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use std::any::{Any, TypeId};
use std::mem::transmute;

#[derive(Default, Copy, Clone)]
pub struct Dynum;

pub trait DynumAdd<N: Any> {
    type Next;
    fn add(self, x: N) -> Self::Next;
}

pub trait DynumGet {
    fn get<X: Any>(&self) -> &X;
    fn get_mut<X: Any>(&mut self) -> &mut X;
}

#[derive(Default)]
pub struct DynumPair<T: Any, N: DynumGet + Any> {
    head: T,
    next: N,
}

impl<U: DynumGet + Any, N: Any> DynumAdd<N> for U {
    type Next = DynumPair<N, Self>;
    #[inline(always)]
    fn add(self, x: N) -> Self::Next {
        DynumPair {
            head: x,
            next: self,
        }
    }
}

impl<T: Any, U: DynumGet + Any> DynumGet for DynumPair<T, U> {
    #[inline(always)]
    fn get<X: Any>(&self) -> &X {
        let (tid, xid): (u64, u64) = unsafe { transmute((TypeId::of::<T>(), TypeId::of::<X>())) };
        if tid == xid {
            unsafe { transmute(&self.head) }
        } else {
            self.next.get::<X>()
        }
    }
    #[inline(always)]
    fn get_mut<X: Any>(&mut self) -> &mut X {
        let (tid, xid): (u64, u64) = unsafe { transmute((TypeId::of::<T>(), TypeId::of::<X>())) };
        if tid == xid {
            unsafe { transmute(&mut self.head) }
        } else {
            self.next.get_mut::<X>()
        }
    }
}

impl DynumGet for Dynum {
    #[inline(always)]
    fn get<X: Any>(&self) -> &X {
        panic!("get: Unregistered type")
    }
    #[inline(always)]
    fn get_mut<X: Any>(&mut self) -> &mut X {
        panic!("get_mut: Unregistered type")
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn it_works() {
        let a = Dynum.add(0u32).add(2i32);
        assert_eq!(a.get::<i32>(), &2);
    }

    #[test]
    #[should_panic]
    fn it_panics() {
        let a = Dynum.add(0u32).add(2i32);
        a.get::<i64>();
    }
}