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use na::{self, Real, Vector3};

use query::{Ray, RayCast, RayIntersection};
use shape::Triangle;
use math::{Isometry, Point};

impl<N: Real> RayCast<N> for Triangle<N> {
    #[inline]
    fn toi_and_normal_with_ray(
        &self,
        m: &Isometry<N>,
        ray: &Ray<N>,
        _: bool,
    ) -> Option<RayIntersection<N>> {
        let ls_ray = ray.inverse_transform_by(m);
        let res = triangle_ray_intersection(self.a(), self.b(), self.c(), &ls_ray);

        res.map(|(mut r, _)| {
            r.normal = m * r.normal;
            r
        })
    }
}

/// Computes the intersection between a triangle and a ray.
///
/// If an intersection is found, the time of impact, the normal and the barycentric coordinates of
/// the intersection point are returned.
pub fn triangle_ray_intersection<N: Real>(
    a: &Point<N>,
    b: &Point<N>,
    c: &Point<N>,
    ray: &Ray<N>,
) -> Option<(RayIntersection<N>, Vector3<N>)> {
    let ab = *b - *a;
    let ac = *c - *a;

    // normal
    let n = ab.cross(&ac);
    let d = na::dot(&n, &ray.dir);

    // the normal and the ray direction are parallel
    if d.is_zero() {
        return None;
    }

    let ap = ray.origin - *a;
    let t = na::dot(&ap, &n);

    // the ray does not intersect the plane defined by the triangle
    if (t < na::zero() && d < na::zero()) || (t > na::zero() && d > na::zero()) {
        return None;
    }

    let d = d.abs();

    //
    // intersection: compute barycentric coordinates
    //
    let e = -ray.dir.cross(&ap);

    let mut v;
    let mut w;
    let toi;
    let normal;

    if t < na::zero() {
        v = -na::dot(&ac, &e);

        if v < na::zero() || v > d {
            return None;
        }

        w = na::dot(&ab, &e);

        if w < na::zero() || v + w > d {
            return None;
        }

        let invd = na::one::<N>() / d;
        toi = -t * invd;
        normal = -na::normalize(&n);
        v = v * invd;
        w = w * invd;
    } else {
        v = na::dot(&ac, &e);

        if v < na::zero() || v > d {
            return None;
        }

        w = -na::dot(&ab, &e);

        if w < na::zero() || v + w > d {
            return None;
        }

        let invd = na::one::<N>() / d;
        toi = t * invd;
        normal = na::normalize(&n);
        v = v * invd;
        w = w * invd;
    }

    Some((
        RayIntersection::new(toi, normal),
        Vector3::new(-v - w + na::one(), v, w),
    ))
}