_lib.rs

#![feature(generators)]

use futures::io::{AsyncReadExt, AsyncRead};

pub fn decode(input: impl AsyncRead) -> impl AsyncRead {
    async_io_macros::async_read! {
        futures::pin_mut!(input);

        loop {
            let mut bytes = [0; 4];
            let len = input.read(&mut bytes).await?;
            if len == 0 {
                break;
            }
            input.read_exact(&mut bytes[len..]).await?;
            for byte in &mut bytes {
                *byte = match *byte {
                    b'A'..=b'Z' => *byte - b'A',
                    b'a'..=b'z' => *byte - b'a' + 26,
                    b'0'..=b'9' => *byte - b'0' + 52,
                    b'+' | b'-' => 62,
                    b'/' | b',' | b'_' => 63,
                    b'=' => b'=',
                    _ => return Err(std::io::Error::new(std::io::ErrorKind::Other, "invalid char")),
                }
            }
            let out = [
                bytes[0] << 2 | bytes[1] >> 4,
                bytes[1] << 4 | bytes[2] >> 2,
                bytes[2] << 6 | bytes[3],
            ];
            let mut out = if bytes[2] == b'=' { &out[..1] } else if bytes[3] == b'=' { &out[..2] } else { &out[..] };
            while !out.is_empty() {
                yield |buffer| {
                    let len = buffer.len().min(out.len());
                    buffer[..len].copy_from_slice(&out[..len]);
                    let (_, tail) = out.split_at(len);
                    out = tail;
                    Ok(len)
                };
            }
        }

        Ok(())
    }
}

#[test]
fn test() {
    let input = b"SGVsbG8gRmVycmlzCg==";
    let decoded = decode(&input[..]);
    futures::pin_mut!(decoded);
    let mut buffer = Vec::new();
    futures::executor::block_on(decoded.read_to_end(&mut buffer)).unwrap();
    assert_eq!(buffer, b"Hello Ferris\n");
}

Cargo.lock

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3

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Cargo.toml

[package]
name = "foo"
version = "0.1.0"
authors = ["Wim Looman <git@nemo157.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
path = "_lib.rs"

[dependencies]
async-io-macros = { git = "https://github.com/Nemo157/async-io-macros" }
futures = "0.3.12"