host-encoding 0.3.4

//! Pure sp-statement-store encoding and decoding functions.
//!
//! No I/O. No network calls. WASM-safe.
//!
//! Implements the binary statement format used by Substrate's statement store pallet.
//! Callers are responsible for providing the current time (as Unix seconds) so that
//! this module remains free of `std::time` and is WASM-safe.

use blake2::digest::Mac;
use blake2::{digest::consts::U32, Blake2b, Digest};

type Blake2b256 = Blake2b<U32>;

/// Topic = 32-byte blake2b hash.
pub type Topic = [u8; 32];

/// Hash a string into a 32-byte Topic (blake2b-256).
pub fn string_to_topic(s: &str) -> Topic {
    let mut hasher = Blake2b256::new();
    hasher.update(s.as_bytes());
    let result = hasher.finalize();
    let mut topic = [0u8; 32];
    topic.copy_from_slice(&result);
    topic
}

// ---------------------------------------------------------------------------
// Statement binary encoding (sp-statement-store compatible)
//
// Substrate format: Compact<u32> field count, then tagged fields in order.
// Field tags: 0=Proof, 1=DecryptionKey, 2=Expiry, 3=Channel, 4-7=Topics, 8=Data
// Each field: u8 tag + SCALE-encoded payload.
// Proof::Sr25519 = tag 0, variant 0, sig[64], signer[32].
// Data = tag 8, Compact<u32> length, bytes.
// Signing payload: same fields without Compact prefix and without Proof field.
// ---------------------------------------------------------------------------

/// A decoded statement from the statement store.
#[derive(Debug, Clone, PartialEq)]
pub struct Statement {
    pub proof_pubkey: Option<[u8; 32]>,
    /// Raw sr25519 (or ed25519) signature over the signing payload.
    /// Present only for proof variants 0 (Sr25519) and 1 (Ed25519).
    pub proof_signature: Option<[u8; 64]>,
    pub decryption_key: Option<Topic>,
    pub channel: Option<Topic>,
    pub priority: u32,
    pub topics: Vec<Topic>,
    pub data: Vec<u8>,
}

/// Encode SCALE Compact<u32>.
pub fn encode_compact_u32(val: u32) -> Vec<u8> {
    if val < 0x40 {
        vec![(val as u8) << 2]
    } else if val < 0x4000 {
        let v = (val << 2) | 0x01;
        vec![v as u8, (v >> 8) as u8]
    } else if val < 0x4000_0000 {
        let v = (val << 2) | 0x02;
        v.to_le_bytes().to_vec()
    } else {
        let mut out = vec![0x03];
        out.extend_from_slice(&val.to_le_bytes());
        out
    }
}

/// Decode SCALE Compact<u32>, returns (value, bytes_consumed).
pub fn decode_compact_u32(data: &[u8]) -> Result<(u32, usize), String> {
    if data.is_empty() {
        return Err("compact: empty".into());
    }
    let mode = data[0] & 0x03;
    match mode {
        0 => Ok(((data[0] >> 2) as u32, 1)),
        1 => {
            if data.len() < 2 {
                return Err("compact: truncated 2-byte".into());
            }
            let v = u16::from_le_bytes([data[0], data[1]]) >> 2;
            Ok((v as u32, 2))
        }
        2 => {
            if data.len() < 4 {
                return Err("compact: truncated 4-byte".into());
            }
            let v = u32::from_le_bytes([data[0], data[1], data[2], data[3]]) >> 2;
            Ok((v, 4))
        }
        3 => {
            if data.len() < 5 {
                return Err("compact: truncated big".into());
            }
            let v = u32::from_le_bytes([data[1], data[2], data[3], data[4]]);
            Ok((v, 5))
        }
        _ => unreachable!(),
    }
}

/// Build the signing payload for a statement.
///
/// Returns `(payload_bytes, num_fields)` where `payload_bytes` are the raw bytes
/// that must be signed (sr25519 context signing or equivalent), and `num_fields`
/// is needed by [`assemble_statement`] to write the SCALE header.
///
/// # Parameters
///
/// - `now_secs`: current Unix timestamp in seconds. Used to compute a 1-hour expiry.
/// - `topics`: at most 4 topics; returns `Err` if more are supplied.
pub fn build_signing_payload(
    now_secs: u64,
    decryption_key: Option<&Topic>,
    channel: Option<&Topic>,
    priority: u32,
    topics: &[Topic],
    data: &[u8],
) -> Result<(Vec<u8>, u32), String> {
    if topics.len() > 4 {
        return Err(format!("too many topics ({}, max 4)", topics.len()));
    }

    // Expiry: upper 32 bits = timestamp (seconds), lower 32 bits = priority.
    let expiry_ts = (now_secs + 3600) as u32;
    let expiry: u64 = ((expiry_ts as u64) << 32) | (priority as u64);

    // Count fields (proof + data fields).
    let mut num_fields: u32 = 1; // proof always present
    if decryption_key.is_some() {
        num_fields += 1;
    }
    num_fields += 1; // expiry always present
    if channel.is_some() {
        num_fields += 1;
    }
    num_fields += topics.len() as u32;
    if !data.is_empty() {
        num_fields += 1;
    }

    // Build signing payload (fields without Compact prefix and without Proof).
    let mut payload = Vec::new();
    if let Some(dk) = decryption_key {
        payload.push(1u8); // tag: DecryptionKey
        payload.extend_from_slice(dk);
    }
    payload.push(2u8); // tag: Expiry
    payload.extend_from_slice(&expiry.to_le_bytes());
    if let Some(ch) = channel {
        payload.push(3u8); // tag: Channel
        payload.extend_from_slice(ch);
    }
    for (i, t) in topics.iter().enumerate() {
        payload.push(4u8 + i as u8); // tag: Topic1..4
        payload.extend_from_slice(t);
    }
    if !data.is_empty() {
        payload.push(8u8); // tag: Data
        payload.extend_from_slice(&encode_compact_u32(data.len() as u32));
        payload.extend_from_slice(data);
    }

    Ok((payload, num_fields))
}

/// Assemble a complete encoded statement from a signing payload and sr25519 signature.
///
/// `signing_payload` and `num_fields` must come from [`build_signing_payload`].
pub fn assemble_statement(
    signing_payload: &[u8],
    num_fields: u32,
    sr25519_pubkey: &[u8; 32],
    signature: &[u8; 64],
) -> Vec<u8> {
    let mut out = Vec::new();

    // Compact<u32> field count prefix.
    out.extend_from_slice(&encode_compact_u32(num_fields));

    // Field 0: AuthenticityProof (Proof::Sr25519 = variant 0)
    out.push(0u8); // tag: AuthenticityProof
    out.push(0u8); // Proof variant 0 = Sr25519
    out.extend_from_slice(signature);
    out.extend_from_slice(sr25519_pubkey);

    // Remaining fields (same as signing_payload).
    out.extend_from_slice(signing_payload);

    out
}

/// Encode a statement into the sp-statement-store SCALE binary format.
///
/// Convenience wrapper that calls [`build_signing_payload`] then signs and
/// assembles in one step. For FFI or two-phase workflows where the signing
/// closure is unavailable, use the split API instead.
///
/// # Parameters
///
/// - `now_secs`: current Unix timestamp in seconds. Used to compute a 1-hour
///   expiry. Callers must supply this to keep the function pure and WASM-safe.
/// - `topics`: at most 4 topics; returns `Err` if more are supplied.
// All 8 parameters are required by the sp-statement-store wire format;
// grouping them into a struct would add churn for callers with no semantic gain.
#[allow(clippy::too_many_arguments)]
pub fn encode_statement(
    now_secs: u64,
    decryption_key: Option<&Topic>,
    channel: Option<&Topic>,
    priority: u32,
    topics: &[Topic],
    data: &[u8],
    sr25519_pubkey: &[u8; 32],
    sr25519_sign: &dyn Fn(&[u8]) -> [u8; 64],
) -> Result<Vec<u8>, String> {
    let (payload, num_fields) =
        build_signing_payload(now_secs, decryption_key, channel, priority, topics, data)?;
    let signature = sr25519_sign(&payload);
    Ok(assemble_statement(
        &payload,
        num_fields,
        sr25519_pubkey,
        &signature,
    ))
}

/// Extract the signing payload from a raw encoded statement.
///
/// The signing payload is the portion of the statement that was signed: all
/// fields except the Compact field-count prefix and the AuthenticityProof field
/// itself.  Callers use this to verify an embedded signature before trusting
/// any other field.
///
/// Returns `Err` if the statement is too short or malformed.
pub fn extract_signing_payload(encoded: &[u8]) -> Result<&[u8], String> {
    if encoded.is_empty() {
        return Err("empty statement".into());
    }
    // Skip the Compact<u32> field-count prefix.
    let (_, compact_len) = decode_compact_u32(encoded)?;

    // The proof field must be first: tag(1B) + variant(1B) + sig[64] + pk[32] = 98 B
    // for variants 0 (Sr25519) and 1 (Ed25519).  We only handle those here because
    // the chat protocol uses Sr25519 exclusively.
    let proof_start = compact_len;
    if encoded.len() < proof_start + 2 {
        return Err("truncated proof header".into());
    }
    let tag = encoded[proof_start];
    if tag != 0 {
        return Err(format!("expected AuthenticityProof tag (0), got {tag}"));
    }
    let variant = encoded[proof_start + 1];
    let proof_body_len = match variant {
        0 | 1 => 96, // sig[64] + pubkey[32]
        2 => 98,     // secp256k1: sig[65] + signer[33]
        3 => 72,     // on-chain: who[32] + block_hash[32] + u64
        _ => return Err(format!("unknown proof variant: {variant}")),
    };
    let payload_start = proof_start + 2 + proof_body_len;
    if encoded.len() < payload_start {
        return Err("truncated proof body".into());
    }
    Ok(&encoded[payload_start..])
}

/// Decode a statement from SCALE binary encoding (sp-statement-store format).
pub fn decode_statement(encoded: &[u8]) -> Result<Statement, String> {
    if encoded.is_empty() {
        return Err("empty statement".into());
    }

    let (num_fields, mut pos) = decode_compact_u32(encoded)?;

    let mut proof_pubkey = None;
    let mut proof_signature = None;
    let mut decryption_key = None;
    let mut channel = None;
    let mut priority = 0u32;
    let mut topics = Vec::new();
    let mut data = Vec::new();

    for _ in 0..num_fields {
        if pos >= encoded.len() {
            return Err("truncated field tag".into());
        }
        let tag = encoded[pos];
        pos += 1;

        match tag {
            0 => {
                // AuthenticityProof
                if pos >= encoded.len() {
                    return Err("truncated proof variant".into());
                }
                let variant = encoded[pos];
                pos += 1;
                match variant {
                    0 | 1 => {
                        // Sr25519 or Ed25519: sig[64] + signer[32]
                        if pos + 96 > encoded.len() {
                            return Err("truncated proof".into());
                        }
                        let mut sig = [0u8; 64];
                        sig.copy_from_slice(&encoded[pos..pos + 64]);
                        proof_signature = Some(sig);
                        let mut pk = [0u8; 32];
                        pk.copy_from_slice(&encoded[pos + 64..pos + 96]);
                        proof_pubkey = Some(pk);
                        pos += 96;
                    }
                    2 => {
                        // Secp256k1: sig[65] + signer[33]
                        if pos + 98 > encoded.len() {
                            return Err("truncated secp proof".into());
                        }
                        pos += 98;
                    }
                    3 => {
                        // OnChain: who[32] + block_hash[32] + u64
                        if pos + 72 > encoded.len() {
                            return Err("truncated onchain proof".into());
                        }
                        let mut pk = [0u8; 32];
                        pk.copy_from_slice(&encoded[pos..pos + 32]);
                        proof_pubkey = Some(pk);
                        pos += 72;
                    }
                    _ => return Err(format!("unknown proof variant: {variant}")),
                }
            }
            1 => {
                // DecryptionKey [32]
                if pos + 32 > encoded.len() {
                    return Err("truncated decryption_key".into());
                }
                let mut dk = [0u8; 32];
                dk.copy_from_slice(&encoded[pos..pos + 32]);
                decryption_key = Some(dk);
                pos += 32;
            }
            2 => {
                // Expiry u64 — upper 32 = timestamp, lower 32 = priority
                if pos + 8 > encoded.len() {
                    return Err("truncated expiry".into());
                }
                let expiry = u64::from_le_bytes([
                    encoded[pos],
                    encoded[pos + 1],
                    encoded[pos + 2],
                    encoded[pos + 3],
                    encoded[pos + 4],
                    encoded[pos + 5],
                    encoded[pos + 6],
                    encoded[pos + 7],
                ]);
                priority = expiry as u32; // lower 32 bits
                pos += 8;
            }
            3 => {
                // Channel [32]
                if pos + 32 > encoded.len() {
                    return Err("truncated channel".into());
                }
                let mut ch = [0u8; 32];
                ch.copy_from_slice(&encoded[pos..pos + 32]);
                channel = Some(ch);
                pos += 32;
            }
            4..=7 => {
                // Topic [32]
                if pos + 32 > encoded.len() {
                    return Err("truncated topic".into());
                }
                let mut t = [0u8; 32];
                t.copy_from_slice(&encoded[pos..pos + 32]);
                topics.push(t);
                pos += 32;
            }
            8 => {
                // Data: Compact<u32> length + bytes
                let (data_len, consumed) =
                    decode_compact_u32(&encoded[pos..]).map_err(|e| format!("data len: {e}"))?;
                pos += consumed;
                let data_len = data_len as usize;
                if pos + data_len > encoded.len() {
                    return Err("truncated data".into());
                }
                data = encoded[pos..pos + data_len].to_vec();
                pos += data_len;
            }
            _ => {
                // Unknown field — can't decode further without knowing size.
                return Err(format!("unknown field tag: {tag}"));
            }
        }
    }

    Ok(Statement {
        proof_pubkey,
        proof_signature,
        decryption_key,
        channel,
        priority,
        topics,
        data,
    })
}

/// Compute a blake2b-256 hash.
pub fn blake2b_256(data: &[u8]) -> [u8; 32] {
    let mut hasher = Blake2b256::new();
    hasher.update(data);
    let result = hasher.finalize();
    let mut out = [0u8; 32];
    out.copy_from_slice(&result);
    out
}

/// Keyed Blake2b-256 hash.
///
/// `key` must be 1..=64 bytes (Blake2b key length constraint).
/// Returns the 32-byte digest, or an error if the key length is invalid.
pub fn blake2b_256_keyed(key: &[u8], data: &[u8]) -> Result<[u8; 32], String> {
    use blake2::Blake2bMac;
    // Reject empty keys explicitly — some Blake2b implementations silently
    // degrade to unkeyed mode for key.len()==0, which would be a security bug.
    if key.is_empty() {
        return Err("blake2b key must not be empty".into());
    }
    let mut mac = <Blake2bMac<U32> as Mac>::new_from_slice(key)
        .map_err(|_| format!("blake2b key length must be 1..=64, got {}", key.len()))?;
    mac.update(data);
    let result = mac.finalize();
    Ok(result.into_bytes().into())
}

/// Derive a 32-byte topic from a context string, a raw AccountId, and optional extra bytes.
///
/// `topic = blake2b_256(SCALE_compact(context.len()) || context || account_id || extra)`
///
/// The context is length-prefixed with SCALE compact encoding to match the
/// deployed iOS protocol (`Data.encode(scaleEncoder:)` produces a SCALE
/// `Vec<u8>` prefix). AccountId is always exactly 32 bytes (enforced by
/// `&[u8; 32]`), so it acts as a fixed-width separator and needs no length
/// prefix.
pub fn derive_topic_from_account(context: &[u8], account_id: &[u8; 32], extra: &[u8]) -> [u8; 32] {
    let ctx_prefix = encode_compact_u32(context.len() as u32);
    let mut input = Vec::with_capacity(ctx_prefix.len() + context.len() + 32 + extra.len());
    input.extend_from_slice(&ctx_prefix);
    input.extend_from_slice(context);
    input.extend_from_slice(account_id);
    input.extend_from_slice(extra);
    blake2b_256(&input)
}

// Re-export shared hex utilities from the crate root.
pub use crate::{hex_decode, hex_encode};

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

    #[test]
    fn test_string_to_topic_deterministic() {
        let t1 = string_to_topic("ss-dothost");
        let t2 = string_to_topic("ss-dothost");
        assert_eq!(t1, t2);
    }

    #[test]
    fn test_string_to_topic_distinct() {
        let t1 = string_to_topic("topic-a");
        let t2 = string_to_topic("topic-b");
        assert_ne!(t1, t2);
    }

    #[test]
    fn test_encode_compact_u32_single_byte() {
        assert_eq!(encode_compact_u32(0), vec![0x00]);
        assert_eq!(encode_compact_u32(1), vec![0x04]);
        assert_eq!(encode_compact_u32(63), vec![0xfc]);
    }

    #[test]
    fn test_decode_compact_u32_empty_returns_error() {
        assert!(decode_compact_u32(&[]).is_err());
    }

    #[test]
    fn test_encode_decode_compact_u32_roundtrip() {
        for val in [0u32, 1, 63, 64, 16383, 16384, 0x3FFF_FFFF] {
            let encoded = encode_compact_u32(val);
            let (decoded, _) = decode_compact_u32(&encoded).unwrap();
            assert_eq!(decoded, val, "roundtrip failed for {val}");
        }
    }

    #[test]
    fn test_build_signing_payload_rejects_too_many_topics() {
        let topic = [0u8; 32];
        let topics = vec![topic; 5];
        let result = build_signing_payload(1_700_000_000, None, None, 0, &topics, b"data");
        assert!(result.is_err());
        assert!(result.unwrap_err().contains("too many topics"));
    }

    #[test]
    fn test_build_and_assemble_matches_encode_statement() {
        let dk = string_to_topic("room-id");
        let ch = string_to_topic("channel-1");
        let topic = string_to_topic("ss-dothost");
        let data = b"hello";
        let pubkey = [0xab; 32];
        let fake_sig = [0xcd; 64];

        let (payload, num_fields) =
            build_signing_payload(1_700_000_000, Some(&dk), Some(&ch), 42, &[topic], data).unwrap();

        let assembled = assemble_statement(&payload, num_fields, &pubkey, &fake_sig);
        let direct = encode_statement(
            1_700_000_000,
            Some(&dk),
            Some(&ch),
            42,
            &[topic],
            data,
            &pubkey,
            &|_| fake_sig,
        )
        .unwrap();

        assert_eq!(assembled, direct);
    }

    #[test]
    fn test_encode_statement_rejects_too_many_topics() {
        let topic = [0u8; 32];
        let topics = vec![topic; 5];
        let pubkey = [0u8; 32];
        let result = encode_statement(
            1_700_000_000,
            None,
            None,
            0,
            &topics,
            b"data",
            &pubkey,
            &|_| [0u8; 64],
        );
        assert!(result.is_err());
        assert!(result.unwrap_err().contains("too many topics"));
    }

    #[test]
    fn test_encode_decode_statement_roundtrip() {
        let decryption_key = string_to_topic("room-id");
        let channel = string_to_topic("channel-1");
        let topic1 = string_to_topic("ss-dothost");
        let topic2 = string_to_topic("presence");
        let data = b"hello world";
        let pubkey = [0xab; 32];
        let fake_sig = [0xcd; 64];

        let encoded = encode_statement(
            1_700_000_000,
            Some(&decryption_key),
            Some(&channel),
            42,
            &[topic1, topic2],
            data,
            &pubkey,
            &|_| fake_sig,
        )
        .unwrap();

        let decoded = decode_statement(&encoded).unwrap();

        assert_eq!(decoded.proof_pubkey, Some(pubkey));
        assert_eq!(decoded.decryption_key, Some(decryption_key));
        assert_eq!(decoded.channel, Some(channel));
        assert_eq!(decoded.priority, 42);
        assert_eq!(decoded.topics.len(), 2);
        assert_eq!(decoded.topics[0], topic1);
        assert_eq!(decoded.topics[1], topic2);
        assert_eq!(decoded.data, data);
    }

    #[test]
    fn test_decode_statement_empty_returns_error() {
        assert!(decode_statement(&[]).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_returns_error() {
        assert!(decode_statement(&[0x04, 0x00]).is_err());
    }

    #[test]
    fn test_hex_encode_decode_roundtrip() {
        let original = vec![0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef];
        let encoded = hex_encode(&original);
        assert_eq!(encoded, "0x0123456789abcdef");
        let decoded = hex_decode(&encoded).unwrap();
        assert_eq!(decoded, original);
    }

    #[test]
    fn test_hex_decode_bare_string() {
        let decoded = hex_decode("deadbeef").unwrap();
        assert_eq!(decoded, vec![0xde, 0xad, 0xbe, 0xef]);
    }

    #[test]
    fn test_hex_decode_rejects_odd_length() {
        assert!(hex_decode("0xabc").is_none());
    }

    #[test]
    fn test_blake2b_256_deterministic() {
        let h1 = blake2b_256(b"test");
        let h2 = blake2b_256(b"test");
        assert_eq!(h1, h2);
        assert_ne!(h1, [0u8; 32]);
    }

    // -----------------------------------------------------------------------
    // encode_compact_u32 — two-byte and four-byte mode coverage
    // -----------------------------------------------------------------------

    #[test]
    fn test_encode_compact_u32_two_byte_mode() {
        // 64 <= val < 16384 → two-byte mode, lower 2 bits = 0b01
        let encoded = encode_compact_u32(64);
        assert_eq!(encoded.len(), 2);
        let (decoded, _) = decode_compact_u32(&encoded).unwrap();
        assert_eq!(decoded, 64);
    }

    #[test]
    fn test_encode_compact_u32_four_byte_mode() {
        // 16384 <= val < 0x40000000 → four-byte mode
        let encoded = encode_compact_u32(16384);
        assert_eq!(encoded.len(), 4);
        let (decoded, _) = decode_compact_u32(&encoded).unwrap();
        assert_eq!(decoded, 16384);
    }

    #[test]
    fn test_encode_compact_u32_big_mode() {
        // val >= 0x40000000 → big mode (5 bytes: 0x03 prefix + 4-byte value)
        let val = 0x4000_0000u32;
        let encoded = encode_compact_u32(val);
        assert_eq!(encoded.len(), 5);
        assert_eq!(encoded[0], 0x03); // big-mode prefix
        let (decoded, _) = decode_compact_u32(&encoded).unwrap();
        assert_eq!(decoded, val);
    }

    // -----------------------------------------------------------------------
    // decode_compact_u32 — truncation error paths
    // -----------------------------------------------------------------------

    #[test]
    fn test_decode_compact_u32_two_byte_truncated_returns_error() {
        // Mode bit = 0b01 but only one byte
        assert!(decode_compact_u32(&[0x01]).is_err());
    }

    #[test]
    fn test_decode_compact_u32_four_byte_truncated_returns_error() {
        // Mode bit = 0b10 but fewer than 4 bytes
        assert!(decode_compact_u32(&[0x02, 0x00, 0x00]).is_err());
    }

    #[test]
    fn test_decode_compact_u32_big_mode_truncated_returns_error() {
        // Mode bit = 0b11 (big mode) but fewer than 5 bytes
        assert!(decode_compact_u32(&[0x03, 0x00, 0x00, 0x00]).is_err());
    }

    // -----------------------------------------------------------------------
    // decode_statement — proof variant coverage
    // -----------------------------------------------------------------------

    /// Helper that builds a signed statement with one data field using a dummy key.
    fn minimal_statement_bytes(proof_variant: u8, proof_bytes: &[u8], data: &[u8]) -> Vec<u8> {
        // Two fields: proof + data (if non-empty), or just proof if data is empty.
        let num_fields: u32 = if data.is_empty() { 1 } else { 2 };
        let mut out = encode_compact_u32(num_fields);
        out.push(0u8); // tag: AuthenticityProof
        out.push(proof_variant);
        out.extend_from_slice(proof_bytes);
        if !data.is_empty() {
            out.push(8u8); // tag: Data
            out.extend_from_slice(&encode_compact_u32(data.len() as u32));
            out.extend_from_slice(data);
        }
        out
    }

    #[test]
    fn test_decode_statement_ed25519_proof_variant() {
        // Variant 1 = Ed25519: sig[64] + signer[32] = 96 bytes, same layout as Sr25519
        let sig = [0x11u8; 64];
        let pk = [0x22u8; 32];
        let mut proof_bytes = Vec::new();
        proof_bytes.extend_from_slice(&sig);
        proof_bytes.extend_from_slice(&pk);

        let encoded = minimal_statement_bytes(1, &proof_bytes, b"");
        let decoded = decode_statement(&encoded).unwrap();
        assert_eq!(decoded.proof_pubkey, Some(pk));
    }

    #[test]
    fn test_decode_statement_secp256k1_proof_variant() {
        // Variant 2 = Secp256k1: sig[65] + signer[33] = 98 bytes
        let proof_bytes = vec![0x33u8; 98];
        let encoded = minimal_statement_bytes(2, &proof_bytes, b"");
        let decoded = decode_statement(&encoded).unwrap();
        // Secp256k1 proof does not set proof_pubkey in the current implementation
        assert_eq!(decoded.proof_pubkey, None);
    }

    #[test]
    fn test_decode_statement_onchain_proof_variant() {
        // Variant 3 = OnChain: who[32] + block_hash[32] + u64[8] = 72 bytes
        let who = [0x44u8; 32];
        let block_hash = [0x55u8; 32];
        let block_num = [0x00u8; 8];
        let mut proof_bytes = Vec::new();
        proof_bytes.extend_from_slice(&who);
        proof_bytes.extend_from_slice(&block_hash);
        proof_bytes.extend_from_slice(&block_num);

        let encoded = minimal_statement_bytes(3, &proof_bytes, b"");
        let decoded = decode_statement(&encoded).unwrap();
        assert_eq!(decoded.proof_pubkey, Some(who));
    }

    #[test]
    fn test_decode_statement_unknown_proof_variant_returns_error() {
        // Variant 99 is not recognized
        let encoded = minimal_statement_bytes(99, &[0u8; 10], b"");
        assert!(decode_statement(&encoded).is_err());
    }

    #[test]
    fn test_decode_statement_unknown_field_tag_returns_error() {
        // Field count = 1, then use tag 9 (unknown after 8)
        let mut out = encode_compact_u32(1);
        out.push(9u8); // unknown tag
        assert!(decode_statement(&out).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_proof_returns_error() {
        // Proof variant 0 needs 96 bytes but only provide 10
        let encoded = minimal_statement_bytes(0, &[0u8; 10], b"");
        assert!(decode_statement(&encoded).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_secp_proof_returns_error() {
        // Variant 2 needs 98 bytes but only provide 10
        let encoded = minimal_statement_bytes(2, &[0u8; 10], b"");
        assert!(decode_statement(&encoded).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_onchain_proof_returns_error() {
        // Variant 3 needs 72 bytes but only provide 10
        let encoded = minimal_statement_bytes(3, &[0u8; 10], b"");
        assert!(decode_statement(&encoded).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_decryption_key_returns_error() {
        // 2 fields: proof (valid) + decryption_key (truncated)
        let mut out = encode_compact_u32(2);
        out.push(0u8); // AuthenticityProof
        out.push(0u8); // Sr25519
        out.extend_from_slice(&[0u8; 96]); // valid proof
        out.push(1u8); // DecryptionKey tag
        out.extend_from_slice(&[0u8; 10]); // only 10 bytes, needs 32
        assert!(decode_statement(&out).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_channel_returns_error() {
        let mut out = encode_compact_u32(2);
        out.push(0u8); // AuthenticityProof
        out.push(0u8); // Sr25519
        out.extend_from_slice(&[0u8; 96]);
        out.push(3u8); // Channel tag
        out.extend_from_slice(&[0u8; 10]); // only 10 bytes, needs 32
        assert!(decode_statement(&out).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_topic_returns_error() {
        let mut out = encode_compact_u32(2);
        out.push(0u8); // AuthenticityProof
        out.push(0u8); // Sr25519
        out.extend_from_slice(&[0u8; 96]);
        out.push(4u8); // Topic1 tag
        out.extend_from_slice(&[0u8; 10]); // only 10 bytes, needs 32
        assert!(decode_statement(&out).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_data_returns_error() {
        let mut out = encode_compact_u32(2);
        out.push(0u8); // AuthenticityProof
        out.push(0u8); // Sr25519
        out.extend_from_slice(&[0u8; 96]);
        out.push(8u8); // Data tag
                       // compact_len claims 50 bytes but we only provide 5
        out.extend_from_slice(&encode_compact_u32(50));
        out.extend_from_slice(&[0u8; 5]);
        assert!(decode_statement(&out).is_err());
    }

    #[test]
    fn test_decode_statement_truncated_expiry_returns_error() {
        let mut out = encode_compact_u32(2);
        out.push(0u8); // AuthenticityProof
        out.push(0u8); // Sr25519
        out.extend_from_slice(&[0u8; 96]);
        out.push(2u8); // Expiry tag
        out.extend_from_slice(&[0u8; 4]); // only 4 bytes, needs 8
        assert!(decode_statement(&out).is_err());
    }

    // -----------------------------------------------------------------------
    // encode_statement — minimal case (no optional fields, no data)
    // -----------------------------------------------------------------------

    #[test]
    fn test_encode_statement_minimal_no_optional_fields() {
        let pubkey = [0xabu8; 32];
        let fake_sig = [0xcdu8; 64];
        let encoded = encode_statement(
            1_700_000_000,
            None, // no decryption key
            None, // no channel
            0,
            &[], // no topics
            &[], // no data
            &pubkey,
            &|_| fake_sig,
        )
        .unwrap();

        // Should decode successfully and have no optional fields
        let decoded = decode_statement(&encoded).unwrap();
        assert_eq!(decoded.proof_pubkey, Some(pubkey));
        assert_eq!(decoded.decryption_key, None);
        assert_eq!(decoded.channel, None);
        assert_eq!(decoded.topics.len(), 0);
        assert_eq!(decoded.data, b"");
    }

    #[test]
    fn test_encode_statement_expiry_encodes_priority_in_lower_bits() {
        let pubkey = [0u8; 32];
        let fake_sig = [0u8; 64];
        let priority = 0x0000_cafe_u32;

        let encoded = encode_statement(
            1_700_000_000,
            None,
            None,
            priority,
            &[],
            &[],
            &pubkey,
            &|_| fake_sig,
        )
        .unwrap();

        let decoded = decode_statement(&encoded).unwrap();
        // The priority is stored in the lower 32 bits of the expiry u64
        assert_eq!(decoded.priority, priority);
    }

    #[test]
    fn test_encode_statement_max_topics_succeeds() {
        let topic = [0u8; 32];
        let topics = vec![topic; 4]; // exactly 4 — maximum allowed
        let pubkey = [0u8; 32];
        let result = encode_statement(1_700_000_000, None, None, 0, &topics, b"", &pubkey, &|_| {
            [0u8; 64]
        });
        assert!(result.is_ok());
        let decoded = decode_statement(&result.unwrap()).unwrap();
        assert_eq!(decoded.topics.len(), 4);
    }

    // -----------------------------------------------------------------------
    // blake2b_256_keyed
    // -----------------------------------------------------------------------

    #[test]
    fn test_blake2b_256_keyed_empty_key_returns_error() {
        let result = blake2b_256_keyed(b"", b"data");
        assert!(result.is_err());
        assert!(result
            .unwrap_err()
            .contains("blake2b key must not be empty"));
    }

    #[test]
    fn test_blake2b_256_keyed_64_byte_key_succeeds() {
        let key = [0xabu8; 64];
        let result = blake2b_256_keyed(&key, b"data");
        assert!(result.is_ok());
        // Non-zero digest confirms something was computed.
        assert_ne!(result.unwrap(), [0u8; 32]);
    }

    #[test]
    fn test_blake2b_256_keyed_65_byte_key_returns_error() {
        let key = [0xabu8; 65];
        let result = blake2b_256_keyed(&key, b"data");
        assert!(result.is_err());
        assert!(result
            .unwrap_err()
            .contains("blake2b key length must be 1..=64, got 65"));
    }

    /// Pinned test vector: blake2b_256_keyed(key=[0x01;32], data=b"polkadot").
    ///
    /// The expected value was computed by running `blake2b_256_keyed` and
    /// recording the output; it detects any future regression in the digest.
    #[test]
    fn test_blake2b_256_keyed_pinned_vector() {
        let key = [0x01u8; 32];
        let data = b"polkadot";
        let digest = blake2b_256_keyed(&key, data).unwrap();
        // Computed once and pinned for regression detection.
        let expected: [u8; 32] = [
            0xdc, 0xbc, 0x39, 0xc6, 0x21, 0xe8, 0xc2, 0x0c, 0x84, 0xc1, 0x81, 0x6b, 0x18, 0x3d,
            0x7c, 0xae, 0x76, 0x11, 0x7b, 0x36, 0x16, 0x0c, 0xd3, 0x3f, 0xda, 0x54, 0x8f, 0x91,
            0x14, 0x49, 0x98, 0x05,
        ];
        assert_eq!(
            digest, expected,
            "keyed blake2b digest must match pinned vector"
        );
    }

    // -----------------------------------------------------------------------
    // derive_topic_from_account
    // -----------------------------------------------------------------------

    /// Verifies that redistributing bytes between `context` and `extra`
    /// changes the output — the length prefix prevents concatenation collisions.
    #[test]
    fn test_derive_topic_from_account_domain_separation() {
        let account_id = [0x42u8; 32];
        // "ab" as context + "c" as extra  vs  "a" as context + "bc" as extra.
        // Without the length prefix these would produce identical inputs.
        let t1 = derive_topic_from_account(b"ab", &account_id, b"c");
        let t2 = derive_topic_from_account(b"a", &account_id, b"bc");
        assert_ne!(
            t1, t2,
            "length-prefixed context must prevent domain collision"
        );
    }

    #[test]
    fn test_derive_topic_from_account_deterministic() {
        let account_id = [0x01u8; 32];
        let t1 = derive_topic_from_account(b"ctx", &account_id, b"extra");
        let t2 = derive_topic_from_account(b"ctx", &account_id, b"extra");
        assert_eq!(t1, t2);
        assert_ne!(t1, [0u8; 32]);
    }

    #[test]
    fn test_derive_topic_from_account_empty_extra() {
        let account_id = [0x77u8; 32];
        let result = derive_topic_from_account(b"context", &account_id, b"");
        assert_ne!(result, [0u8; 32]);
    }

    /// Verify that derive_topic_from_account uses SCALE compact encoding for the
    /// context length prefix, matching the deployed iOS protocol.
    ///
    /// For context = "chat-request" (12 bytes), SCALE compact encodes 12 as a
    /// single byte: 12 * 4 = 48 = 0x30.
    #[test]
    fn test_derive_topic_from_account_uses_scale_compact_prefix() {
        let context = b"chat-request";
        let account_id = [0x01u8; 32];
        let extra = 0u64.to_le_bytes();

        // Build the expected preimage manually with SCALE compact prefix.
        let mut expected_input = Vec::new();
        expected_input.push(0x30); // SCALE compact(12) = single byte
        expected_input.extend_from_slice(context);
        expected_input.extend_from_slice(&account_id);
        expected_input.extend_from_slice(&extra);

        let expected = blake2b_256(&expected_input);
        let actual = derive_topic_from_account(context, &account_id, &extra);
        assert_eq!(
            actual, expected,
            "derive_topic_from_account must use SCALE compact prefix (iOS compatibility)"
        );
    }

    #[test]
    fn test_derive_topic_from_account_empty_context() {
        let account_id = [0x01u8; 32];
        let mut expected_input = Vec::new();
        expected_input.push(0x00); // SCALE compact(0)
        expected_input.extend_from_slice(&account_id);
        let expected = blake2b_256(&expected_input);
        let actual = derive_topic_from_account(b"", &account_id, b"");
        assert_eq!(actual, expected);
    }

    #[test]
    fn test_derive_topic_from_account_two_byte_compact_context() {
        // Length 64 triggers 2-byte SCALE compact: (64<<2)|1 = 257 = [0x01, 0x01]
        let context = vec![b'x'; 64];
        let account_id = [0x02u8; 32];
        let mut expected_input = Vec::new();
        expected_input.extend_from_slice(&encode_compact_u32(64));
        expected_input.extend_from_slice(&context);
        expected_input.extend_from_slice(&account_id);
        let expected = blake2b_256(&expected_input);
        let actual = derive_topic_from_account(&context, &account_id, b"");
        assert_eq!(actual, expected);
    }
}