//! Utility functions to quickly encode and decode `&[u8]` to and from framed messages.
//!
//! Framed messages consist of the following items:
//!
//! ```txt
//! | len: u32 of data.len() | data: binary data |
//! ```
//!
//! The data stored after the length consists of the following items:
//!
//! ```txt
//! | checksum: [u8; 32] sha256 hash of `raw_data` | raw_data: &[u8] |
//! ```


use sha2::{Digest, Sha256};

#[derive(Debug, Clone, thiserror::Error)]
pub enum DecodeError {
    /// There were not enough bytes to determine the length of the data slice.
    #[error("Invalid length: {0}")]
    InvalidLength(std::array::TryFromSliceError),

    /// There were not enough bytes to read a checksum of the data slice.
    #[error("Invalid checksum: {0} bytes")]
    InvalidChecksum(std::array::TryFromSliceError),

    /// There were not enough bytes to read the rest of the data.
    #[error("Incorrect length of internal data: {0}, expected at least: {1}")]
    IncorrectLength(usize, u32),

    /// The provided checksum of the data did not match the locally-generated checksum.
    #[error("Checksum did not match: Their {0} != Our {1}")]
    BadChecksum(String, String),
}

#[derive(Debug, Clone, thiserror::Error)]
pub enum EncodeError {
    /// The given input was larger than could be encoded by this protocol.
    #[error("Input too large to encode: {0}")]
    InputTooLarge(usize),
}

const LEN_SIZE: usize = std::mem::size_of::<u32>();

fn hash(data: &[u8]) -> Vec<u8> {
    let mut hashobj = Sha256::new();
    hashobj.update(data);
    hashobj.finalize().to_vec()
}

/// Encode a given `&[u8]` to a framed message.
///
/// # Errors
/// An error may be returned if the given `data` is more than [`u32::MAX`] bytes. This is a
/// constraint on a protocol level.
pub fn try_encode(data: &[u8]) -> Result<Vec<u8>, EncodeError> {
    let hash = hash(data);
    let content = hash.iter().chain(data.iter()).copied().collect::<Vec<_>>();
    let mut result = (u32::try_from(content.len())
        .map_err(|_| EncodeError::InputTooLarge(content.len()))?)
    .to_be_bytes()
    .to_vec();
    result.extend(content);
    Ok(result)
}

/// Decode a framed message into a `Vec<u8>`.
///
/// # Errors
/// An error may be returned if:
/// * The given `data` does not contain enough data to parse a length,
/// * The given `data` does not contain the given length's worth of data,
/// * The given `data` has a checksum that does not match what we build locally.
pub fn try_decode(data: &[u8]) -> Result<Vec<u8>, DecodeError> {
    // check length and advance data pointer beyond length check
    let len_bytes: [u8; LEN_SIZE] = data[..LEN_SIZE]
        .try_into()
        .map_err(DecodeError::InvalidLength)?;
    let len = u32::from_be_bytes(len_bytes);
    if len as usize + LEN_SIZE > data.len() {
        return Err(DecodeError::IncorrectLength(data.len() - LEN_SIZE, len));
    }
    let data = &data[LEN_SIZE..];

    let checksum: &[u8; 32] = &data[..32]
        .try_into()
        .map_err(DecodeError::InvalidChecksum)?;

    let content = &data[32..];
    let our_checksum = hash(content);
    if our_checksum != checksum {
        return Err(DecodeError::BadChecksum(
            hex::encode(checksum),
            hex::encode(our_checksum),
        ));
    }

    Ok(content.to_vec())
}

#[cfg(test)]
mod tests {
    use super::{try_encode, try_decode, DecodeError};

    #[test]
    fn stable_interface() {
        let data = (0..255).collect::<Vec<u8>>();
        insta::assert_debug_snapshot!(try_encode(&data[..]));
    }

    #[test]
    fn equivalency() -> Result<(), DecodeError> {
        let data = (0..255).collect::<Vec<u8>>();
        assert_eq!(try_decode(&try_encode(&data[..]).unwrap())?, data);
        Ok(())
    }

    #[test]
    fn allows_extra_data() -> Result<(), DecodeError> {
        let data = (0..255).collect::<Vec<u8>>();
        let mut encoded = try_encode(&data[..]).unwrap();
        // Throw on some extra data
        encoded.extend(0..255);
        assert_eq!(try_decode(&try_encode(&data[..]).unwrap())?, data);
        Ok(())
    }

    #[test]
    fn error_on_smaller_data() {
        // Data sliced by 1 byte
        let data = (0..255).collect::<Vec<u8>>();
        let encoded = try_encode(&data[..]).unwrap();
        let error = try_decode(&encoded[..data.len() - 1]);
        assert!(error.is_err());

        // Data includes length and checksum
        let error = try_decode(&encoded[..super::LEN_SIZE + 256 / 8]);
        assert!(error.is_err());

        // Data only includes length
        let data = 12u32.to_be_bytes();
        let error = try_decode(&data[..]);
        assert!(error.is_err());
    }

    #[test]
    fn error_on_invalid_checksum() {
        let data = (0..255).collect::<Vec<u8>>();
        let mut encoded = try_encode(&data[..]).unwrap();
        assert_ne!(encoded[super::LEN_SIZE + 1], 0);
        encoded[super::LEN_SIZE + 1] = 0;

        let error = try_decode(&data[..]);
        assert!(error.is_err());
    }
}