midpoint 0.1.5

Extension traits offering implementations of midpoint [currently, only for primitive integers].
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use primitive_promotion::PrimitivePromotionExt as PP;

try_impl_generic_const_fn_for_trait!(
    MidpointViaPrimitivePromotionExt::midpoint_via_primitive_promotion
);

/// Extension trait providing implementation of midpoint algorithm via [primitive promotion][PP].
/// For primitive integers, the result is rounded towards zero.
pub trait MidpointViaPrimitivePromotionExt: PP {
    /// Returns midpoint using algorithm based on [primitive promotion][PP].
    /// For primitive integers, the result is rounded towards zero.
    ///
    /// # Example
    ///
    /// ```
    /// use midpoint::MidpointViaPrimitivePromotionExt;
    ///
    /// let result: i32 = (-3).midpoint_via_primitive_promotion(&-2);
    /// assert_eq!(result, -2);
    /// ```
    #[must_use]
    fn midpoint_via_primitive_promotion(&self /*lhs_ref*/, rhs_ref: &Self) -> Self;
}

macro_rules! impl_midpoint_fn_for_t {
    () => {
        fn midpoint_via_primitive_promotion(&self /*lhs_ref*/, rhs_ref: &Self) -> Self {
            // At the time of writing, explicit dereferencing is necessary because
            // `<&u8 as Add<&u8>>::add` is not yet stable as a const fn
            // and requires `#![feature(const_ops)]`
            //
            // Rust unstable book entry:
            // https://doc.rust-lang.org/beta/unstable-book/library-features/const-ops.html
            let (lhs, rhs) = (
                *self as <Self as PP>::PrimitivePromotion,
                *rhs_ref as <Self as PP>::PrimitivePromotion,
            );
            ((lhs + rhs) / 2) as Self
        }
    };
}

// u128 and i128 don't have a primitive promotion
impl_for_prim_ints_with_prim_promotion!(
    trait = MidpointViaPrimitivePromotionExt,
    fn macro = impl_midpoint_fn_for_t
);

#[cfg(test)]
mod tests {
    use crate::MidpointViaPrimitivePromotionExt;

    #[test]
    fn midpoint_via_primitive_promotion_rounds_towards_zero_including_when_args_are_positive() {
        let result: i32 = 2.midpoint_via_primitive_promotion(&3);
        assert_eq!(result, 2);
    }

    #[test]
    fn midpoint_via_primitive_promotion_rounds_towards_zero_including_when_args_are_negative() {
        let result: i32 = (-3).midpoint_via_primitive_promotion(&-2);
        assert_eq!(result, -2);
    }
}