Json: Efficiency of bytes encoding

[elliottslaughter]

Is serde_json integrated with serde_bytes? I was hoping that using serde_bytes would allow me to encode Vec<u8> somewhat intelligently (e.g. as a base64-encoded string, or similar). However, a very simple test seems to indicate that it doesn't do anything at all:

plain {"c":[123,124,125]}
bytes {"c":[123,124,125]}

This encoding results in an expansion ratio of up to 4:1 (depending on the values of the bytes encoded), rather than 4:3 in a base64 encoding. Needless to say, in some use cases this can be really significant reduction in message size.

The source of the test is below.

#[macro_use]
extern crate serde_derive;
extern crate serde_bytes;
extern crate serde_json;

#[derive(Serialize, Deserialize, Debug)]
pub struct MyPlain {
    c: Vec<u8>
}

#[derive(Serialize, Deserialize, Debug)]
pub struct MyBytes {
    #[serde(with = "serde_bytes")]
    c: Vec<u8>
}

fn main() {
    let plain = MyPlain { c: vec![123, 124, 125] };
    let plain_json = serde_json::to_string(&plain).unwrap();
    println!("plain {}", plain_json);

    let bytes = MyBytes { c: vec![123, 124, 125] };
    let bytes_json = serde_json::to_string(&bytes).unwrap();
    println!("bytes {}", bytes_json);
}

[dtolnay]

The default encoding is not going to change but you can plug in base64 if you want.

#[derive(Serialize, Deserialize, Debug)]
struct MyBytes {
    #[serde(with = "base64")]
    c: Vec<u8>
}

mod base64 {
    extern crate base64;
    use serde::{Serializer, de, Deserialize, Deserializer};

    pub fn serialize<S>(bytes: &[u8], serializer: S) -> Result<S::Ok, S::Error>
        where S: Serializer
    {
        serializer.serialize_str(&base64::encode(bytes))

        // Could also use a wrapper type with a Display implementation to avoid
        // allocating the String.
        //
        // serializer.collect_str(&Base64(bytes))
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<Vec<u8>, D::Error>
        where D: Deserializer<'de>
    {
        let s = <&str>::deserialize(deserializer)?;
        base64::decode(s).map_err(de::Error::custom)
    }
}

I filed https://github.com/alicemaz/rust-base64/issues/46 to consider providing these functions directly in the base64 crate.

[elliottslaughter]

That seems to work, thanks.

[dl00]

what happens if we have 2 serialization libs (for example serde_json and bincode) and we want to encode base64 in json, but not in bincode?

[clarfonthey]

@dl00 that's what Serializer::is_human_readable is for

[anguslees]

To save others from searching, the "Display" version of the above is:

pub fn serialize<S>(bytes: &[u8], serializer: S) -> Result<S::Ok, S::Error>
    where S: Serializer
{
    serializer.collect_str(&base64::display::Base64Display::standard(bytes))
}