// Ray-sphere intersection.
// ro: ray origin, rd: ray direction (must be normalized).
// center: sphere center, radius: sphere radius.
// Returns t of the nearest positive intersection, or -1.0 if no hit.

struct RaySphereHit {
    t:   f32,   // distance along ray to nearest hit (negative = miss)
    hit: bool,
};

fn ray_sphere_intersection(ro: vec3f, rd: vec3f,
                           center: vec3f, radius: f32) -> RaySphereHit {
    let oc   = ro - center;
    let b    = dot(oc, rd);
    let c    = dot(oc, oc) - radius * radius;
    let disc = b * b - c;
    if (disc < 0.0) { return RaySphereHit(-1.0, false); }
    let t = -b - sqrt(disc);
    if (t < 0.0)    { return RaySphereHit(-1.0, false); }
    return RaySphereHit(t, true);
}