lighthouse/beacon_chain/utils/ssz/src/decode.rs

use super::LENGTH_BYTES;

#[derive(Debug, PartialEq)]
pub enum DecodeError {
    TooShort,
    TooLong,
}

pub trait Decodable: Sized {
    fn ssz_decode(bytes: &[u8], index: usize) -> Result<(Self, usize), DecodeError>;
}

/// Decode the given bytes for the given type
///
/// The single ssz encoded value will be decoded as the given type at the
/// given index.
pub fn decode_ssz<T>(ssz_bytes: &[u8], index: usize) -> Result<(T, usize), DecodeError>
where
    T: Decodable,
{
    if index >= ssz_bytes.len() {
        return Err(DecodeError::TooShort);
    }
    T::ssz_decode(ssz_bytes, index)
}

/// Decode a vector (list) of encoded bytes.
///
/// Each element in the list will be decoded and placed into the vector.
pub fn decode_ssz_list<T>(ssz_bytes: &[u8], index: usize) -> Result<(Vec<T>, usize), DecodeError>
where
    T: Decodable,
{
    if index + LENGTH_BYTES > ssz_bytes.len() {
        return Err(DecodeError::TooShort);
    };

    // get the length
    let serialized_length = match decode_length(ssz_bytes, index, LENGTH_BYTES) {
        Err(v) => return Err(v),
        Ok(v) => v,
    };

    let final_len: usize = index + LENGTH_BYTES + serialized_length;

    if final_len > ssz_bytes.len() {
        return Err(DecodeError::TooShort);
    };

    let mut tmp_index = index + LENGTH_BYTES;
    let mut res_vec: Vec<T> = Vec::new();

    while tmp_index < final_len {
        match T::ssz_decode(ssz_bytes, tmp_index) {
            Err(v) => return Err(v),
            Ok(v) => {
                tmp_index = v.1;
                res_vec.push(v.0);
            }
        };
    }

    Ok((res_vec, final_len))
}

/// Given some number of bytes, interpret the first four
/// bytes as a 32-bit big-endian integer and return the
/// result.
pub fn decode_length(
    bytes: &[u8],
    index: usize,
    length_bytes: usize,
) -> Result<usize, DecodeError> {
    if bytes.len() < index + length_bytes {
        return Err(DecodeError::TooShort);
    };
    let mut len: usize = 0;
    for (i, byte) in bytes
        .iter()
        .enumerate()
        .take(index + length_bytes)
        .skip(index)
    {
        let offset = (index + length_bytes - i - 1) * 8;
        len |= (*byte as usize) << offset;
    }
    Ok(len)
}

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

    #[test]
    fn test_ssz_decode_length() {
        let decoded = decode_length(&vec![0, 0, 0, 1], 0, LENGTH_BYTES);
        assert_eq!(decoded.unwrap(), 1);

        let decoded = decode_length(&vec![0, 0, 1, 0], 0, LENGTH_BYTES);
        assert_eq!(decoded.unwrap(), 256);

        let decoded = decode_length(&vec![0, 0, 1, 255], 0, LENGTH_BYTES);
        assert_eq!(decoded.unwrap(), 511);

        let decoded = decode_length(&vec![255, 255, 255, 255], 0, LENGTH_BYTES);
        assert_eq!(decoded.unwrap(), 4294967295);
    }

    #[test]
    fn test_encode_decode_length() {
        let params: Vec<usize> = vec![
            0,
            1,
            2,
            3,
            7,
            8,
            16,
            2 ^ 8,
            2 ^ 8 + 1,
            2 ^ 16,
            2 ^ 16 + 1,
            2 ^ 24,
            2 ^ 24 + 1,
            2 ^ 32,
        ];
        for i in params {
            let decoded = decode_length(&encode_length(i, LENGTH_BYTES), 0, LENGTH_BYTES).unwrap();
            assert_eq!(i, decoded);
        }
    }

    #[test]
    fn test_decode_ssz_list() {
        // u16
        let v: Vec<u16> = vec![10, 10, 10, 10];
        let decoded: (Vec<u16>, usize) =
            decode_ssz_list(&vec![0, 0, 0, 8, 0, 10, 0, 10, 0, 10, 0, 10], 0).unwrap();

        assert_eq!(decoded.0, v);
        assert_eq!(decoded.1, 12);

        // u32
        let v: Vec<u32> = vec![10, 10, 10, 10];
        let decoded: (Vec<u32>, usize) = decode_ssz_list(
            &vec![
                0, 0, 0, 16, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10,
            ],
            0,
        ).unwrap();
        assert_eq!(decoded.0, v);
        assert_eq!(decoded.1, 20);

        // u64
        let v: Vec<u64> = vec![10, 10, 10, 10];
        let decoded: (Vec<u64>, usize) = decode_ssz_list(
            &vec![
                0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 10, 0, 0, 0, 0, 0, 0, 0, 10, 0, 0, 0, 0, 0, 0, 0,
                10, 0, 0, 0, 0, 0, 0, 0, 10,
            ],
            0,
        ).unwrap();
        assert_eq!(decoded.0, v);
        assert_eq!(decoded.1, 36);

        // Check that it can accept index
        let v: Vec<usize> = vec![15, 15, 15, 15];
        let decoded: (Vec<usize>, usize) = decode_ssz_list(
            &vec![
                0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 15, 0, 0, 0, 0, 0,
                0, 0, 15, 0, 0, 0, 0, 0, 0, 0, 15, 0, 0, 0, 0, 0, 0, 0, 15,
            ],
            10,
        ).unwrap();
        assert_eq!(decoded.0, v);
        assert_eq!(decoded.1, 46);

        // Check that length > bytes throws error
        let decoded: Result<(Vec<usize>, usize), DecodeError> =
            decode_ssz_list(&vec![0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 15], 0);
        assert_eq!(decoded, Err(DecodeError::TooShort));

        // Check that incorrect index throws error
        let decoded: Result<(Vec<usize>, usize), DecodeError> =
            decode_ssz_list(&vec![0, 0, 0, 0, 0, 0, 0, 15], 16);
        assert_eq!(decoded, Err(DecodeError::TooShort));
    }
}