//! Server fill policy: what the ServerFill descriptor means, and what bytes it
//! puts in an echo reply's payload region.
//!
//! Descriptor *strings* are wire data and `irtt-proto` carries them verbatim;
//! deciding what one means is server policy and lives here. The result is a
//! [`FillMode`], which is deliberately crate-private: it is neither wire format
//! nor public configuration.

use irtt_proto::{echo_packet_len, PacketLayout, Params, ServerFill};

/// The bytes of this server's default fill, repeated: `69 72 74 74`, or `irtt`.
///
/// The value matches the descriptor the clean evidence records the reference
/// server defaulting to, so a client that recognizes it sees what it expects.
/// Its use here is nevertheless `irtt-rs` policy: payload bytes carry no
/// protocol meaning, and nothing about interoperability requires this pattern
/// over any other.
const DEFAULT_FILL_PATTERN: &[u8] = b"irtt";

/// The descriptor naming [`DEFAULT_FILL_PATTERN`], as returned in an open reply
/// whose explicit request this server could not honor.
///
/// Held as a constant beside the bytes rather than derived from them, so the
/// descriptor a client is told and the bytes it is sent cannot drift apart
/// through a formatting or parsing step neither of them needs.
pub(crate) const DEFAULT_FILL_DESCRIPTOR: &str = "pattern:69727474";

/// The mode prefix of a repeating hexadecimal pattern descriptor.
const PATTERN_PREFIX: &str = "pattern:";

/// How this server fills the payload region of one session's echo replies.
///
/// Crate-private on purpose. It is an interpretation of a wire string, not the
/// string itself and not a configuration surface, so nothing outside the crate
/// has any use for it.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum FillMode {
    /// Write no fill of the server's own. The payload region is zeroes — see
    /// [`FillMode::payload`].
    None,
    /// Fill the payload region with bytes from the operating system's random
    /// source.
    Random,
    /// Fill the payload region with these bytes, repeated. Never empty.
    Pattern(Vec<u8>),
}

impl FillMode {
    /// This server's default fill, used by a session whose client expressed no
    /// preference and by one whose explicit descriptor could not be honored.
    pub(crate) fn default_fill() -> Self {
        Self::Pattern(DEFAULT_FILL_PATTERN.to_vec())
    }

    /// The mode a descriptor asks for, or `None` if it names none.
    ///
    /// The three mode names are matched exactly and case-sensitively; only a
    /// pattern's hexadecimal body is case-insensitive. An unrecognized name, a
    /// differently-cased one, and a `pattern:` whose body is empty, odd-length
    /// or not hexadecimal all return `None`, and negotiation replaces them with
    /// the default. Nothing here rejects an open, panics, or half-parses a
    /// descriptor into a partial pattern.
    fn parse(descriptor: &str) -> Option<Self> {
        match descriptor {
            "none" => Some(Self::None),
            "rand" => Some(Self::Random),
            _ => descriptor
                .strip_prefix(PATTERN_PREFIX)
                .and_then(parse_hex_pattern)
                .map(Self::Pattern),
        }
    }

    /// The `len` payload bytes an echo reply carries under this mode.
    ///
    /// **A pattern always starts at its first byte.** The phase resets for every
    /// reply, which is `irtt-rs` policy: the clean evidence records the
    /// reference implementation advancing one phase continuously across replies,
    /// sessions and listeners, and states equally clearly that payload phase has
    /// no protocol significance and that a conforming client must not depend on
    /// it. Resetting is therefore interoperability-equivalent, and it buys
    /// determinism — no global mutable fill state, no coupling between sessions
    /// or listeners, and a reply whose bytes a test can state exactly.
    ///
    /// Infallible by construction. A pattern and an empty region cannot fail,
    /// and a random draw that does falls back to zeroes rather than failing the
    /// reply.
    pub(crate) fn payload(&self, len: usize) -> Vec<u8> {
        match self {
            // No allocation and no copy: the encoder already zero-fills the
            // whole region between the field block and the negotiated length,
            // so an empty payload *is* the zero payload. Zero is the policy —
            // the clean evidence records the reference server leaving this
            // region as unspecified residual buffer content, which returned
            // other traffic's bytes, and a compatible server must not.
            Self::None => Vec::new(),
            Self::Random => random_payload(len, getrandom::fill),
            Self::Pattern(pattern) => pattern.iter().copied().cycle().take(len).collect(),
        }
    }
}

/// The pattern a `pattern:` descriptor's body names, or `None` if the body is
/// not a whole number of hexadecimal bytes.
///
/// A descriptor is at most [`MAX_SERVER_FILL_BYTES`] on the wire, so this
/// decodes at most twelve bytes and complexity is beside the point. Both digit
/// cases are accepted; nothing else is, including a non-ASCII character, whose
/// bytes are all rejected as digits.
///
/// [`MAX_SERVER_FILL_BYTES`]: irtt_proto::MAX_SERVER_FILL_BYTES
fn parse_hex_pattern(body: &str) -> Option<Vec<u8>> {
    let digits = body.as_bytes();
    if digits.is_empty() || !digits.len().is_multiple_of(2) {
        return None;
    }
    digits
        .chunks_exact(2)
        .map(|pair| Some(hex_digit(pair[0])? << 4 | hex_digit(pair[1])?))
        .collect()
}

/// The value of one ASCII hexadecimal digit.
fn hex_digit(digit: u8) -> Option<u8> {
    match digit {
        b'0'..=b'9' => Some(digit - b'0'),
        b'a'..=b'f' => Some(digit - b'a' + 10),
        b'A'..=b'F' => Some(digit - b'A' + 10),
        _ => None,
    }
}

/// `len` random bytes drawn with `draw`, or `len` zeroes if the draw failed.
///
/// `draw` is a parameter rather than a direct call so the failure path is
/// assertable without a production hook: the caller passes the operating
/// system's random source and one unit test passes a scripted one. An empty
/// region asks the source for nothing at all.
///
/// **A failed draw zero-fills and is not an error.** Payload bytes carry no
/// protocol meaning, so a random source having a bad afternoon must not cost a
/// session its reply, let alone take the server down: the reply is still
/// structurally valid and interoperable. The buffer is rewritten rather than
/// trusted, because a failed draw leaves its contents unspecified.
///
/// The bytes exist to vary the payload, and nothing here is a security claim.
/// `getrandom` is used because the crate already depends on it; session tokens,
/// which *are* security state, keep their own separate source.
fn random_payload<E>(len: usize, draw: impl FnOnce(&mut [u8]) -> Result<(), E>) -> Vec<u8> {
    let mut payload = vec![0; len];
    if len > 0 && draw(&mut payload).is_err() {
        payload.fill(0);
    }
    payload
}

/// Settles what fill a session will actually use, restricting `params` only
/// where this server could not honor what was asked for.
///
/// Four cases, and the distinction between the first two and the last is the
/// point:
///
/// - **Absent or empty descriptor.** The client expressed no preference, so
///   `params` is left exactly as it arrived — absent stays absent, an explicit
///   empty value stays empty — and the session uses this server's default fill.
///   Writing the default descriptor into the reply would manufacture a
///   restriction out of a request that made none, and a strict client rejects a
///   descriptor it never asked for.
/// - **A recognized, well-formed descriptor.** Honored exactly, and returned
///   byte-for-byte, including a pattern body whose hexadecimal case differs from
///   the decoded bytes. `irtt-rs` accepts every valid `none`, `rand` and
///   `pattern:` descriptor: this is deliberately more permissive than the
///   reference server's default glob allow-list, which is upstream policy rather
///   than an interoperability requirement, and refusing valid modes would create
///   strict-negotiation failures while preventing nothing — the payload carries
///   no protocol meaning, the descriptor is bounded to 32 wire bytes, a pattern
///   is tiny, `none` is safely zero-filled here, and a random fill is bounded by
///   the negotiated packet length like every other reply.
/// - **An unrecognized or malformed explicit descriptor.** The server really did
///   change what was asked for, so it says so: `params.server_fill` becomes
///   [`DEFAULT_FILL_DESCRIPTOR`] and the session uses the default fill. A strict
///   client is then free to reject the session — the restriction is honest, and
///   hiding it would be worse.
///
/// The descriptor is parsed exactly once, here, so echo processing never looks
/// at a string.
pub(crate) fn negotiate_server_fill(params: &mut Params) -> FillMode {
    let Some(requested) = params
        .server_fill
        .as_ref()
        .filter(|fill| !fill.value.is_empty())
    else {
        return FillMode::default_fill();
    };
    match FillMode::parse(&requested.value) {
        Some(mode) => mode,
        None => {
            params.server_fill = Some(ServerFill {
                value: DEFAULT_FILL_DESCRIPTOR.to_owned(),
            });
            FillMode::default_fill()
        }
    }
}

/// The payload region of one echo reply under `params`, in bytes.
///
/// Derived from `irtt-proto`'s own layout and sizing rather than from
/// `params.length`, which is neither the packet length nor the payload length:
/// the mandatory field block, the negotiated statistics and timestamps, and
/// authentication's 16 bytes all consume packet space, and a length below the
/// field block asks for no payload at all.
///
/// An unrepresentable length yields zero rather than an error. An acknowledged
/// session cannot reach it — the open path already required [`echo_packet_len`]
/// to succeed — and if one somehow did, encoding the reply would fail on that
/// same call, which is where the failure belongs.
///
/// [`echo_packet_len`]: irtt_proto::echo_packet_len
pub(crate) fn echo_payload_len(hmac: bool, params: &Params) -> usize {
    let header_len = PacketLayout::echo(hmac, params).header_len();
    echo_packet_len(hmac, params).map_or(0, |len| len.saturating_sub(header_len))
}

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

    #[test]
    fn recognized_descriptors_parse_to_their_mode() {
        for (descriptor, expected) in [
            ("none", FillMode::None),
            ("rand", FillMode::Random),
            ("pattern:00", FillMode::Pattern(vec![0x00])),
            ("pattern:ff00", FillMode::Pattern(vec![0xff, 0x00])),
            // Only the hexadecimal body is case-insensitive.
            ("pattern:AaBb", FillMode::Pattern(vec![0xaa, 0xbb])),
            (
                "pattern:69727474",
                FillMode::Pattern(DEFAULT_FILL_PATTERN.to_vec()),
            ),
        ] {
            assert_eq!(FillMode::parse(descriptor), Some(expected), "{descriptor}");
        }
    }

    #[test]
    fn unrecognized_and_malformed_descriptors_parse_to_nothing() {
        for descriptor in [
            "",
            "bogus",
            // The mode names themselves are case-sensitive.
            "RAND",
            "None",
            "Pattern:aabb",
            "rand ",
            "pattern:",
            "pattern:f",
            "pattern:abc",
            "pattern:zz",
            "pattern:0g",
            // A multi-byte character is not two hexadecimal digits, whatever
            // its byte length.
            "pattern:é",
        ] {
            assert_eq!(FillMode::parse(descriptor), None, "{descriptor:?}");
        }
    }

    #[test]
    fn the_default_descriptor_and_the_default_pattern_agree() {
        // The two constants are held separately so nothing has to parse a
        // string per open; this is what keeps them from drifting.
        assert_eq!(
            FillMode::parse(DEFAULT_FILL_DESCRIPTOR),
            Some(FillMode::default_fill())
        );
    }

    #[test]
    fn a_pattern_repeats_from_its_first_byte_and_stops_at_the_region() {
        let mode = FillMode::Pattern(vec![0xaa, 0xbb]);
        assert_eq!(mode.payload(0), Vec::<u8>::new());
        assert_eq!(mode.payload(1), vec![0xaa]);
        assert_eq!(
            mode.payload(7),
            vec![0xaa, 0xbb, 0xaa, 0xbb, 0xaa, 0xbb, 0xaa]
        );
    }

    #[test]
    fn no_fill_leaves_the_region_to_the_encoder() {
        // Not a length-sized run of zeroes: the encoder has already zeroed the
        // region, and copying zeroes over zeroes would only cost an allocation.
        assert!(FillMode::None.payload(16).is_empty());
    }

    #[test]
    fn a_failed_random_draw_yields_zeroes() {
        // The real source cannot be made to fail without a production hook, and
        // the fallback itself is the policy under test: a failed draw must
        // produce a full-length payload of zeroes rather than an error or the
        // buffer's unspecified contents.
        let scribble = |buffer: &mut [u8]| {
            buffer.fill(0xab);
            Err(())
        };
        assert_eq!(random_payload(6, scribble), vec![0; 6]);
    }

    #[test]
    fn a_successful_random_draw_is_propagated_whole() {
        // Deterministic on purpose. Nothing here asserts that real random bytes
        // are nonzero, distinct or well distributed; those are properties of
        // the operating system's source, and testing them would be a coin flip
        // dressed up as an assertion.
        let drawn = [1, 2, 3, 4, 5];
        let source = |buffer: &mut [u8]| -> Result<(), ()> {
            assert_eq!(buffer.len(), drawn.len(), "the whole region is offered");
            buffer.copy_from_slice(&drawn);
            Ok(())
        };
        assert_eq!(random_payload(drawn.len(), source), drawn);
    }

    #[test]
    fn an_empty_region_never_reaches_the_random_source() {
        let source = |_: &mut [u8]| -> Result<(), ()> { panic!("no bytes were needed") };
        assert!(random_payload(0, source).is_empty());
    }
}