module.exports = slerp

/**
 * Performs a spherical linear interpolation between two quat
 *
 * @param {quat} out the receiving quaternion
 * @param {quat} a the first operand
 * @param {quat} b the second operand
 * @param {Number} t interpolation amount between the two inputs
 * @returns {quat} out
 */
function slerp (out, a, b, t) {
  // benchmarks:
  //    http://jsperf.com/quaternion-slerp-implementations

  var ax = a[0], ay = a[1], az = a[2], aw = a[3],
    bx = b[0], by = b[1], bz = b[2], bw = b[3]

  var omega, cosom, sinom, scale0, scale1

  // calc cosine
  cosom = ax * bx + ay * by + az * bz + aw * bw
  // adjust signs (if necessary)
  if (cosom < 0.0) {
    cosom = -cosom
    bx = -bx
    by = -by
    bz = -bz
    bw = -bw
  }
  // calculate coefficients
  if ((1.0 - cosom) > 0.000001) {
    // standard case (slerp)
    omega = Math.acos(cosom)
    sinom = Math.sin(omega)
    scale0 = Math.sin((1.0 - t) * omega) / sinom
    scale1 = Math.sin(t * omega) / sinom
  } else {
    // "from" and "to" quaternions are very close
    //  ... so we can do a linear interpolation
    scale0 = 1.0 - t
    scale1 = t
  }
  // calculate final values
  out[0] = scale0 * ax + scale1 * bx
  out[1] = scale0 * ay + scale1 * by
  out[2] = scale0 * az + scale1 * bz
  out[3] = scale0 * aw + scale1 * bw

  return out
}