use crate::{
    matrices::Matrix4x4,
    tuples::{cross, Tuple},
};

/// Create a matrix representing a eye at `from` looking at `to`, with an `up`
/// as the up vector.
pub fn view_transform(from: Tuple, to: Tuple, up: Tuple) -> Matrix4x4 {
    let forward = (to - from).normalize();
    let left = cross(forward, up.normalize());
    let true_up = cross(left, forward);
    Matrix4x4::new(
        [left.x, left.y, left.z, 0.],
        [true_up.x, true_up.y, true_up.z, 0.],
        [-forward.x, -forward.y, -forward.z, 0.],
        [0., 0., 0., 1.],
    ) * Matrix4x4::translation(-from.x, -from.y, -from.z)
}

#[cfg(test)]
mod tests {
    use crate::{
        matrices::{identity, scaling, translation, Matrix4x4},
        transformations::view_transform,
        tuples::{point, vector},
    };
    #[test]
    fn default_orientation() {
        // The transformation matrix for the default orientation.
        let from = point(0., 0., 0.);
        let to = point(0., 0., -1.);
        let up = vector(0., 1., 0.);
        let t = view_transform(from, to, up);
        assert_eq!(t, identity());
    }
    #[test]
    fn looking_positive_z() {
        // A view transformation matrix looking in positive z direction.
        let from = point(0., 0., 0.);
        let to = point(0., 0., 1.);
        let up = vector(0., 1., 0.);
        let t = view_transform(from, to, up);
        assert_eq!(t, scaling(-1., 1., -1.));
    }
    #[test]
    fn transformation_moves_world() {
        // The view transformation moves the world.
        let from = point(0., 0., 8.);
        let to = point(0., 0., 0.);
        let up = vector(0., 1., 0.);
        let t = view_transform(from, to, up);
        assert_eq!(t, translation(0., 0., -8.));
    }
    #[test]
    fn arbitrary_view() {
        // An arbitrary view transformation.
        let from = point(1., 3., 2.);
        let to = point(4., -2., 8.);
        let up = vector(1., 1., 0.);
        let t = view_transform(from, to, up);
        assert_eq!(
            t,
            Matrix4x4::new(
                [-0.50709, 0.50709, 0.67612, -2.36643],
                [0.76772, 0.60609, 0.12122, -2.82843],
                [-0.35857, 0.59761, -0.71714, 0.],
                [0., 0., 0., 1.],
            )
        );
    }
}