cggmp21.rs/hex_or_bin/hex_or_bin.rs

/// (De)serializes byte arrays as hex-string for human-readable formats (like
/// json) or as raw bytes otherwise
///
/// # Motivation
/// We want byte arrays to be serialized as bytes for binary formats, to keep
/// size of serialized data minimal. For that purpose, [`serde_with::Bytes`]
/// should suffice. However, serializing data as bytes on human-readable formats
/// (like json) is not efficient nor is it human-readable: bytes are serialized
/// as list of integers. Hex encoding is preferred for such formats as it's more
/// compact and readable.
///
/// # Private API
/// `HexOrBin` is shared between several crates in the project, however we do not
/// publicly expose it. Although it works perfectly fine in our case, it may not
/// work well sometimes due to limitations.
///
/// # Limitations
/// * Only works with byte arrays that have staticly-known size. `Array::default()`
///   should return `Array` filled with zeroes
/// * Using `HexOrBin` compiles, but deseralization basically always fails except for
///   deserializing empty arrays
/// * Only defined for arrays that implement [`Default`] trait. Note that `[u8; N]`
///   implements this trait only for limited amount of `N`.
pub struct HexOrBin;

impl<T> serde_with::SerializeAs<T> for HexOrBin
where
    T: AsRef<[u8]>,
{
    fn serialize_as<S>(source: &T, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        if serializer.is_human_readable() {
            serializer.serialize_str(&hex::encode(source))
        } else {
            serializer.serialize_bytes(source.as_ref())
        }
    }
}

impl<'de, T> serde_with::DeserializeAs<'de, T> for HexOrBin
where
    T: Default + AsMut<[u8]>,
{
    fn deserialize_as<D>(deserializer: D) -> Result<T, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        struct Visitor<T> {
            expect_hex: bool,
            out: T,
            _ph: core::marker::PhantomData<T>,
        }
        impl<T> serde::de::Visitor<'_> for Visitor<T>
        where
            T: AsMut<[u8]>,
        {
            type Value = T;

            fn expecting(&self, formatter: &mut core::fmt::Formatter) -> core::fmt::Result {
                if self.expect_hex {
                    formatter.write_str("hex-encoded byte string")
                } else {
                    formatter.write_str("byte string")
                }
            }

            fn visit_bytes<E>(mut self, v: &[u8]) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                if self.expect_hex {
                    return Err(E::invalid_value(
                        serde::de::Unexpected::Bytes(v),
                        &"expected hex-encoded bytes",
                    ));
                }
                let out_len = self.out.as_mut().len();
                if out_len != v.len() {
                    return Err(E::invalid_length(v.len(), &ExpectedLen(out_len)));
                }
                self.out.as_mut().copy_from_slice(v);
                Ok(self.out)
            }

            fn visit_str<E>(mut self, v: &str) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                if !self.expect_hex {
                    return Err(E::invalid_value(
                        serde::de::Unexpected::Str(v),
                        &"expected raw bytes",
                    ));
                }

                hex::decode_to_slice(v, self.out.as_mut()).map_err(E::custom)?;

                Ok(self.out)
            }
        }

        if deserializer.is_human_readable() {
            deserializer.deserialize_str(Visitor {
                expect_hex: true,
                out: Default::default(),
                _ph: Default::default(),
            })
        } else {
            deserializer.deserialize_bytes(Visitor {
                expect_hex: false,
                out: Default::default(),
                _ph: Default::default(),
            })
        }
    }
}

struct ExpectedLen(usize);
impl serde::de::Expected for ExpectedLen {
    fn fmt(&self, formatter: &mut core::fmt::Formatter) -> core::fmt::Result {
        write!(formatter, "{}", self.0)
    }
}