/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! Machinery to compute distances between animatable values.

use app_units::Au;
use euclid::Size2D;
use std::iter::Sum;
use std::ops::Add;

/// A trait to compute squared distances between two animatable values.
///
/// This trait is derivable with `#[derive(ComputeSquaredDistance)]`. The derived
/// implementation uses a `match` expression with identical patterns for both
/// `self` and `other`, calling `ComputeSquaredDistance::compute_squared_distance`
/// on each fields of the values.
///
/// If a variant is annotated with `#[animation(error)]`, the corresponding
/// `match` arm is not generated.
///
/// If the two values are not similar, an error is returned unless a fallback
/// function has been specified through `#[distance(fallback)]`.
pub trait ComputeSquaredDistance {
    /// Computes the squared distance between two animatable values.
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()>;
}

/// A distance between two animatable values.
#[derive(Clone, Copy, Debug)]
pub enum SquaredDistance {
    /// Represented as the square root of the squared distance.
    Sqrt(f64),
    /// Represented as the squared distance itself.
    Value(f64),
}

impl ComputeSquaredDistance for u16 {
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        Ok(SquaredDistance::Sqrt(((*self as f64) - (*other as f64)).abs()))
    }
}

impl ComputeSquaredDistance for i32 {
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        Ok(SquaredDistance::Sqrt((*self - *other).abs() as f64))
    }
}

impl ComputeSquaredDistance for f32 {
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        Ok(SquaredDistance::Sqrt((*self - *other).abs() as f64))
    }
}

impl ComputeSquaredDistance for f64 {
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        Ok(SquaredDistance::Sqrt((*self - *other).abs()))
    }
}

impl ComputeSquaredDistance for Au {
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        self.0.compute_squared_distance(&other.0)
    }
}

impl<T> ComputeSquaredDistance for Option<T>
    where T: ComputeSquaredDistance
{
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        match (self.as_ref(), other.as_ref()) {
            (Some(this), Some(other)) => this.compute_squared_distance(other),
            (None, None) => Ok(SquaredDistance::Value(0.)),
            _ => Err(()),
        }
    }
}

impl<T> ComputeSquaredDistance for Size2D<T>
    where T: ComputeSquaredDistance
{
    #[inline]
    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
        Ok(self.width.compute_squared_distance(&other.width)? + self.height.compute_squared_distance(&other.height)?)
    }
}

impl SquaredDistance {
    /// Returns the square root of this squared distance.
    pub fn sqrt(self) -> f64 {
        match self {
            SquaredDistance::Sqrt(this) => this,
            SquaredDistance::Value(this) => this.sqrt(),
        }
    }
}

impl From<SquaredDistance> for f64 {
    #[inline]
    fn from(distance: SquaredDistance) -> Self {
        match distance {
            SquaredDistance::Sqrt(this) => this * this,
            SquaredDistance::Value(this) => this,
        }
    }
}

impl Add for SquaredDistance {
    type Output = Self;

    #[inline]
    fn add(self, rhs: Self) -> Self {
        SquaredDistance::Value(f64::from(self) + f64::from(rhs))
    }
}

impl Sum for SquaredDistance {
    fn sum<I>(mut iter: I) -> Self
    where
        I: Iterator<Item = Self>,
    {
        let first = match iter.next() {
            Some(first) => first,
            None => return SquaredDistance::Value(0.),
        };
        iter.fold(first, Add::add)
    }
}