use crate::header::HeaderMap;

// xor-shift
#[cfg(not(target_arch = "wasm32"))]
pub(crate) fn fast_random() -> u64 {
    use std::cell::Cell;
    use std::collections::hash_map::RandomState;
    use std::hash::{BuildHasher, Hasher};
    use std::num::Wrapping;

    thread_local! {
        static RNG: Cell<Wrapping<u64>> = Cell::new(Wrapping(seed()));
    }

    fn seed() -> u64 {
        let seed = RandomState::new();

        let mut out = 0;
        let mut cnt = 0;
        while out == 0 {
            cnt += 1;
            let mut hasher = seed.build_hasher();
            hasher.write_usize(cnt);
            out = hasher.finish();
        }
        out
    }

    RNG.with(|rng| {
        let mut n = rng.get();
        debug_assert_ne!(n.0, 0);
        n ^= n >> 12;
        n ^= n << 25;
        n ^= n >> 27;
        rng.set(n);
        n.0.wrapping_mul(0x2545_f491_4f6c_dd1d)
    })
}

pub(crate) fn replace_headers(dst: &mut HeaderMap, src: HeaderMap) {
    // IntoIter of HeaderMap yields (Option<HeaderName>, HeaderValue).
    // The first time a name is yielded, it will be Some(name), and if
    // there are more values with the same name, the next yield will be
    // None.
    //
    // TODO: a complex exercise would be to optimize this to only
    // require 1 hash/lookup of the key, but doing something fancy
    // with header::Entry...

    let mut prev_name = None;
    for (key, value) in src {
        match key {
            Some(key) => {
                dst.insert(key.clone(), value);
                prev_name = Some(key);
            }
            None => match prev_name {
                Some(ref key) => {
                    dst.append(key.clone(), value);
                }
                None => unreachable!("HeaderMap::into_iter yielded None first"),
            },
        }
    }
}