use super::error::WalError;

/// Magic bytes identifying a tidalDB WAL batch frame: "TIDL" in LE byte order.
///
/// Stored as `[0x44, 0x4C, 0x49, 0x54]` which is `0x54494C44` as a u32 LE.
/// This allows `u32::from_le_bytes(magic) == 0x54494C44` to validate.
pub const MAGIC: [u8; 4] = [0x44, 0x4C, 0x49, 0x54];

/// Current wire format version.
pub const FORMAT_VERSION: u8 = 1;

/// Record type discriminant for signal events.
pub const RECORD_TYPE_SIGNAL: u8 = 0x01;

/// Size of the batch header in bytes (one cache line).
pub const HEADER_SIZE: usize = 64;

/// Size of a single event record in bytes.
pub const EVENT_SIZE: usize = 21;

/// Maximum number of events in a single batch.
pub const MAX_EVENTS_PER_BATCH: u16 = 256;

/// Decoded batch header.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BatchHeader {
    pub version: u8,
    pub flags: u8,
    pub event_count: u16,
    pub first_seq: u64,
    pub batch_timestamp: u64,
    pub payload_len: u32,
    pub checksum: [u8; 32],
}

/// A single signal event record in wire format.
#[derive(Debug, Clone, PartialEq)]
pub struct EventRecord {
    pub entity_id: u64,
    pub signal_type: u8,
    pub weight: f32,
    pub timestamp_nanos: u64,
}

impl EventRecord {
    /// Serialize this event into the 21-byte wire format.
    #[must_use]
    pub fn to_bytes(&self) -> [u8; EVENT_SIZE] {
        let mut buf = [0u8; EVENT_SIZE];
        buf[0..8].copy_from_slice(&self.entity_id.to_le_bytes());
        buf[8] = self.signal_type;
        buf[9..13].copy_from_slice(&self.weight.to_le_bytes());
        buf[13..21].copy_from_slice(&self.timestamp_nanos.to_le_bytes());
        buf
    }

    /// Deserialize an event from 21 bytes of wire format.
    ///
    /// # Errors
    ///
    /// Returns `WalError::Corruption` if the slice is not exactly 21 bytes.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, WalError> {
        if bytes.len() != EVENT_SIZE {
            return Err(WalError::Corruption {
                message: format!(
                    "event record: expected {EVENT_SIZE} bytes, got {}",
                    bytes.len()
                ),
            });
        }
        let entity_id =
            u64::from_le_bytes(bytes[0..8].try_into().map_err(|_| WalError::Corruption {
                message: "event record: invalid entity_id bytes".into(),
            })?);
        let signal_type = bytes[8];
        let weight =
            f32::from_le_bytes(bytes[9..13].try_into().map_err(|_| WalError::Corruption {
                message: "event record: invalid weight bytes".into(),
            })?);
        let timestamp_nanos =
            u64::from_le_bytes(bytes[13..21].try_into().map_err(|_| WalError::Corruption {
                message: "event record: invalid timestamp bytes".into(),
            })?);
        Ok(Self {
            entity_id,
            signal_type,
            weight,
            timestamp_nanos,
        })
    }
}

/// Encode a batch of events into the WAL wire format.
///
/// Produces a byte vector containing the 64-byte header followed by
/// tightly packed 21-byte event records. The BLAKE3 checksum covers
/// `header[0..32] || event_bytes`.
///
/// # Errors
///
/// Returns `WalError::Corruption` if `events` is empty or exceeds
/// `MAX_EVENTS_PER_BATCH`.
pub fn encode_batch(
    events: &[EventRecord],
    first_seq: u64,
    batch_ts: u64,
) -> Result<Vec<u8>, WalError> {
    let event_count = events.len();
    if event_count == 0 || event_count > usize::from(MAX_EVENTS_PER_BATCH) {
        return Err(WalError::Corruption {
            message: format!(
                "batch event count {event_count} out of range [1, {MAX_EVENTS_PER_BATCH}]"
            ),
        });
    }

    let payload_len = event_count * EVENT_SIZE;
    let total_len = HEADER_SIZE + payload_len;
    let mut buf = vec![0u8; total_len];

    // Write header fields [0..32]
    buf[0..4].copy_from_slice(&MAGIC);
    buf[4] = FORMAT_VERSION;
    buf[5] = 0; // flags: reserved
    #[allow(clippy::cast_possible_truncation)]
    let count_u16 = event_count as u16;
    buf[6..8].copy_from_slice(&count_u16.to_le_bytes());
    buf[8..16].copy_from_slice(&first_seq.to_le_bytes());
    buf[16..24].copy_from_slice(&batch_ts.to_le_bytes());
    #[allow(clippy::cast_possible_truncation)]
    let payload_len_u32 = payload_len as u32;
    buf[24..28].copy_from_slice(&payload_len_u32.to_le_bytes());
    // [28..32] reserved, already zeroed

    // Write event records starting at offset 64
    for (i, event) in events.iter().enumerate() {
        let offset = HEADER_SIZE + i * EVENT_SIZE;
        buf[offset..offset + EVENT_SIZE].copy_from_slice(&event.to_bytes());
    }

    // Compute BLAKE3 over header[0..32] || event_bytes
    let checksum = compute_checksum(&buf[0..32], &buf[HEADER_SIZE..]);
    buf[32..64].copy_from_slice(checksum.as_bytes());

    Ok(buf)
}

/// Decode a batch from raw bytes.
///
/// Two-phase validation:
/// - Phase 1: magic bytes, version, payload length bounds
/// - Phase 2: BLAKE3 checksum verification
///
/// # Errors
///
/// Returns `WalError::Corruption` on any validation failure.
pub fn decode_batch(bytes: &[u8]) -> Result<(BatchHeader, Vec<EventRecord>), WalError> {
    if bytes.len() < HEADER_SIZE {
        return Err(WalError::Corruption {
            message: format!(
                "batch too short for header: {} bytes, need {HEADER_SIZE}",
                bytes.len()
            ),
        });
    }

    // Phase 1: structural validation
    if bytes[0..4] != MAGIC {
        return Err(WalError::Corruption {
            message: "invalid magic bytes".into(),
        });
    }

    let version = bytes[4];
    if version != FORMAT_VERSION {
        return Err(WalError::Corruption {
            message: format!("unsupported format version: {version}"),
        });
    }

    let flags = bytes[5];
    let event_count =
        u16::from_le_bytes(bytes[6..8].try_into().map_err(|_| WalError::Corruption {
            message: "invalid event_count bytes".into(),
        })?);

    if event_count == 0 || event_count > MAX_EVENTS_PER_BATCH {
        return Err(WalError::Corruption {
            message: format!("event count {event_count} out of range [1, {MAX_EVENTS_PER_BATCH}]"),
        });
    }

    let first_seq =
        u64::from_le_bytes(bytes[8..16].try_into().map_err(|_| WalError::Corruption {
            message: "invalid first_seq bytes".into(),
        })?);

    let batch_timestamp =
        u64::from_le_bytes(bytes[16..24].try_into().map_err(|_| WalError::Corruption {
            message: "invalid batch_timestamp bytes".into(),
        })?);

    let payload_len =
        u32::from_le_bytes(bytes[24..28].try_into().map_err(|_| WalError::Corruption {
            message: "invalid payload_len bytes".into(),
        })?);

    let expected_payload = u32::from(event_count) * EVENT_SIZE as u32;
    if payload_len != expected_payload {
        return Err(WalError::Corruption {
            message: format!(
                "payload_len {payload_len} != event_count {event_count} * {EVENT_SIZE}"
            ),
        });
    }

    let total_len = HEADER_SIZE + payload_len as usize;
    if bytes.len() < total_len {
        return Err(WalError::Corruption {
            message: format!(
                "batch truncated: have {} bytes, need {total_len}",
                bytes.len()
            ),
        });
    }

    // Extract stored checksum
    let mut checksum = [0u8; 32];
    checksum.copy_from_slice(&bytes[32..64]);

    // Phase 2: BLAKE3 verification
    let event_bytes = &bytes[HEADER_SIZE..total_len];
    let computed = compute_checksum(&bytes[0..32], event_bytes);
    if computed.as_bytes() != &checksum {
        return Err(WalError::Corruption {
            message: "BLAKE3 checksum mismatch".into(),
        });
    }

    // Parse event records
    let mut events = Vec::with_capacity(usize::from(event_count));
    for i in 0..usize::from(event_count) {
        let offset = i * EVENT_SIZE;
        let event = EventRecord::from_bytes(&event_bytes[offset..offset + EVENT_SIZE])?;
        events.push(event);
    }

    let header = BatchHeader {
        version,
        flags,
        event_count,
        first_seq,
        batch_timestamp,
        payload_len,
        checksum,
    };

    Ok((header, events))
}

/// Compute the BLAKE3 checksum for a batch.
///
/// Input: `header_prefix[0..32] || event_bytes`.
/// The hash field at `[32..64]` is NOT part of the hash input.
fn compute_checksum(header_prefix: &[u8], event_bytes: &[u8]) -> blake3::Hash {
    let mut hasher = blake3::Hasher::new();
    hasher.update(header_prefix);
    hasher.update(event_bytes);
    hasher.finalize()
}

/// Compute the per-event content hash used for deduplication.
///
/// Returns the first 128 bits of the BLAKE3 hash of the 21-byte event record.
///
/// # Panics
///
/// Cannot panic. The `expect` is on a `try_into` converting a 16-byte slice
/// (from a 32-byte BLAKE3 hash) into `[u8; 16]`, which is infallible.
#[must_use]
pub fn event_content_hash(event: &EventRecord) -> u128 {
    let bytes = event.to_bytes();
    let hash = blake3::hash(&bytes);
    let hash_bytes: &[u8; 32] = hash.as_bytes();
    u128::from_le_bytes(
        hash_bytes[..16]
            .try_into()
            .expect("BLAKE3 hash is always 32 bytes; first 16 is infallible"),
    )
}

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

    fn sample_event(id: u64) -> EventRecord {
        EventRecord {
            entity_id: id,
            signal_type: RECORD_TYPE_SIGNAL,
            weight: 1.0,
            timestamp_nanos: 1_000_000_000,
        }
    }

    #[test]
    fn event_record_roundtrip() {
        let event = sample_event(42);
        let bytes = event.to_bytes();
        assert_eq!(bytes.len(), EVENT_SIZE);
        let decoded = EventRecord::from_bytes(&bytes).expect("decode should succeed");
        assert_eq!(decoded, event);
    }

    #[test]
    fn event_record_wrong_size() {
        let result = EventRecord::from_bytes(&[0u8; 10]);
        assert!(result.is_err());
    }

    #[test]
    fn encode_decode_roundtrip_single() {
        let events = vec![sample_event(1)];
        let encoded = encode_batch(&events, 100, 999).expect("encode should succeed");
        assert_eq!(encoded.len(), HEADER_SIZE + EVENT_SIZE);

        let (header, decoded_events) = decode_batch(&encoded).expect("decode should succeed");
        assert_eq!(header.version, FORMAT_VERSION);
        assert_eq!(header.event_count, 1);
        assert_eq!(header.first_seq, 100);
        assert_eq!(header.batch_timestamp, 999);
        assert_eq!(header.payload_len, EVENT_SIZE as u32);
        assert_eq!(decoded_events.len(), 1);
        assert_eq!(decoded_events[0], events[0]);
    }

    #[test]
    fn encode_decode_roundtrip_multi() {
        let events: Vec<EventRecord> = (0..50).map(sample_event).collect();
        let encoded = encode_batch(&events, 1, 42).expect("encode should succeed");
        let (header, decoded) = decode_batch(&encoded).expect("decode should succeed");
        assert_eq!(header.event_count, 50);
        assert_eq!(decoded.len(), 50);
        for (original, decoded_ev) in events.iter().zip(decoded.iter()) {
            assert_eq!(original, decoded_ev);
        }
    }

    #[test]
    fn encode_empty_batch_fails() {
        let result = encode_batch(&[], 1, 1);
        assert!(result.is_err());
    }

    #[test]
    fn encode_oversized_batch_fails() {
        let events: Vec<EventRecord> = (0..=u64::from(MAX_EVENTS_PER_BATCH))
            .map(sample_event)
            .collect();
        let result = encode_batch(&events, 1, 1);
        assert!(result.is_err());
    }

    #[test]
    fn corrupt_payload_byte_fails_blake3() {
        let events = vec![sample_event(1), sample_event(2)];
        let mut encoded = encode_batch(&events, 1, 1).expect("encode should succeed");
        // Flip a byte in the payload (event data area)
        let payload_offset = HEADER_SIZE + 5;
        encoded[payload_offset] ^= 0xFF;
        let result = decode_batch(&encoded);
        assert!(result.is_err());
        let err_msg = result.expect_err("should fail").to_string();
        assert!(err_msg.contains("checksum"));
    }

    #[test]
    fn corrupt_header_field_fails_blake3() {
        let events = vec![sample_event(1)];
        let mut encoded = encode_batch(&events, 1, 1).expect("encode should succeed");
        // Corrupt the first_seq field in the header (byte 8)
        encoded[10] ^= 0xFF;
        let result = decode_batch(&encoded);
        assert!(result.is_err());
    }

    #[test]
    fn invalid_magic_detected() {
        let events = vec![sample_event(1)];
        let mut encoded = encode_batch(&events, 1, 1).expect("encode should succeed");
        encoded[0] = 0xFF; // corrupt magic
        let result = decode_batch(&encoded);
        assert!(result.is_err());
        let err_msg = result.expect_err("should fail").to_string();
        assert!(err_msg.contains("magic"));
    }

    #[test]
    fn truncated_header_detected() {
        let result = decode_batch(&[0u8; 32]);
        assert!(result.is_err());
    }

    #[test]
    fn truncated_payload_detected() {
        let events = vec![sample_event(1)];
        let encoded = encode_batch(&events, 1, 1).expect("encode should succeed");
        // Truncate: give header but only partial payload
        let result = decode_batch(&encoded[..HEADER_SIZE + 5]);
        assert!(result.is_err());
    }

    #[test]
    fn magic_bytes_are_tidl() {
        // Verify 0x54494C44 LE = "TIDL"
        assert_eq!(u32::from_le_bytes(MAGIC), 0x5449_4C44);
    }

    #[test]
    fn event_content_hash_deterministic() {
        let event = sample_event(42);
        let h1 = event_content_hash(&event);
        let h2 = event_content_hash(&event);
        assert_eq!(h1, h2);
    }

    #[test]
    fn event_content_hash_differs_for_different_events() {
        let h1 = event_content_hash(&sample_event(1));
        let h2 = event_content_hash(&sample_event(2));
        assert_ne!(h1, h2);
    }

    #[test]
    fn header_size_is_cache_line() {
        assert_eq!(HEADER_SIZE, 64);
    }

    #[test]
    fn event_size_is_21() {
        assert_eq!(EVENT_SIZE, 21);
    }

    mod proptests {
        use super::*;
        use proptest::prelude::*;

        fn arb_event() -> impl Strategy<Value = EventRecord> {
            (any::<u64>(), any::<u8>(), any::<f32>(), any::<u64>()).prop_map(
                |(entity_id, signal_type, weight, timestamp_nanos)| EventRecord {
                    entity_id,
                    signal_type,
                    weight,
                    timestamp_nanos,
                },
            )
        }

        proptest! {
            #[test]
            fn event_roundtrip(event in arb_event()) {
                let bytes = event.to_bytes();
                let decoded = EventRecord::from_bytes(&bytes)?;
                prop_assert_eq!(decoded.entity_id, event.entity_id);
                prop_assert_eq!(decoded.signal_type, event.signal_type);
                // f32 NaN != NaN, so compare bits
                prop_assert_eq!(
                    decoded.weight.to_bits(),
                    event.weight.to_bits()
                );
                prop_assert_eq!(decoded.timestamp_nanos, event.timestamp_nanos);
            }

            #[test]
            fn batch_roundtrip(
                events in proptest::collection::vec(arb_event(), 1..=100),
                first_seq in any::<u64>(),
                batch_ts in any::<u64>(),
            ) {
                let encoded = encode_batch(&events, first_seq, batch_ts)?;
                let (header, decoded) = decode_batch(&encoded)?;
                prop_assert_eq!(header.first_seq, first_seq);
                prop_assert_eq!(header.batch_timestamp, batch_ts);
                prop_assert_eq!(decoded.len(), events.len());
                for (orig, dec) in events.iter().zip(decoded.iter()) {
                    prop_assert_eq!(orig.entity_id, dec.entity_id);
                    prop_assert_eq!(orig.signal_type, dec.signal_type);
                    prop_assert_eq!(orig.weight.to_bits(), dec.weight.to_bits());
                    prop_assert_eq!(orig.timestamp_nanos, dec.timestamp_nanos);
                }
            }

            #[test]
            fn corrupt_any_payload_byte_fails(
                events in proptest::collection::vec(arb_event(), 1..=50),
                corrupt_offset in 0usize..1050,
            ) {
                let encoded = encode_batch(&events, 1, 1)?;
                let payload_start = HEADER_SIZE;
                let payload_end = encoded.len();
                let payload_size = payload_end - payload_start;
                if payload_size == 0 {
                    return Ok(());
                }
                let actual_offset = payload_start + (corrupt_offset % payload_size);
                let mut corrupted = encoded;
                corrupted[actual_offset] ^= 0xFF;
                let result = decode_batch(&corrupted);
                prop_assert!(result.is_err());
            }
        }
    }
}