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use anyhow::Result;
use frunk_core::hlist::{HCons, HNil};

/// Convenient network layer builder
pub struct NetworkBuilder<T, I>(pub(crate) T, pub(crate) std::marker::PhantomData<I>);

impl<T, I> NetworkBuilder<T, I>
where
    T: DeferredInitializationList,
{
    /// Initializes all layers into a tuple of components
    pub fn build(self) -> Result<T::Output> {
        self.0.initialize()
    }
}

/// Lazy initialization when building layers with [`NetworkBuilder`]
pub trait DeferredInitialization {
    type Initialized;

    fn initialize(self) -> Result<Self::Initialized>;
}

/// List of lazy initializers for [`NetworkBuilder`]
pub trait DeferredInitializationList {
    type Output;

    fn initialize(self) -> Result<Self::Output>;
}

type BaseLayer<T> = HCons<T, HNil>;

impl<T0> DeferredInitializationList for BaseLayer<T0>
where
    T0: DeferredInitialization,
{
    type Output = T0::Initialized;

    fn initialize(self) -> Result<Self::Output> {
        self.head.initialize()
    }
}

impl<T0, T1> DeferredInitializationList for HCons<T1, BaseLayer<T0>>
where
    T0: DeferredInitialization,
    T1: DeferredInitialization,
{
    type Output = (T0::Initialized, T1::Initialized);

    fn initialize(self) -> Result<Self::Output> {
        let t1 = self.head.initialize()?;
        let t0 = self.tail.head.initialize()?;
        Ok((t0, t1))
    }
}

impl<T0, T1, T2> DeferredInitializationList for HCons<T2, HCons<T1, BaseLayer<T0>>>
where
    T0: DeferredInitialization,
    T1: DeferredInitialization,
    T2: DeferredInitialization,
{
    type Output = (T0::Initialized, T1::Initialized, T2::Initialized);

    fn initialize(self) -> Result<Self::Output> {
        let t2 = self.head.initialize()?;
        let t1 = self.tail.head.initialize()?;
        let t0 = self.tail.tail.head.initialize()?;
        Ok((t0, t1, t2))
    }
}

impl<T0, T1, T2, T3> DeferredInitializationList for HCons<T3, HCons<T2, HCons<T1, BaseLayer<T0>>>>
where
    T0: DeferredInitialization,
    T1: DeferredInitialization,
    T2: DeferredInitialization,
    T3: DeferredInitialization,
{
    type Output = (
        T0::Initialized,
        T1::Initialized,
        T2::Initialized,
        T3::Initialized,
    );

    fn initialize(self) -> Result<Self::Output> {
        let t3 = self.head.initialize()?;
        let t2 = self.tail.head.initialize()?;
        let t1 = self.tail.tail.head.initialize()?;
        let t0 = self.tail.tail.tail.head.initialize()?;
        Ok((t0, t1, t2, t3))
    }
}