clay-borg/crates/cb-events/src/lib.rs

//! cb-events — event envelope, append-only log, snapshots, canonical
//! serialization, and state hashing (GameKernel §2.4, K4, K7, K9–K11).

use cb_kernel::{EventSeq, GameId};
use serde::{de::DeserializeOwned, Deserialize, Serialize};
use sha2::{Digest, Sha256};

/// Versioned event envelope (GameKernel K4).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Envelope<E> {
    pub seq: EventSeq,
    pub game_id: GameId,
    pub round: u8,
    pub schema_ver: u16,
    pub payload: E,
}

/// In-memory append-only event log. File-backed storage arrives in a later
/// loop pass behind the same interface (AM-11 pairs this with storage
/// impls under one conformance suite).
#[derive(Debug, Default)]
pub struct EventLog<E> {
    events: Vec<Envelope<E>>,
}

impl<E> EventLog<E> {
    pub fn new() -> Self {
        Self { events: Vec::new() }
    }

    /// Append with sequence enforcement: envelopes must arrive in strictly
    /// increasing `seq` order (GameKernel K11).
    pub fn append(&mut self, envelope: Envelope<E>) -> Result<(), LogError> {
        if let Some(last) = self.events.last() {
            if envelope.seq.0 != last.seq.0 + 1 {
                return Err(LogError::NonMonotonicSeq {
                    expected: last.seq.0 + 1,
                    got: envelope.seq.0,
                });
            }
        }
        self.events.push(envelope);
        Ok(())
    }

    pub fn len(&self) -> usize {
        self.events.len()
    }

    pub fn is_empty(&self) -> bool {
        self.events.is_empty()
    }

    pub fn iter(&self) -> impl Iterator<Item = &Envelope<E>> {
        self.events.iter()
    }
}

#[derive(Debug, PartialEq, Eq)]
pub enum LogError {
    NonMonotonicSeq { expected: u64, got: u64 },
}

impl core::fmt::Display for LogError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            LogError::NonMonotonicSeq { expected, got } => {
                write!(f, "non-monotonic event seq: expected {expected}, got {got}")
            }
        }
    }
}

/// Canonical serialization: serde_json with struct-declaration field order
/// and ordered maps (kernel state uses BTreeMap per GameKernel K6), so the
/// same state always yields the same bytes.
pub fn canonical_bytes<T: Serialize>(value: &T) -> Vec<u8> {
    serde_json::to_vec(value).expect("canonical serialization must not fail")
}

/// SHA-256 state hash over the canonical serialization (GameKernel K7).
pub fn state_hash<T: Serialize>(value: &T) -> [u8; 32] {
    let mut hasher = Sha256::new();
    hasher.update(canonical_bytes(value));
    hasher.finalize().into()
}

/// Hex form of [`state_hash`], for scenario `expect.state_hash` fields.
pub fn state_hash_hex<T: Serialize>(value: &T) -> String {
    let hash = state_hash(value);
    let mut out = String::with_capacity(64);
    for byte in hash {
        use core::fmt::Write;
        write!(out, "{byte:02x}").expect("writing to String cannot fail");
    }
    out
}

/// A snapshot pairs the canonical state bytes with the last included
/// event (GameKernel K9).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Snapshot {
    pub through: EventSeq,
    pub state: Vec<u8>,
}

impl Snapshot {
    pub fn take<T: Serialize>(state: &T, through: EventSeq) -> Self {
        Self {
            through,
            state: canonical_bytes(state),
        }
    }

    pub fn restore<T: DeserializeOwned>(&self) -> Result<T, serde_json::Error> {
        serde_json::from_slice(&self.state)
    }
}

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

    #[test]
    fn log_rejects_seq_gaps() {
        let mut log: EventLog<u8> = EventLog::new();
        let env = |seq| Envelope {
            seq: EventSeq(seq),
            game_id: GameId(1),
            round: 1,
            schema_ver: 1,
            payload: 0u8,
        };
        log.append(env(0)).unwrap();
        log.append(env(1)).unwrap();
        assert_eq!(
            log.append(env(3)),
            Err(LogError::NonMonotonicSeq {
                expected: 2,
                got: 3
            })
        );
    }

    /// K7: identical state → identical hash; any change → different hash.
    #[test]
    fn state_hash_is_stable_and_sensitive() {
        let mut a = BTreeMap::new();
        a.insert("stress", 2u8);
        let mut b = BTreeMap::new();
        b.insert("stress", 2u8);
        assert_eq!(state_hash_hex(&a), state_hash_hex(&b));
        b.insert("stress", 3u8);
        assert_ne!(state_hash_hex(&a), state_hash_hex(&b));
    }

    /// K9: snapshot → restore is identity on the canonical form.
    #[test]
    fn snapshot_roundtrip() {
        let mut state = BTreeMap::new();
        state.insert("round".to_string(), 3u8);
        let snap = Snapshot::take(&state, EventSeq(17));
        let restored: BTreeMap<String, u8> = snap.restore().unwrap();
        assert_eq!(state, restored);
        assert_eq!(state_hash_hex(&state), state_hash_hex(&restored));
    }
}