//! Path attributes: what an UPDATE says about the routes it announces
//! (RFC 4271 §5 and those that added to it), each with how it is read
//! from the wire and written to it.
//!
//! Everything here is re-exported from [`crate::message`], which is where
//! the rest of the tree names it from.

use std::net::{Ipv4Addr, Ipv6Addr};

use bitfield::bitfield;
use bytes::{BufMut, BytesMut};
use nom::Err::Failure;
use nom::number::complete::{be_u8, be_u16, be_u32};
use nom::{IResult, Parser};
use serde::Serialize;
use serde_repr::Serialize_repr;

use bgp_primitives::constants::{Afi, AfiSafi, IpVersion, Safi};
use bgp_primitives::ip_prefix::{IpPrefix, Nlri};
use bgp_primitives::parser::{BgpParserError, EncodeError, ParserContext};

use crate::message::{AS_TRANS, write_nlri};

bitfield! {
    #[derive(Clone, Debug, Serialize, PartialEq, Eq)]
    pub struct PathAttributeFlags(u8);
    impl new;
    u8;
    pub optional, set_optional: 7;
    pub transitive, set_transitive: 6;
    pub partial, set_partial: 5;
    pub extended_length, set_extended_length: 4;
}

/// Path attribute type codes.
pub mod attr_type {
    pub const ORIGIN: u8 = 1;
    pub const AS_PATH: u8 = 2;
    pub const NEXT_HOP: u8 = 3;
    pub const MULTI_EXIT_DISC: u8 = 4;
    pub const LOCAL_PREF: u8 = 5;
    pub const ATOMIC_AGGREGATE: u8 = 6;
    pub const AGGREGATOR: u8 = 7;
    pub const COMMUNITIES: u8 = 8;
    pub const ORIGINATOR_ID: u8 = 9;
    pub const CLUSTER_LIST: u8 = 10;
    pub const MP_REACH_NLRI: u8 = 14;
    pub const MP_UNREACH_NLRI: u8 = 15;
    pub const EXTENDED_COMMUNITIES: u8 = 16;
    pub const AS4_PATH: u8 = 17;
    pub const AS4_AGGREGATOR: u8 = 18;
    pub const LARGE_COMMUNITIES: u8 = 32;
    pub const ONLY_TO_CUSTOMER: u8 = 35;
}

#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub enum PathAttribute {
    Origin(OriginPathAttribute),
    AsPath(AsPathAttribute),
    NextHop(NextHopPathAttribute),
    MultiExitDisc(MultiExitDiscPathAttribute),
    LocalPref(LocalPrefPathAttribute),
    AtomicAggregate(AtomicAggregatePathAttribute),
    Aggregator(AggregatorPathAttribute),
    Communities(CommunitiesPathAttribute),
    MpReachNlri(MpReachNlriPathAttribute),
    MpUnreachNlri(MpUnreachNlriPathAttribute),
    ExtendedCommunities(ExtendedCommunitiesPathAttribute),
    /// The AS path with four-octet AS numbers, for a route that crosses
    /// speakers that only handle two-octet ones (RFC 6793 §3).
    As4Path(AsPathAttribute),
    LargeCommunities(LargeCommunitiesPathAttribute),
    /// Only to Customer (RFC 9234 §5): the route has been handed to a
    /// customer, a peer or a route server client by the AS with this
    /// number, and from there on may only go down to customers.
    OnlyToCustomer(u32),
    /// An attribute this implementation does not interpret, kept as it
    /// came so that it can be passed on.
    UnknownPathAttribute {
        flags: PathAttributeFlags,
        type_code: u8,
        payload: Vec<u8>,
    },
}

/// The Optional and Transitive flags that the attribute with this type code
/// is defined to have, for the attributes whose definition we know. A
/// received attribute with other values is in error (RFC 7606 §3.c).
pub(crate) fn defined_flags(type_code: u8) -> Option<(bool, bool)> {
    use attr_type::*;
    const WELL_KNOWN: (bool, bool) = (false, true);
    const OPTIONAL_TRANSITIVE: (bool, bool) = (true, true);
    const OPTIONAL_NON_TRANSITIVE: (bool, bool) = (true, false);
    Some(match type_code {
        ORIGIN | AS_PATH | NEXT_HOP | LOCAL_PREF | ATOMIC_AGGREGATE => WELL_KNOWN,
        MULTI_EXIT_DISC | ORIGINATOR_ID | CLUSTER_LIST | MP_REACH_NLRI | MP_UNREACH_NLRI => {
            OPTIONAL_NON_TRANSITIVE
        }
        AGGREGATOR | COMMUNITIES | EXTENDED_COMMUNITIES | AS4_PATH | AS4_AGGREGATOR
        | LARGE_COMMUNITIES | ONLY_TO_CUSTOMER => OPTIONAL_TRANSITIVE,
        _ => return None,
    })
}

/// A parsed value, provided the parser used all of it.
fn whole<T>((rest, parsed): (&[u8], T)) -> Result<T, nom::Err<BgpParserError<&[u8]>>> {
    if rest.is_empty() {
        Ok(parsed)
    } else {
        Err(Failure(BgpParserError::Malformed(format!(
            "{} octets left over",
            rest.len()
        ))))
    }
}

/// One attribute as it lies on the wire, split out but not interpreted.
#[derive(Debug, Clone)]
pub struct RawAttribute<'a> {
    pub flags: PathAttributeFlags,
    pub type_code: u8,
    pub value: &'a [u8],
    /// The whole attribute, header and value.
    pub wire: &'a [u8],
}

/// Why the bytes at the front of a buffer are not an attribute.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum NotAnAttribute {
    /// Too few octets for flags, type and length.
    NoHeader,
    /// The length says more than there is.
    Truncated {
        type_code: u8,
        length: usize,
        available: usize,
    },
}

impl std::fmt::Display for NotAnAttribute {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::NoHeader => f.write_str("too few octets for an attribute"),
            Self::Truncated {
                type_code,
                length,
                available,
            } => write!(
                f,
                "attribute {type_code} has length {length} but only {available} octets remain"
            ),
        }
    }
}

/// Splits the attribute at the front of `buf` from what follows it.
pub fn split_attribute(buf: &[u8]) -> Result<(RawAttribute<'_>, &[u8]), NotAnAttribute> {
    let [flags, type_code, rest @ ..] = buf else {
        return Err(NotAnAttribute::NoHeader);
    };
    let flags = PathAttributeFlags(*flags);
    let (length, rest) = match (flags.extended_length(), rest) {
        (true, [hi, lo, rest @ ..]) => (usize::from(u16::from_be_bytes([*hi, *lo])), rest),
        (false, [length, rest @ ..]) => (usize::from(*length), rest),
        _ => return Err(NotAnAttribute::NoHeader),
    };
    let Some((value, rest)) = rest.split_at_checked(length) else {
        return Err(NotAnAttribute::Truncated {
            type_code: *type_code,
            length,
            available: rest.len(),
        });
    };
    let raw = RawAttribute {
        flags,
        type_code: *type_code,
        value,
        wire: &buf[..buf.len() - rest.len()],
    };
    Ok((raw, rest))
}

impl PathAttribute {
    pub fn type_code(&self) -> u8 {
        use attr_type::*;
        match self {
            Self::Origin(_) => ORIGIN,
            Self::AsPath(_) => AS_PATH,
            Self::NextHop(_) => NEXT_HOP,
            Self::MultiExitDisc(_) => MULTI_EXIT_DISC,
            Self::LocalPref(_) => LOCAL_PREF,
            Self::AtomicAggregate(_) => ATOMIC_AGGREGATE,
            Self::Aggregator(_) => AGGREGATOR,
            Self::Communities(_) => COMMUNITIES,
            Self::MpReachNlri(_) => MP_REACH_NLRI,
            Self::MpUnreachNlri(_) => MP_UNREACH_NLRI,
            Self::ExtendedCommunities(_) => EXTENDED_COMMUNITIES,
            Self::As4Path(_) => AS4_PATH,
            Self::LargeCommunities(_) => LARGE_COMMUNITIES,
            Self::OnlyToCustomer(_) => ONLY_TO_CUSTOMER,
            Self::UnknownPathAttribute { type_code, .. } => *type_code,
        }
    }

    /// The flags to send the attribute with, but for Extended Length, which
    /// depends on how long the value turns out to be.
    fn flags(&self) -> PathAttributeFlags {
        match self {
            // As they came: the Partial bit has to survive (RFC 4271 §5).
            Self::UnknownPathAttribute { flags, .. } => flags.clone(),
            known => {
                let (optional, transitive) = defined_flags(known.type_code())
                    .expect("every attribute with a variant has defined flags");
                PathAttributeFlags::new(optional, transitive, false, false)
            }
        }
    }

    /// Parses one attribute, header and value, from the front of `buf`.
    /// The flags are not checked against the attribute's definition;
    /// [`UpdateMessage::decode`] does that for attributes off a session.
    pub fn from_wire<'a>(
        ctx: &ParserContext,
        buf: &'a [u8],
    ) -> IResult<&'a [u8], Self, BgpParserError<&'a [u8]>> {
        let (raw, rest) =
            split_attribute(buf).map_err(|e| Failure(BgpParserError::Malformed(e.to_string())))?;
        let attribute = Self::parse_value(ctx, raw.flags, raw.type_code, raw.value)?;
        Ok((rest, attribute))
    }

    /// Makes the attribute with this type code from its value. `value` is
    /// exactly the value, and all of it has to be used. A type code without
    /// a variant of its own is kept as bytes.
    pub(crate) fn parse_value<'a>(
        ctx: &ParserContext,
        flags: PathAttributeFlags,
        type_code: u8,
        value: &'a [u8],
    ) -> Result<Self, nom::Err<BgpParserError<&'a [u8]>>> {
        use attr_type::*;
        Ok(match type_code {
            ORIGIN => Self::Origin(whole(OriginPathAttribute::from_wire(value)?)?),
            AS_PATH => Self::AsPath(whole(AsPathAttribute::from_wire(ctx, value)?)?),
            NEXT_HOP => Self::NextHop(whole(NextHopPathAttribute::from_wire(value)?)?),
            MULTI_EXIT_DISC => {
                Self::MultiExitDisc(whole(MultiExitDiscPathAttribute::from_wire(value)?)?)
            }
            LOCAL_PREF => Self::LocalPref(whole(LocalPrefPathAttribute::from_wire(value)?)?),
            ATOMIC_AGGREGATE => {
                Self::AtomicAggregate(whole((value, AtomicAggregatePathAttribute {}))?)
            }
            AGGREGATOR => Self::Aggregator(whole(AggregatorPathAttribute::from_wire(ctx, value)?)?),
            COMMUNITIES => Self::Communities(whole(CommunitiesPathAttribute::from_wire(value)?)?),
            MP_REACH_NLRI => {
                Self::MpReachNlri(whole(MpReachNlriPathAttribute::from_wire(ctx, value)?)?)
            }
            MP_UNREACH_NLRI => {
                Self::MpUnreachNlri(whole(MpUnreachNlriPathAttribute::from_wire(ctx, value)?)?)
            }
            EXTENDED_COMMUNITIES => Self::ExtendedCommunities(whole(
                ExtendedCommunitiesPathAttribute::from_wire(value)?,
            )?),
            // Always four octets wide, whatever the session's AS_PATH is.
            AS4_PATH => Self::As4Path(whole(AsPathAttribute::from_wire_with_width(true, value)?)?),
            LARGE_COMMUNITIES => {
                Self::LargeCommunities(whole(LargeCommunitiesPathAttribute::from_wire(value)?)?)
            }
            ONLY_TO_CUSTOMER => {
                let asn = <[u8; 4]>::try_from(value).map_err(|_| {
                    malformed(format!(
                        "Only to Customer is {} octets long, not four",
                        value.len()
                    ))
                })?;
                Self::OnlyToCustomer(u32::from_be_bytes(asn))
            }
            _ => Self::UnknownPathAttribute {
                flags,
                type_code,
                payload: value.to_vec(),
            },
        })
    }

    /// Writes the attribute, header and value, to the end of `out`.
    pub fn to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) -> Result<(), EncodeError> {
        // A header with a one-octet length first; whether the value needs
        // the two-octet one is known once it is written.
        let start = out.len();
        let mut flags = self.flags();
        out.put_u8(0);
        out.put_u8(self.type_code());
        out.put_u8(0);
        let value_start = out.len();
        self.value_to_wire(ctx, out)?;
        let length = out.len() - value_start;

        if let Ok(short) = u8::try_from(length)
            && !flags.extended_length()
        {
            out[start + 2] = short;
        } else {
            let long = u16::try_from(length).map_err(|_| EncodeError::TooLong {
                what: "path attribute",
                len: length,
                max: u16::MAX.into(),
            })?;
            flags.set_extended_length(true);
            // Move the value up by one octet to make room.
            out.put_u8(0);
            out.copy_within(value_start..value_start + length, value_start + 1);
            out[start + 2..start + 4].copy_from_slice(&long.to_be_bytes());
        }
        out[start] = flags.0;
        Ok(())
    }

    fn value_to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) -> Result<(), EncodeError> {
        match self {
            Self::Origin(a) => a.to_wire(out),
            Self::AsPath(a) => a.to_wire(ctx, out)?,
            Self::NextHop(a) => a.to_wire(out),
            Self::MultiExitDisc(a) => a.to_wire(out),
            Self::LocalPref(a) => a.to_wire(out),
            Self::AtomicAggregate(_) => {}
            Self::Aggregator(a) => a.to_wire(ctx, out),
            Self::Communities(a) => a.to_wire(out),
            Self::MpReachNlri(a) => a.to_wire(ctx, out),
            Self::MpUnreachNlri(a) => a.to_wire(ctx, out),
            Self::ExtendedCommunities(a) => a.to_wire(out),
            Self::As4Path(a) => a.to_wire_with_width(true, out)?,
            Self::LargeCommunities(a) => a.to_wire(out),
            Self::OnlyToCustomer(asn) => out.put_u32(*asn),
            Self::UnknownPathAttribute { payload, .. } => out.put_slice(payload),
        }
        Ok(())
    }
}

/// An Only to Customer attribute with this AS number, as it is on the
/// wire: for adding to attributes that are kept as bytes.
pub fn only_to_customer_wire(asn: u32) -> [u8; 7] {
    const OPTIONAL_TRANSITIVE: u8 = 0xc0;
    let [a, b, c, d] = asn.to_be_bytes();
    [
        OPTIONAL_TRANSITIVE,
        attr_type::ONLY_TO_CUSTOMER,
        4,
        a,
        b,
        c,
        d,
    ]
}

type Parsed<'a, T> = IResult<&'a [u8], T, BgpParserError<&'a [u8]>>;

fn malformed<'a>(detail: String) -> nom::Err<BgpParserError<&'a [u8]>> {
    Failure(BgpParserError::Malformed(detail))
}

/// ORIGIN: how the route came into BGP (RFC 4271 §5.1.1).
#[derive(Debug, PartialEq, Eq, Copy, Clone, Serialize_repr)]
#[repr(u8)]
pub enum OriginPathAttribute {
    IGP = 0,
    EGP = 1,
    INCOMPLETE = 2,
}

impl OriginPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, byte) = be_u8(buf)?;
        let origin = match byte {
            0 => Self::IGP,
            1 => Self::EGP,
            2 => Self::INCOMPLETE,
            other => return Err(malformed(format!("unknown ORIGIN {other}"))),
        };
        Ok((buf, origin))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        out.put_u8(*self as u8);
    }
}

/// AS_PATH: the ASes a route has passed through, as a list of segments
/// (RFC 4271 §4.3). Each segment is a type (1 for AS_SET, 2 for
/// AS_SEQUENCE), a count in one octet, and that many AS numbers.
#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct AsPathAttribute {
    pub segments: Vec<AsPathSegment>,
}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct AsPathSegment {
    /// True for an AS_SEQUENCE, false for an AS_SET.
    pub ordered: bool,
    pub path: Vec<u32>,
}

impl AsPathAttribute {
    pub fn from_asns(asns: Vec<u32>) -> PathAttribute {
        let segment = AsPathSegment {
            ordered: true,
            path: asns,
        };
        PathAttribute::AsPath(AsPathAttribute {
            segments: vec![segment],
        })
    }

    /// How long the path is, as RFC 4271 §9.1.2.2 counts: one for each AS
    /// of a sequence, one for a whole set.
    pub fn hops(&self) -> usize {
        self.segments
            .iter()
            .map(|segment| {
                if segment.ordered {
                    segment.path.len()
                } else {
                    1
                }
            })
            .sum()
    }

    /// The path as it really is, given what a two-octet session carried:
    /// this, with AS_TRANS wherever a number did not fit, and `wide`, the
    /// AS4_PATH that has the numbers themselves for the path's tail
    /// (RFC 6793 §4.2.3). What `wide` does not cover is kept from the
    /// front of this.
    ///
    /// `None` if `wide` is longer than the path, which is not to be used:
    /// some speaker on the way that knew nothing of AS4_PATH changed the
    /// one and not the other.
    pub fn with_as4_path(&self, wide: &AsPathAttribute) -> Option<AsPathAttribute> {
        let mut keep = self.hops().checked_sub(wide.hops())?;
        let mut segments = Vec::with_capacity(self.segments.len() + wide.segments.len());
        for segment in &self.segments {
            if keep == 0 {
                break;
            }
            let (hops, taken) = if segment.ordered {
                let taken = segment.path.len().min(keep);
                (taken, &segment.path[..taken])
            } else {
                (1, &segment.path[..])
            };
            keep -= hops;
            segments.push(AsPathSegment {
                ordered: segment.ordered,
                path: taken.to_vec(),
            });
        }
        segments.extend(wide.segments.iter().cloned());
        Some(AsPathAttribute { segments })
    }

    /// Parses an AS_PATH with AS numbers as wide as the session negotiated
    /// (RFC 6793).
    pub fn from_wire<'a>(ctx: &ParserContext, buf: &'a [u8]) -> Parsed<'a, Self> {
        Self::from_wire_with_width(ctx.four_octet_asn, buf)
    }

    fn from_wire_with_width(four_octet: bool, buf: &[u8]) -> Parsed<'_, Self> {
        let parse_segment = |buf| -> Parsed<'_, AsPathSegment> {
            let (buf, typ) = be_u8(buf)?;
            let (buf, len) = be_u8(buf)?;
            // AS_SET and AS_SEQUENCE. The confederation segment types of
            // RFC 5065 are an error from a peer outside a confederation,
            // and this speaker is never inside one.
            if typ != 1 && typ != 2 {
                return Err(malformed(format!(
                    "unrecognised AS path segment type {typ}"
                )));
            }
            if len == 0 {
                return Err(malformed("AS path segment with no AS numbers".into()));
            }
            let count = usize::from(len);
            let (buf, path): (_, Vec<u32>) = if four_octet {
                nom::multi::many_m_n(count, count, be_u32).parse(buf)?
            } else {
                nom::multi::many_m_n(count, count, be_u16)
                    .parse(buf)
                    .map(|(buf, asns)| (buf, asns.into_iter().map(u32::from).collect()))?
            };
            Ok((
                buf,
                AsPathSegment {
                    ordered: typ == 2,
                    path,
                },
            ))
        };

        // Not many0: that stops quietly at a segment it cannot read, and a
        // truncated segment is an error.
        let mut segments = Vec::new();
        let mut buf = buf;
        while !buf.is_empty() {
            let (rest, segment) = parse_segment(buf)?;
            segments.push(segment);
            buf = rest;
        }
        Ok((buf, Self { segments }))
    }

    /// Writes the path with AS numbers as wide as the session negotiated.
    /// On a two-octet session an AS number that does not fit is written as
    /// AS_TRANS (RFC 6793 §4.2.2); [`AsPathAttribute::as4_path`] is what
    /// carries the real numbers alongside.
    pub fn to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) -> Result<(), EncodeError> {
        self.to_wire_with_width(ctx.four_octet_asn, out)
    }

    fn to_wire_with_width(&self, four_octet: bool, out: &mut BytesMut) -> Result<(), EncodeError> {
        for segment in &self.segments {
            out.put_u8(if segment.ordered { 2 } else { 1 });
            let count = u8::try_from(segment.path.len()).map_err(|_| EncodeError::TooLong {
                what: "AS path segment",
                len: segment.path.len(),
                max: u8::MAX.into(),
            })?;
            out.put_u8(count);
            for asn in &segment.path {
                if four_octet {
                    out.put_u32(*asn);
                } else {
                    out.put_u16(u16::try_from(*asn).unwrap_or(AS_TRANS));
                }
            }
        }
        Ok(())
    }

    /// The AS4_PATH to send with this path on a two-octet session, if one is
    /// needed: it is when some AS number does not fit in two octets and so
    /// goes out as AS_TRANS in the AS_PATH itself (RFC 6793 §4.2.2).
    pub fn as4_path(&self) -> Option<PathAttribute> {
        self.segments
            .iter()
            .flat_map(|segment| &segment.path)
            .any(|asn| u16::try_from(*asn).is_err())
            .then(|| PathAttribute::As4Path(self.clone()))
    }
}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct NextHopPathAttribute(pub Ipv4Addr);

impl NextHopPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, address) = be_u32(buf)?;
        Ok((buf, Self(Ipv4Addr::from(address))))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        out.put_u32(self.0.into());
    }
}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct MultiExitDiscPathAttribute(pub u32);

impl MultiExitDiscPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, value) = be_u32(buf)?;
        Ok((buf, Self(value)))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        out.put_u32(self.0);
    }
}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct LocalPrefPathAttribute(pub u32);

impl LocalPrefPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, value) = be_u32(buf)?;
        Ok((buf, Self(value)))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        out.put_u32(self.0);
    }
}

/// ATOMIC_AGGREGATE: a marker with no value (RFC 4271 §5.1.6).
#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct AtomicAggregatePathAttribute {}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct AggregatorPathAttribute {
    pub asn: u32,
    pub ip: Ipv4Addr,
}

impl AggregatorPathAttribute {
    /// The AS number is as wide as the session's AS numbers (RFC 6793), so
    /// the attribute is six or eight octets.
    pub fn from_wire<'a>(ctx: &ParserContext, buf: &'a [u8]) -> Parsed<'a, Self> {
        let (buf, asn) = if ctx.four_octet_asn {
            be_u32(buf)?
        } else {
            be_u16(buf).map(|(buf, asn)| (buf, u32::from(asn)))?
        };
        let (buf, ip) = be_u32(buf)?;
        Ok((
            buf,
            Self {
                asn,
                ip: Ipv4Addr::from(ip),
            },
        ))
    }

    /// On a two-octet session an AS number that does not fit is written as
    /// AS_TRANS (RFC 6793 §4.2.2).
    pub fn to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) {
        if ctx.four_octet_asn {
            out.put_u32(self.asn);
        } else {
            out.put_u16(u16::try_from(self.asn).unwrap_or(AS_TRANS));
        }
        out.put_u32(self.ip.into());
    }
}

#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct CommunitiesPathAttribute(pub Vec<(u16, u16)>);

impl CommunitiesPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        // At least one: an empty COMMUNITIES is malformed (RFC 7606 §7.8).
        let (buf, values) = nom::multi::many1(|i| (be_u16, be_u16).parse(i)).parse(buf)?;
        Ok((buf, CommunitiesPathAttribute(values)))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        for value in &self.0 {
            out.put_u16(value.0);
            out.put_u16(value.1);
        }
    }
}

/// EXTENDED COMMUNITIES (RFC 4360).
#[derive(Debug, PartialEq, Eq, Clone, Serialize)]
pub struct ExtendedCommunitiesPathAttribute {
    pub extended_communities: Vec<ExtendedCommunity>,
}

impl ExtendedCommunitiesPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (communities, rest) = buf.as_chunks::<8>();
        if communities.is_empty() || !rest.is_empty() {
            return Err(malformed(format!(
                "{} octets are not a list of extended communities",
                buf.len()
            )));
        }
        Ok((
            rest,
            Self {
                extended_communities: communities.iter().copied().map(ExtendedCommunity).collect(),
            },
        ))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        for community in &self.extended_communities {
            out.put_slice(&community.0);
        }
    }
}

/// One extended community: eight octets, of which the first is the type
/// and, for most types, the second a sub-type (RFC 4360 §2). How the rest
/// divides into an administrator and a value depends on the type, and is
/// left to whoever acts on a particular community.
#[derive(Debug, PartialEq, Eq, Clone, Copy, Serialize)]
pub struct ExtendedCommunity(pub [u8; 8]);

impl ExtendedCommunity {
    pub fn typ(&self) -> u8 {
        self.0[0]
    }

    pub fn sub_typ(&self) -> u8 {
        self.0[1]
    }
}

#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub struct LargeCommunitiesPathAttribute {
    pub communities: Vec<LargeCommunity>,
}

impl LargeCommunitiesPathAttribute {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, communities) = nom::multi::many1(LargeCommunity::from_wire).parse(buf)?;
        Ok((buf, Self { communities }))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        for community in &self.communities {
            community.to_wire(out);
        }
    }
}

#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub struct LargeCommunity {
    pub global_admin: u32,
    pub data_1: u32,
    pub data_2: u32,
}

impl LargeCommunity {
    pub fn from_wire(buf: &[u8]) -> Parsed<'_, Self> {
        let (buf, global_admin) = be_u32(buf)?;
        let (buf, data_1) = be_u32(buf)?;
        let (buf, data_2) = be_u32(buf)?;
        Ok((
            buf,
            Self {
                global_admin,
                data_1,
                data_2,
            },
        ))
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        out.put_u32(self.global_admin);
        out.put_u32(self.data_1);
        out.put_u32(self.data_2);
    }
}

/// The next hop of routes in MP_REACH_NLRI.
#[derive(Debug, Clone, Copy, Serialize, PartialEq, Eq, Hash)]
pub enum NlriNextHop {
    Ipv4(Ipv4Addr),
    /// A global IPv6 address.
    Ipv6(Ipv6Addr),
    /// A global IPv6 address and a link-local one (RFC 2545 §3).
    Ipv6WithLl {
        global: Ipv6Addr,
        link_local: Ipv6Addr,
    },
}

impl std::fmt::Display for NlriNextHop {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            NlriNextHop::Ipv4(addr) => write!(f, "{addr}"),
            NlriNextHop::Ipv6(addr) => write!(f, "{addr}"),
            NlriNextHop::Ipv6WithLl { global, link_local } => {
                write!(f, "{global} (link-local {link_local})")
            }
        }
    }
}

impl NlriNextHop {
    /// Reads a next hop that is all of `bytes`. `None` if they are not the
    /// length of one.
    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
        let v6 = |bytes: &[u8]| <[u8; 16]>::try_from(bytes).ok().map(Ipv6Addr::from);
        match bytes.len() {
            4 => <[u8; 4]>::try_from(bytes)
                .ok()
                .map(|a| Self::Ipv4(a.into())),
            16 => v6(bytes).map(Self::Ipv6),
            32 => Some(Self::Ipv6WithLl {
                global: v6(&bytes[..16])?,
                link_local: v6(&bytes[16..])?,
            }),
            _ => None,
        }
    }

    pub fn to_wire(&self, out: &mut BytesMut) {
        match self {
            NlriNextHop::Ipv4(address) => out.put_slice(&address.octets()),
            NlriNextHop::Ipv6(address) => out.put_slice(&address.octets()),
            NlriNextHop::Ipv6WithLl { global, link_local } => {
                out.put_slice(&global.octets());
                out.put_slice(&link_local.octets());
            }
        }
    }

    /// How many octets the next hop is on the wire.
    pub fn wire_len(&self) -> u8 {
        match self {
            NlriNextHop::Ipv4(_) => 4,
            NlriNextHop::Ipv6(_) => 16,
            NlriNextHop::Ipv6WithLl { .. } => 32,
        }
    }
}

/// Parses one prefix as an UPDATE names it: with a path identifier in
/// front if the session uses them for the family (RFC 7911 §3).
fn parse_nlri(version: IpVersion, with_path_id: bool, buf: &[u8]) -> Parsed<'_, Nlri> {
    let (buf, path_id) = if with_path_id { be_u32(buf)? } else { (buf, 0) };
    let (buf, prefix_len) = be_u8(buf)?;
    let byte_len = usize::from(prefix_len.div_ceil(8));
    let (buf, prefix_bytes) = nom::bytes::take(byte_len).parse(buf)?;
    let prefix =
        IpPrefix::new(version, prefix_bytes, prefix_len).map_err(|e| malformed(e.to_string()))?;
    Ok((buf, Nlri { prefix, path_id }))
}

/// Parses a field that is nothing but prefixes, to its end.
pub(crate) fn parse_nlri_list(
    version: IpVersion,
    with_path_id: bool,
    mut buf: &[u8],
) -> Parsed<'_, Vec<Nlri>> {
    // Not many0: that stops quietly at a prefix it cannot read.
    let mut nlri = Vec::new();
    while !buf.is_empty() {
        let (rest, parsed) = parse_nlri(version, with_path_id, buf)?;
        nlri.push(parsed);
        buf = rest;
    }
    Ok((buf, nlri))
}

/// The IP version of the prefixes that make up the NLRI of `family`.
///
/// Any family can be named in MP_REACH_NLRI and MP_UNREACH_NLRI, but only
/// unicast and multicast routes of IPv4 and IPv6 are plain prefixes, and
/// those are the only NLRI this parser reads. A peer does not send routes
/// of a family that was not negotiated, so the others do not come up.
fn ip_version_of_nlri<'a>(
    family: AfiSafi,
) -> Result<IpVersion, nom::Err<BgpParserError<&'a [u8]>>> {
    let unsupported = || malformed(format!("NLRI of {family} is not supported"));
    if family.safi != Safi::UNICAST && family.safi != Safi::MULTICAST {
        return Err(unsupported());
    }
    IpVersion::try_from(family.afi).map_err(|_| unsupported())
}

fn parse_family(buf: &[u8]) -> Parsed<'_, AfiSafi> {
    let (buf, afi) = be_u16(buf)?;
    let (buf, safi) = be_u8(buf)?;
    Ok((buf, AfiSafi::new(Afi(afi), Safi(safi))))
}

/// MP_REACH_NLRI: routes of a family other than the one in the message
/// body, with their next hop (RFC 4760 §3).
#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub struct MpReachNlriPathAttribute {
    pub family: AfiSafi,
    pub next_hop: NlriNextHop,
    pub prefixes: Vec<Nlri>,
}

impl MpReachNlriPathAttribute {
    pub fn from_wire<'a>(ctx: &ParserContext, buf: &'a [u8]) -> Parsed<'a, Self> {
        let (buf, family) = parse_family(buf)?;
        let version = ip_version_of_nlri(family)?;

        let (buf, next_hop_len) = be_u8(buf)?;
        let (buf, next_hop) = nom::bytes::take(usize::from(next_hop_len)).parse(buf)?;
        // For IPv6 routes, a global address, optionally followed by a
        // link-local one (RFC 2545). For IPv4 routes an IPv4 address, or
        // the same as for IPv6: routes over a link that has only IPv6
        // addresses (RFC 8950). Which of the two it is shows in its length.
        let next_hop = NlriNextHop::from_bytes(next_hop)
            .filter(|next_hop| {
                version == IpVersion::Ipv4 || !matches!(next_hop, NlriNextHop::Ipv4(_))
            })
            .ok_or_else(|| {
                malformed(format!(
                    "a next hop of {next_hop_len} octets is not valid for {family}"
                ))
            })?;

        // Reserved octet (formerly the SNPA count), for every family.
        let (buf, _) = be_u8(buf)?;
        let (buf, prefixes) = parse_nlri_list(version, ctx.has_add_path(family), buf)?;
        Ok((
            buf,
            Self {
                family,
                next_hop,
                prefixes,
            },
        ))
    }

    pub fn to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) {
        out.put_u16(self.family.afi.0);
        out.put_u8(self.family.safi.0);
        out.put_u8(self.next_hop.wire_len());
        self.next_hop.to_wire(out);
        out.put_u8(0);
        write_nlri(&self.prefixes, ctx.has_add_path(self.family), out);
    }
}

/// MP_UNREACH_NLRI: withdrawals of routes of a family other than the one
/// in the message body (RFC 4760 §4).
#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub struct MpUnreachNlriPathAttribute {
    pub family: AfiSafi,
    pub prefixes: Vec<Nlri>,
}

impl MpUnreachNlriPathAttribute {
    pub fn from_wire<'a>(ctx: &ParserContext, buf: &'a [u8]) -> Parsed<'a, Self> {
        let (buf, family) = parse_family(buf)?;
        let version = ip_version_of_nlri(family)?;
        let (buf, prefixes) = parse_nlri_list(version, ctx.has_add_path(family), buf)?;
        Ok((buf, Self { family, prefixes }))
    }

    pub fn to_wire(&self, ctx: &ParserContext, out: &mut BytesMut) {
        out.put_u16(self.family.afi.0);
        out.put_u8(self.family.safi.0);
        write_nlri(&self.prefixes, ctx.has_add_path(self.family), out);
    }
}