Some of the Quaternion Extensions for Unity

QuaternionExtensions.cs

using UnityEngine;

public static class QuaternionExtensions
{
    /// <summary>
    /// Converts rotation into its euler angles representaion (in degrees) in range [-180, 180]. 
    /// </summary>
    /// <param name="q"></param>
    /// <returns></returns>
    public static Vector3 ToEulerAnglesInRange180(this Quaternion q)
    {
        return new Vector3()
        {
            x = WrapAngle(q.eulerAngles.x),
            y = WrapAngle(q.eulerAngles.y),
            z = WrapAngle(q.eulerAngles.z)
        };

        float WrapAngle(float angle)
        {
            angle %= 360;
            return angle > 180 ? angle - 360 : angle;
        }
    }

    public static Quaternion ShortestRotation(Quaternion a, Quaternion b)
    {
        if (Quaternion.Dot(a, b) < 0)

        {
            return a * Quaternion.Inverse(Multiply(b, -1));
        }

        else return a * Quaternion.Inverse(b);
    }

    public static Quaternion Multiply(Quaternion input, float scalar)
    {
        return new Quaternion(input.x * scalar, input.y * scalar, input.z * scalar, input.w * scalar);
    }
}