2-step FFT?

medium-fft.lisp

@@ -0,0 +1,40 @@
+(defconstant +blocking-factor+ 1)
+
+(defun medium-fft (small-fft twiddle size
+                   vec dst tmp ck-factors)
+  (declare (type function small-fft)
+           (type size size)
+           (type complex-sample-array vec dst tmp
+                 twiddle ck-factors))
+  (let ((half-size (ash 1 (truncate (integer-length (1- size))
+                                    2))))
+    (loop for i of-type index below half-size do
+      (loop for j of-type index below half-size
+            for k of-type index from i by half-size
+            do (setf (ref tmp j) (ref vec k)))
+      (funcall small-fft
+               dst tmp twiddle
+               (* i half-size)   0))
+    (map-into dst #'* dst ck-factors)
+    (loop for i of-type index below half-size do
+      (loop for j of-type index below half-size
+            for k of-type index from i by half-size
+            do (setf (ref tmp j) (ref dst k)))
+      (funcall small-fft
+               tmp tmp twiddle
+               half-size 0)
+      (loop for j of-type index from half-size
+            for k of-type index from i by half-size
+            below size
+            do (setf (ref dst k) (ref tmp j))))
+    dst))
+
+(let ((fun (compile nil `(lambda (dst src twiddle startd starts)
+                           ,(gen-fft/n 16))))
+      (twiddle (bordeaux-fft::make-twiddle-factors 16 1))
+      (ck-factors (bordeaux-fft::make-cooley-tuckey-factors 16 16 1))
+      (src *vec*)
+      (dst (make-array 256 :element-type 'complex-sample))
+      (tmp (make-array 256 :element-type 'complex-sample)))
+  (medium-fft fun twiddle 256
+              src dst tmp ck-factors))