signv4 1.0.0

AWS Signature Version 4 implementation for Rust
Documentation 
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use hmac::{Hmac, Mac, digest::FixedOutput};
use sha2::Sha256;

type HmacSha256 = Hmac<Sha256>;

pub(crate) struct HmacMsg<P>
where
    P: AsRef<[u8]>,
{
    chain: Vec<Vec<P>>,
}

impl<P> HmacMsg<P>
where
    P: AsRef<[u8]>,
{
    pub(crate) fn new(data: Vec<P>) -> Self {
        Self { chain: vec![data] }
    }

    pub(crate) fn segment(&mut self, data: P) -> &mut Self {
        self.chain.push(vec![data]);
        self
    }

    #[allow(unused)]
    fn segment_vec(&mut self, data: Vec<P>) -> &mut Self {
        self.chain.push(data);
        self
    }

    pub(crate) fn finalize_fixed(&self) -> Vec<u8> {
        let res: Option<Vec<u8>> = self.chain.iter().fold(None, |acc, segment| {
            let mut iter = segment.iter();
            let mut hmac = match acc {
                None => {
                    let f = iter.next().unwrap();
                    HmacSha256::new_from_slice(f.as_ref())
                }
                Some(prev) => HmacSha256::new_from_slice(prev.as_ref()),
            }
            .unwrap();
            iter.for_each(|p| hmac.update(p.as_ref()));
            let v = hmac.finalize_fixed().to_vec();
            Some(v)
        });
        res.unwrap()
    }
}