idft   xtlang

;; complex in -> real out
(bind-func idft
  (lambda (in:Complexd* out:double* len)
    (let ((n:i64 0) (k:i64 0) (tmp 0.0) (cpx:Complexd* null) (N (i64tod len)))
      (dotimes (n len)
        (pset! out n 0.0)
        (dotimes (k len)
          (set! cpx (pref-ptr in k))
          (set! tmp (* (i64tod (* k n)) (/ TWOPI N)))
          (pset! out n
                 (+ (pref out n)
                    (- (* (tref cpx 0) (cos tmp))
                       (* (tref cpx 1) (sin tmp))))))
        ;; scale
        (pset! out n (/ (pref out n) N)))
      void)))