http_srv/http/encoding/
stream.rs

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use std::io::Read;

/// Generator function
pub trait Generator<'a,T> : FnMut() -> Option<T> + 'a {}
impl<'a,T,R> Generator<'a,R> for T
where T: FnMut() -> Option<R> + 'a {}

/// Reader that takes a [Generator] function
/// and reads values from it
pub struct StreamReader<'a,T> {
    gen: Box<dyn Generator<'a,T>>,
}

impl<'a,T> StreamReader<'a,T> {
    pub fn new(gen: impl Generator<'a,T>) -> Self {
        let gen = Box::new(gen);
        Self { gen }
    }
}

impl Read for StreamReader<'_,u8> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        for (i,e) in buf.iter_mut().enumerate() {
            match (self.gen)() {
                Some(b) => *e = b,
                None => return Ok(i),
            }
        };
        Ok(buf.len())
    }
}