ggmath/vector/generics/
length.rs

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use std::mem::{transmute, transmute_copy};

use super::*;

/// Sealed trait for ```ScalarCount```s that are valid as vector lengths.
///
/// Vectors can only have lengths 2, 3, or 4 because internally vector fns have differently optimized implementations for each length.
///
/// This trait is implemented by ```ScalarCount<2/3/4>``` and is used to validate that a generic vector length is either 2, 3, or 4 with ```where ScalarCount<N>: VecLen```.
///
/// # Examples
/// ```
/// // Line is generic over dimension count.
/// use ggmath::vec::*;
///
/// struct Line<const N: usize>
/// where
///     ScalarCount<N>: VecLen,
/// {
///     start: VecN<N, f32>,
///     end: VecN<N, f32>,
/// }
///
/// type Line2D = Line<2>;
/// type Line3D = Line<3>;
/// type Line4D = Line<4>;
///
/// // Using an N that isn't 2, 3, or 4. Wont work.
/// struct InvalidStruct {
///     line: Line<6>, // ERROR: the trait bound `ScalarCount<6>: VecLen<6>` is not satisfied
/// }
/// ```
#[allow(private_bounds)]
pub trait VecLen: Seal + Sized + 'static + Send + Sync {
    /// Used by [```Vector```] to determine the inner-type of ```Vector<N, T, VecAligned>```.
    type InnerAlignedVector<T: Scalar>: Construct;
}

/// Count of scalars that may or may not be a [```VecLen```].
///
/// Vectors can only have lengths 2, 3, or 4 because internally vector fns have differently optimized implementations for each length.
///
/// Only ```ScalarCount<2/3/4>``` implements ```VecLen```.
/// this is used to validate that a generic vector length is either 2, 3, or 4 with ```where ScalarCount<N>: VecLen```.
///
/// # Examples
/// ```
/// // Line is generic over dimension count.
/// struct Line<const N: usize>
/// where
///     ScalarCount<N>: VecLen,
/// {
///     start: FVecN<N>,
///     end: FVecN<N>,
/// }
///
/// type Line2D = Line<2>;
/// type Line3D = Line<3>;
/// type Line4D = Line<4>;
///
/// // Using an N that isn't 2, 3, or 4. Wont work.
/// struct InvalidStruct {
///     line: Line<6>, // ERROR: the trait bound `ScalarCount<6>: VecLen<6>` is not satisfied
/// }
/// ```
pub struct ScalarCount<const VALUE: usize>;

pub enum LengthResolvedVector<T: Scalar, A: VecAlignment> {
    Vec2(Vector<2, T, A>),
    Vec3(Vector<3, T, A>),
    Vec4(Vector<4, T, A>),
}
pub enum LengthResolvedVectorRef<'a, T: Scalar, A: VecAlignment> {
    Vec2(&'a Vector<2, T, A>),
    Vec3(&'a Vector<3, T, A>),
    Vec4(&'a Vector<4, T, A>),
}
pub enum LengthResolvedVectorMut<'a, T: Scalar, A: VecAlignment> {
    Vec2(&'a mut Vector<2, T, A>),
    Vec3(&'a mut Vector<3, T, A>),
    Vec4(&'a mut Vector<4, T, A>),
}

impl<const N: usize, T: Scalar, A: VecAlignment> Vector<N, T, A>
where
    ScalarCount<N>: VecLen,
{
    #[inline(always)]
    pub fn resolve_length(self) -> LengthResolvedVector<T, A> {
        unsafe {
            match N {
                2 => LengthResolvedVector::Vec2(transmute_copy(&self)),
                3 => LengthResolvedVector::Vec3(transmute_copy(&self)),
                4 => LengthResolvedVector::Vec4(transmute_copy(&self)),
                n => panic!("invalid vector length: '{n}'"),
            }
        }
    }
    #[inline(always)]
    pub fn resolve_length_ref(&self) -> LengthResolvedVectorRef<T, A> {
        unsafe {
            match N {
                2 => LengthResolvedVectorRef::Vec2(transmute(self)),
                3 => LengthResolvedVectorRef::Vec3(transmute(self)),
                4 => LengthResolvedVectorRef::Vec4(transmute(self)),
                n => panic!("invalid vector length: '{n}'"),
            }
        }
    }
    #[inline(always)]
    pub fn resolve_length_mut(&mut self) -> LengthResolvedVectorMut<T, A> {
        unsafe {
            match N {
                2 => LengthResolvedVectorMut::Vec2(transmute(self)),
                3 => LengthResolvedVectorMut::Vec3(transmute(self)),
                4 => LengthResolvedVectorMut::Vec4(transmute(self)),
                n => panic!("invalid vector length: '{n}'"),
            }
        }
    }

    #[inline(always)]
    pub fn from_resolved_length_fns(
        f_2: impl FnOnce() -> Vector<2, T, A>,
        f_3: impl FnOnce() -> Vector<3, T, A>,
        f_4: impl FnOnce() -> Vector<4, T, A>,
    ) -> Self {
        unsafe {
            match N {
                2 => transmute_copy(&f_2()),
                3 => transmute_copy(&f_3()),
                4 => transmute_copy(&f_4()),
                n => panic!("invalid vector length: '{n}'"),
            }
        }
    }
}

impl VecLen for ScalarCount<2> {
    type InnerAlignedVector<T: Scalar> = <T as ScalarInnerAlignedVecs>::InnerAlignedVec2;
}
impl VecLen for ScalarCount<3> {
    type InnerAlignedVector<T: Scalar> = T::InnerAlignedVec4;
}
impl VecLen for ScalarCount<4> {
    type InnerAlignedVector<T: Scalar> = T::InnerAlignedVec4;
}

trait Seal {}
impl<const N: usize> Seal for ScalarCount<N> {}