kcode-k1-transaction 0.1.1

use kcode_k1_transaction_store::TxId;

pub const SUBSYSTEM_BYTES: usize = 20;
pub const PUBLIC_KEY_BYTES: usize = 32;
pub const SIGNATURE_BYTES: usize = 64;
pub const MIN_TRANSACTION_BYTES: usize = 136;
pub const GENESIS_PARENT: TxId = TxId::from_bytes([0xff; 12]);

#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct SubsystemId {
    bytes: [u8; SUBSYSTEM_BYTES],
}

impl SubsystemId {
    pub fn from_bytes(bytes: [u8; SUBSYSTEM_BYTES]) -> Result<Self, String> {
        std::str::from_utf8(&bytes).map_err(|error| error.to_string())?;
        Ok(Self { bytes })
    }

    #[allow(clippy::should_implement_trait)]
    pub fn from_str(value: &str) -> Result<Self, String> {
        let bytes: [u8; SUBSYSTEM_BYTES] = value
            .as_bytes()
            .try_into()
            .map_err(|_| "subsystem ID must contain exactly 20 bytes".to_owned())?;
        Self::from_bytes(bytes)
    }

    pub const fn as_bytes(&self) -> &[u8; SUBSYSTEM_BYTES] {
        &self.bytes
    }

    pub fn as_str(&self) -> &str {
        std::str::from_utf8(&self.bytes).expect("SubsystemId always contains valid UTF-8")
    }
}

pub struct Transaction<'a> {
    bytes: &'a [u8],
    subsystem: SubsystemId,
}

impl<'a> Transaction<'a> {
    pub fn parse(bytes: &'a [u8]) -> Result<Self, String> {
        if bytes.len() < MIN_TRANSACTION_BYTES {
            return Err(format!(
                "transaction must contain at least {MIN_TRANSACTION_BYTES} bytes"
            ));
        }

        let subsystem_bytes = bytes[52..72]
            .try_into()
            .map_err(|_| "invalid subsystem ID".to_owned())?;
        let subsystem = SubsystemId::from_bytes(subsystem_bytes)?;
        Ok(Self { bytes, subsystem })
    }

    pub fn parent(&self) -> TxId {
        TxId::from_bytes(self.bytes[..12].try_into().expect("fixed parent range"))
    }

    pub fn timestamp(&self) -> u64 {
        u64::from_le_bytes(
            self.bytes[12..20]
                .try_into()
                .expect("fixed timestamp range"),
        )
    }

    pub fn creator(&self) -> &[u8; PUBLIC_KEY_BYTES] {
        self.bytes[20..52].try_into().expect("fixed creator range")
    }

    pub fn subsystem(&self) -> SubsystemId {
        self.subsystem
    }

    pub fn payload(&self) -> &'a [u8] {
        &self.bytes[72..self.bytes.len() - SIGNATURE_BYTES]
    }

    pub fn signature(&self) -> &'a [u8; SIGNATURE_BYTES] {
        self.bytes[self.bytes.len() - SIGNATURE_BYTES..]
            .try_into()
            .expect("fixed signature range")
    }

    pub fn signing_bytes(&self) -> &'a [u8] {
        &self.bytes[..self.bytes.len() - SIGNATURE_BYTES]
    }
}

pub fn build_signed_transaction<F>(
    parent: TxId,
    timestamp: u64,
    creator: [u8; PUBLIC_KEY_BYTES],
    subsystem: SubsystemId,
    payload: &[u8],
    signer: F,
) -> Result<Vec<u8>, String>
where
    F: FnOnce(&[u8]) -> Result<[u8; SIGNATURE_BYTES], String>,
{
    let final_len = MIN_TRANSACTION_BYTES
        .checked_add(payload.len())
        .ok_or_else(|| "transaction length exceeds usize".to_owned())?;
    let mut bytes = Vec::with_capacity(final_len);

    bytes.extend_from_slice(parent.as_bytes());
    bytes.extend_from_slice(&timestamp.to_le_bytes());
    bytes.extend_from_slice(&creator);
    bytes.extend_from_slice(subsystem.as_bytes());
    bytes.extend_from_slice(payload);

    let signature = signer(&bytes)?;
    bytes.extend_from_slice(&signature);

    debug_assert_eq!(bytes.len(), final_len);
    debug_assert_eq!(bytes.capacity(), final_len);
    Ok(bytes)
}

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

    fn transaction(payload: &[u8], subsystem: [u8; SUBSYSTEM_BYTES]) -> Vec<u8> {
        let mut bytes = Vec::new();
        bytes.extend_from_slice(&[1; 12]);
        bytes.extend_from_slice(&0x0102_0304_0506_0708_u64.to_le_bytes());
        bytes.extend_from_slice(&[2; PUBLIC_KEY_BYTES]);
        bytes.extend_from_slice(&subsystem);
        bytes.extend_from_slice(payload);
        bytes.extend_from_slice(&[3; SIGNATURE_BYTES]);
        bytes
    }

    #[test]
    fn parses_exact_fields_and_empty_payload() {
        let subsystem = *b"12345678901234567890";
        let bytes = transaction(&[], subsystem);
        let parsed = Transaction::parse(&bytes).unwrap();

        assert_eq!(parsed.parent().into_bytes(), [1; 12]);
        assert_eq!(parsed.timestamp(), 0x0102_0304_0506_0708);
        assert_eq!(parsed.creator(), &[2; PUBLIC_KEY_BYTES]);
        assert_eq!(parsed.subsystem().as_bytes(), &subsystem);
        assert_eq!(parsed.payload(), &[]);
        assert_eq!(parsed.signature(), &[3; SIGNATURE_BYTES]);
        assert_eq!(parsed.signing_bytes(), &bytes[..72]);
        assert_eq!(bytes.len(), MIN_TRANSACTION_BYTES);
    }

    #[test]
    fn preserves_payload_signature_and_signing_prefix() {
        let bytes = transaction(b"payload", *b"abcdefghijklmnopqrst");
        let parsed = Transaction::parse(&bytes).unwrap();

        assert_eq!(parsed.payload(), b"payload");
        assert_eq!(parsed.signature(), &[3; SIGNATURE_BYTES]);
        assert_eq!(
            parsed.signing_bytes(),
            &bytes[..bytes.len() - SIGNATURE_BYTES]
        );
    }

    #[test]
    fn builds_exact_signed_transaction_and_calls_signer_once() {
        let parent = TxId::from_bytes([9; 12]);
        let timestamp = 0x0102_0304_0506_0708;
        let creator = [7; PUBLIC_KEY_BYTES];
        let subsystem = SubsystemId::from_bytes(*b"abcdefghijklmnopqrst").unwrap();
        let payload = b"payload";
        let calls = Cell::new(0);

        let bytes =
            build_signed_transaction(parent, timestamp, creator, subsystem, payload, |prefix| {
                calls.set(calls.get() + 1);

                let mut expected = Vec::new();
                expected.extend_from_slice(parent.as_bytes());
                expected.extend_from_slice(&timestamp.to_le_bytes());
                expected.extend_from_slice(&creator);
                expected.extend_from_slice(subsystem.as_bytes());
                expected.extend_from_slice(payload);
                assert_eq!(prefix, expected);

                Ok([5; SIGNATURE_BYTES])
            })
            .unwrap();

        assert_eq!(calls.get(), 1);
        assert_eq!(bytes.len(), MIN_TRANSACTION_BYTES + payload.len());
        assert_eq!(bytes.capacity(), bytes.len());
        assert_eq!(
            &bytes[bytes.len() - SIGNATURE_BYTES..],
            &[5; SIGNATURE_BYTES]
        );

        let parsed = Transaction::parse(&bytes).unwrap();
        assert_eq!(parsed.parent(), parent);
        assert_eq!(parsed.timestamp(), timestamp);
        assert_eq!(parsed.creator(), &creator);
        assert_eq!(parsed.subsystem(), subsystem);
        assert_eq!(parsed.payload(), payload);
        assert_eq!(parsed.signature(), &[5; SIGNATURE_BYTES]);
    }

    #[test]
    fn builds_empty_payload_transaction() {
        let subsystem = SubsystemId::from_bytes(*b"12345678901234567890").unwrap();
        let bytes = build_signed_transaction(
            GENESIS_PARENT,
            0,
            [0; PUBLIC_KEY_BYTES],
            subsystem,
            &[],
            |_| Ok([0; SIGNATURE_BYTES]),
        )
        .unwrap();

        assert_eq!(bytes.len(), MIN_TRANSACTION_BYTES);
        assert!(Transaction::parse(&bytes).unwrap().payload().is_empty());
    }

    #[test]
    fn propagates_signer_error_unchanged() {
        let subsystem = SubsystemId::from_bytes(*b"12345678901234567890").unwrap();
        let calls = Cell::new(0);

        let result = build_signed_transaction(
            GENESIS_PARENT,
            0,
            [0; PUBLIC_KEY_BYTES],
            subsystem,
            b"payload",
            |_| {
                calls.set(calls.get() + 1);
                Err("signing failed exactly".to_owned())
            },
        );

        assert_eq!(calls.get(), 1);
        assert_eq!(result.unwrap_err(), "signing failed exactly");
    }

    #[test]
    fn rejects_short_transactions() {
        assert!(Transaction::parse(&[0; MIN_TRANSACTION_BYTES - 1]).is_err());
    }

    #[test]
    fn accepts_exact_multibyte_utf8_subsystem() {
        let value = "éééééééééé";
        let subsystem = SubsystemId::from_str(value).unwrap();

        assert_eq!(subsystem.as_str(), value);
        assert_eq!(subsystem.as_bytes().len(), SUBSYSTEM_BYTES);
    }

    #[test]
    fn rejects_wrong_subsystem_byte_length() {
        assert!(SubsystemId::from_str("short").is_err());
        assert!(SubsystemId::from_str("123456789012345678901").is_err());
    }

    #[test]
    fn rejects_invalid_subsystem_utf8() {
        assert!(SubsystemId::from_bytes([0xff; SUBSYSTEM_BYTES]).is_err());
    }

    #[test]
    fn rejects_transaction_with_invalid_subsystem_utf8() {
        let bytes = transaction(&[], [0xff; SUBSYSTEM_BYTES]);

        assert!(Transaction::parse(&bytes).is_err());
    }

    #[test]
    fn exposes_genesis_sentinel() {
        assert_eq!(GENESIS_PARENT.into_bytes(), [0xff; 12]);
    }
}