

    #[test]
    fn unpack_rejects_zero_length_data_for_both_header_types() {
        for header_type in [constants::HEADER_1, constants::HEADER_2] {
            let packet = RawPacket::pack(
                PacketFlags {
                    header_type,
                    context_flag: constants::FLAG_UNSET,
                    transport_type: constants::TRANSPORT_BROADCAST,
                    destination_type: constants::DESTINATION_SINGLE,
                    packet_type: constants::PACKET_TYPE_DATA,
                },
                0,
                &[0x42; 16],
                (header_type == constants::HEADER_2).then_some(&[0x24; 16]),
                constants::CONTEXT_NONE,
                b"",
            )
            .unwrap();
            assert!(matches!(
                RawPacket::unpack(&packet.raw),
                Err(PacketError::ZeroLengthData)
            ));
        }
    }

    #[test]
    fn unpack_enforces_pathfinder_hop_boundary_but_pack_remains_permissive() {
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };
        let accepted = RawPacket::pack(flags, 127, &[1; 16], None, 0, b"x").unwrap();
        assert_eq!(RawPacket::unpack(&accepted.raw).unwrap().hops, 127);

        for hops in [128, 255] {
            let packed = RawPacket::pack(flags, hops, &[1; 16], None, 0, b"x").unwrap();
            assert!(matches!(
                RawPacket::unpack(&packed.raw),
                Err(PacketError::InvalidHopCount(value)) if value == hops
            ));
        }
    }

    #[test]
    fn test_pack_exceeds_mtu() {
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };
        let data = [0u8; 500]; // way too much data
        let result = RawPacket::pack(flags, 0, &[0; 16], None, 0, &data);
        assert!(result.is_err());
    }

    #[test]
    fn test_header2_missing_transport_id() {
        let flags = PacketFlags {
            header_type: constants::HEADER_2,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_TRANSPORT,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_ANNOUNCE,
        };
        let result = RawPacket::pack(flags, 0, &[0; 16], None, 0, b"data");
        assert!(result.is_err());
    }

    #[test]
    fn test_hashable_part_header1_masks_upper_flags() {
        let dest_hash = [0xCC; 16];
        let flags = PacketFlags {
            header_type: constants::HEADER_1,
            context_flag: constants::FLAG_SET,
            transport_type: constants::TRANSPORT_BROADCAST,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_DATA,
        };

        let pkt =
            RawPacket::pack(flags, 0, &dest_hash, None, constants::CONTEXT_NONE, b"test").unwrap();
        let hashable = pkt.get_hashable_part();

        // First byte should have upper 4 bits masked out
        assert_eq!(hashable[0], pkt.raw[0] & 0x0F);
        // Rest should be raw[2:]
        assert_eq!(&hashable[1..], &pkt.raw[2..]);
    }

    #[test]
    fn test_hashable_part_header2_strips_transport_id() {
        let dest_hash = [0xDD; 16];
        let transport_id = [0xEE; 16];
        let flags = PacketFlags {
            header_type: constants::HEADER_2,
            context_flag: constants::FLAG_UNSET,
            transport_type: constants::TRANSPORT_TRANSPORT,
            destination_type: constants::DESTINATION_SINGLE,
            packet_type: constants::PACKET_TYPE_ANNOUNCE,
        };

        let pkt = RawPacket::pack(
            flags,
            0,
            &dest_hash,
            Some(&transport_id),
            constants::CONTEXT_NONE,
            b"data",
        )
        .unwrap();
        let hashable = pkt.get_hashable_part();

        // First byte: flags masked
        assert_eq!(hashable[0], pkt.raw[0] & 0x0F);
        // Should skip transport_id: raw[18:] = dest_hash + context + data
        assert_eq!(&hashable[1..], &pkt.raw[18..]);
    }
}