module.exports = rotateX

/**
 * Rotates a quaternion by the given angle about the X axis
 *
 * @param {quat} out quat receiving operation result
 * @param {quat} a quat to rotate
 * @param {number} rad angle (in radians) to rotate
 * @returns {quat} out
 */
function rotateX (out, a, rad) {
  rad *= 0.5

  var ax = a[0], ay = a[1], az = a[2], aw = a[3],
    bx = Math.sin(rad), bw = Math.cos(rad)

  out[0] = ax * bw + aw * bx
  out[1] = ay * bw + az * bx
  out[2] = az * bw - ay * bx
  out[3] = aw * bw - ax * bx
  return out
}