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/// Distance between singletons.
#[allow(clippy::len_without_is_empty)]
pub trait Measure<U = Self> {
    type Len: Default
        + std::ops::Add<Output = Self::Len>
        + std::ops::Sub<Output = Self::Len>
        + PartialEq;

    /// Returns the length of the given element.
    fn len(&self) -> Self::Len;

    /// Returns the distance to the given other element.
    fn distance(&self, other: &U) -> Self::Len;
}

impl Measure for char {
    type Len = u64;

    fn len(&self) -> u64 {
        1
    }

    fn distance(&self, other: &char) -> u64 {
        let mut a = *self as u64;
        let mut b = *other as u64;

        if a > b {
            std::mem::swap(&mut a, &mut b);
        }

        if (..=0xd7ff).contains(&a) && (0xe000..).contains(&b) {
            (b - 0xd000 + 1) + (0xd7ff - a)
        } else {
            b - a
        }
    }
}

macro_rules! impl_measure {
    // Measure for type `$ty`.
    // `$cast` is a type that can handle the subtraction of two elements
    // without overflowing.
    // `$len` is a type that can handle the size of the entire domain of `$ty`.
    (@refl $ty:ty, $cast:ty, $len:ty) => {
        impl_measure!($ty, $ty, $cast, $len);
    };
    (@both $ty1:ty, $ty2:ty, $cast:ty, $len:ty) => {
        impl_measure!($ty1, $ty2, $cast, $len);
        impl_measure!($ty2, $ty1, $cast, $len);
    };
    ($ty1:ty, $ty2:ty, $cast:ty, $len:ty) => {
        impl Measure<$ty2> for $ty1 {
            type Len = $len;

            fn len(&self) -> $len {
                1
            }

            fn distance(&self, other: &$ty2) -> $len {
                let a = *self as $cast;
                let b = *other as $cast;

                if a > b {
                    (a - b) as $len
                } else {
                    (b - a) as $len
                }
            }
        }
    };
}

// All of those are generated by the `generate-measures.rb` script
// to avoid mistakes.
impl_measure!(@refl u8, u8, u16);
impl_measure!(@refl u16, u16, u32);
impl_measure!(@refl u32, u32, u64);
impl_measure!(@refl u64, u64, u128);
impl_measure!(@refl i8, i16, u8);
impl_measure!(@refl i16, i32, u16);
impl_measure!(@refl i32, i64, u32);
impl_measure!(@refl i64, i128, u64);
impl_measure!(@both u8, u16, u16, u32);
impl_measure!(@both u8, u32, u32, u64);
impl_measure!(@both u8, u64, u64, u128);
impl_measure!(@both u8, i8, i16, u16);
impl_measure!(@both u8, i16, i32, u16);
impl_measure!(@both u8, i32, i64, u32);
impl_measure!(@both u8, i64, i128, u64);
impl_measure!(@both u16, u32, u32, u64);
impl_measure!(@both u16, u64, u64, u128);
impl_measure!(@both u16, i8, i32, u32);
impl_measure!(@both u16, i16, i32, u32);
impl_measure!(@both u16, i32, i64, u32);
impl_measure!(@both u16, i64, i128, u64);
impl_measure!(@both u32, u64, u64, u128);
impl_measure!(@both u32, i8, i64, u64);
impl_measure!(@both u32, i16, i64, u64);
impl_measure!(@both u32, i32, i64, u64);
impl_measure!(@both u32, i64, i128, u64);
impl_measure!(@both u64, i8, i128, u128);
impl_measure!(@both u64, i16, i128, u128);
impl_measure!(@both u64, i32, i128, u128);
impl_measure!(@both u64, i64, i128, u128);
impl_measure!(@both i8, i16, i32, u16);
impl_measure!(@both i8, i32, i64, u32);
impl_measure!(@both i8, i64, i128, u64);
impl_measure!(@both i16, i32, i64, u32);
impl_measure!(@both i16, i64, i128, u64);
impl_measure!(@both i32, i64, i128, u64);
#[cfg(target_pointer_width = "8")]
impl_measure!(@refl usize, u8, u16);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, u8, u8, u16);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, u16, u16, u32);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, u32, u32, u64);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, u64, u64, u128);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, i8, i16, u16);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, i16, i32, u16);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, i32, i64, u32);
#[cfg(target_pointer_width = "8")]
impl_measure!(@both usize, i64, i128, u64);
#[cfg(target_pointer_width = "16")]
impl_measure!(@refl usize, u16, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, u8, u16, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, u16, u16, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, u32, u32, u64);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, u64, u64, u128);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, i8, i32, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, i16, i32, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, i32, i64, u32);
#[cfg(target_pointer_width = "16")]
impl_measure!(@both usize, i64, i128, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@refl usize, u32, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, u8, u32, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, u16, u32, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, u32, u32, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, u64, u64, u128);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, i8, i64, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, i16, i64, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, i32, i64, u64);
#[cfg(target_pointer_width = "32")]
impl_measure!(@both usize, i64, i128, u64);
#[cfg(target_pointer_width = "64")]
impl_measure!(@refl usize, u64, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, u8, u64, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, u16, u64, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, u32, u64, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, u64, u64, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, i8, i128, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, i16, i128, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, i32, i128, u128);
#[cfg(target_pointer_width = "64")]
impl_measure!(@both usize, i64, i128, u128);

macro_rules! impl_f_measure {
    ($ty:ty, $zero:expr, $min:expr, $max:expr) => {
        impl Measure<$ty> for $ty {
            type Len = $ty;

            fn len(&self) -> $ty {
                $zero
            }

            fn distance(&self, other: &$ty) -> $ty {
                if self.is_infinite() || other.is_infinite() {
                    $max
                } else {
                    let a = *self as $ty;
                    let b = *other as $ty;

                    if a > b {
                        (a - b) as $ty
                    } else {
                        (b - a) as $ty
                    }
                }
            }
        }
    };
}

impl_f_measure!(f32, 0.0f32, f32::NEG_INFINITY, f32::INFINITY);
impl_f_measure!(f64, 0.0f64, f64::NEG_INFINITY, f64::INFINITY);

#[cfg(feature = "ordered-float")]
mod ordered_float {
    use super::{Measure, PartialEnum};
    use ordered_float::NotNan;

    impl_f_measure!(
        NotNan<f32>,
        unsafe { NotNan::new_unchecked(0.0f32) },
        unsafe { NotNan::new_unchecked(f32::NEG_INFINITY) },
        unsafe { NotNan::new_unchecked(f32::INFINITY) }
    );

    impl_f_measure!(
        NotNan<f64>,
        unsafe { NotNan::new_unchecked(0.0f64) },
        unsafe { NotNan::new_unchecked(f64::NEG_INFINITY) },
        unsafe { NotNan::new_unchecked(f64::INFINITY) }
    );
}