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```// Copyright 2016 The CGMath Developers. For a full listing of the authors,
// refer to the Cargo.toml file at the top-level directory of this distribution.
//
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//
// Unless required by applicable law or agreed to in writing, software
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and

use rand::{Rand, Rng};
use num_traits::cast;

use structure::*;

use approx::ApproxEq;
use quaternion::Quaternion;
use num::BaseFloat;

/// A set of [Euler angles] representing a rotation in three-dimensional space.
///
/// This type is marked as `#[repr(C, packed)]`.
///
/// The axis rotation sequence is XYZ. That is, the rotation is first around
/// the X axis, then the Y axis, and lastly the Z axis (using intrinsic
/// rotations). Since all three rotation axes are used, the angles are
/// Tait–Bryan angles rather than proper Euler angles.
///
/// # Ranges
///
/// - x: [-pi, pi]
/// - y: [-pi/2, pi/2]
/// - z: [-pi, pi]
///
/// # Defining rotations using Euler angles
///
/// Note that while [Euler angles] are intuitive to define, they are prone to
/// [gimbal lock] and are challenging to interpolate between. Instead we
/// recommend that you convert them to a more robust representation, such as a
/// quaternion or or rotation matrix. To this end, `From<Euler<A>>` conversions
/// are provided for the following types:
///
/// - [`Basis3`](struct.Basis3.html)
/// - [`Matrix3`](struct.Matrix3.html)
/// - [`Matrix4`](struct.Matrix4.html)
/// - [`Quaternion`](struct.Quaternion.html)
///
/// For example, to define a quaternion that applies the following:
///
/// 1. a 90° rotation around the _x_ axis
/// 2. a 45° rotation around the _y_ axis
/// 3. a 15° rotation around the _z_ axis
///
/// you can use the following code:
///
/// ```
/// use cgmath::{Deg, Euler, Quaternion};
///
/// let rotation = Quaternion::from(Euler {
///     x: Deg(90.0),
///     y: Deg(45.0),
///     z: Deg(15.0),
/// });
/// ```
///
/// [Euler angles]: https://en.wikipedia.org/wiki/Euler_angles
/// [gimbal lock]: https://en.wikipedia.org/wiki/Gimbal_lock#Gimbal_lock_in_applied_mathematics
/// [convert]: #defining-rotations-using-euler-angles
#[repr(C, packed)]
#[derive(Copy, Clone, Debug)]
#[derive(PartialEq, Eq)]
#[cfg_attr(feature = "rustc-serialize", derive(RustcEncodable, RustcDecodable))]
#[cfg_attr(feature = "eders", derive(Serialize, Deserialize))]
pub struct Euler<A: Angle> {
/// The angle to apply around the _x_ axis. Also known at the _pitch_.
pub x: A,
/// The angle to apply around the _y_ axis. Also known at the _yaw_.
pub y: A,
/// The angle to apply around the _z_ axis. Also known at the _roll_.
pub z: A,
}

impl<A: Angle> Euler<A> {
/// Construct a set of euler angles.
///
/// # Arguments
///
/// * `x` - The angle to apply around the _x_ axis. Also known at the _pitch_.
/// * `y` - The angle to apply around the _y_ axis. Also known at the _yaw_.
/// * `z` - The angle to apply around the _z_ axis. Also known at the _roll_.
pub fn new(x: A, y: A, z: A) -> Euler<A> {
Euler { x: x, y: y, z: z }
}
}

impl<S: BaseFloat> From<Quaternion<S>> for Euler<Rad<S>> {
fn from(src: Quaternion<S>) -> Euler<Rad<S>> {
let sig: S = cast(0.499).unwrap();
let two: S = cast(2).unwrap();
let one: S = cast(1).unwrap();

let (qw, qx, qy, qz) = (src.s, src.v.x, src.v.y, src.v.z);
let (sqw, sqx, sqy, sqz) = (qw * qw, qx * qx, qy * qy, qz * qz);

let unit = sqx + sqz + sqy + sqw;
let test = qx * qz + qy * qw;

// We set x to zero and z to the value, but the other way would work too.
if test > sig * unit {
// x + z = 2 * atan(x / w)
Euler {
}
} else if test < -sig * unit {
// x - z = 2 * atan(x / w)
Euler {
}
} else {
// Using the quat-to-matrix equation from either
// http://www.euclideanspace.com/maths/geometry/rotations/conversions/quaternionToMatrix/index.htm
// or equation 15 on page 7 of
// http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770024290.pdf
// to fill in the equations on page A-2 of the NASA document gives the below.
Euler {
x: Rad::atan2(two * (-qy * qz + qx * qw), one - two * (sqx + sqy)),
y: Rad::asin(two * (qx * qz + qy * qw)),
z: Rad::atan2(two * (-qx * qy + qz * qw), one - two * (sqy + sqz)),
}
}
}
}

impl<A: Angle> ApproxEq for Euler<A> {
type Epsilon = A::Unitless;

#[inline]
fn approx_eq_eps(&self, other: &Euler<A>, epsilon: &A::Unitless) -> bool {
self.x.approx_eq_eps(&other.x, epsilon) &&
self.y.approx_eq_eps(&other.y, epsilon) &&
self.z.approx_eq_eps(&other.z, epsilon)
}
}

impl<A: Angle + Rand> Rand for Euler<A> {
#[inline]
fn rand<R: Rng>(rng: &mut R) -> Euler<A> {
Euler { x: rng.gen(), y: rng.gen(), z: rng.gen() }
}
}
```