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| 1 | + |
| 2 | +(defun coefficient (time-index freq-index dft-len) |
| 3 | + "Calculates a single twiddle factor for the Fourier Transform." |
| 4 | + (exp (- (/ (* #c( 0 1) 2.0 pi time-index freq-index) |
| 5 | + dft-len)))) |
| 6 | + |
| 7 | +(defun dft (data) |
| 8 | + "Performs the Discrete Fourier Transform" |
| 9 | + (let ((dft-len (length data))) |
| 10 | + (loop for freq-index from 0 below dft-len collect |
| 11 | + (loop for time-index from 0 below dft-len sum |
| 12 | + (* (coefficient time-index freq-index dft-len) (elt data time-index)))))) |
| 13 | + |
| 14 | +(defun merge-sub-ffts (evens odds) |
| 15 | + "Combines the FFTs of the even and odd indices" |
| 16 | + (let* ((fft-length (+ (length evens) (length odds))) |
| 17 | + (indices (loop for i from 0 below (length odds) collect i)) |
| 18 | + (odd-terms (mapcar #'(lambda (v i) (* v (coefficient 1.0 i fft-length))) |
| 19 | + odds |
| 20 | + indices))) |
| 21 | + (concatenate 'list |
| 22 | + (mapcar #'+ evens odd-terms) |
| 23 | + (mapcar #'- evens odd-terms)))) |
| 24 | + |
| 25 | +(defun cooley-tukey-rec (data) |
| 26 | + "Performs the DFT using the recursive Cooley-Tukey method." |
| 27 | + (if (<= (length data) 1) |
| 28 | + data |
| 29 | + (loop for i from 0 below (length data) |
| 30 | + if (evenp i) |
| 31 | + collect (elt data i) into evens |
| 32 | + else |
| 33 | + collect (elt data i) into odds |
| 34 | + finally |
| 35 | + (return (merge-sub-ffts (cooley-tukey-rec evens) |
| 36 | + (cooley-tukey-rec odds)))))) |
| 37 | + |
| 38 | +(defun reverse-bits (value num-bits) |
| 39 | + "Reverses the bits of a value" |
| 40 | + (if (= num-bits 1) |
| 41 | + value |
| 42 | + (let* ((num-low-bits (floor (/ num-bits 2))) |
| 43 | + (num-high-bits (- num-bits num-low-bits)) |
| 44 | + (bit-mask (- (expt 2 num-low-bits) 1)) |
| 45 | + (lower-half (logand value bit-mask)) |
| 46 | + (upper-half (ash value (- num-low-bits)))) |
| 47 | + (logior (ash (reverse-bits lower-half num-low-bits) num-high-bits) |
| 48 | + (reverse-bits upper-half num-high-bits))))) |
| 49 | + |
| 50 | +(defun bit-shuffle-indices (data) |
| 51 | + "Rearanges the elements in a list according to their bit-reversed indices." |
| 52 | + (loop |
| 53 | + with num-bits = (floor (log (length data) 2)) |
| 54 | + for i from 0 below (length data) |
| 55 | + collect (elt data (reverse-bits i num-bits)))) |
| 56 | + |
| 57 | +(defun butterfly (a b coeff) |
| 58 | + "Calculates a single butterfly." |
| 59 | + (values (+ a (* coeff b)) (- a (* coeff b)))) |
| 60 | + |
| 61 | +(defun butterfly-group (data start stride) |
| 62 | + "Calculates a single group of butterflies." |
| 63 | + (dotimes (i stride) |
| 64 | + (let* ((a-index (+ start i)) |
| 65 | + (b-index (+ start i stride)) |
| 66 | + (a (elt data a-index)) |
| 67 | + (b (elt data b-index)) |
| 68 | + (coeff (coefficient 1.0 i (* 2 stride)))) |
| 69 | + (multiple-value-bind (sum difference) (butterfly a b coeff) |
| 70 | + (setf (elt data a-index) sum) |
| 71 | + (setf (elt data b-index) difference))))) |
| 72 | + |
| 73 | +(defun cooley-tukey-tailrec (data stride) |
| 74 | + "Actual iterative implementation of the Cooley-Tukey method." |
| 75 | + (if (>= stride (length data)) |
| 76 | + data |
| 77 | + (progn |
| 78 | + (loop for i from 0 below (length data) by (* 2 stride) do |
| 79 | + (butterfly-group data i stride)) |
| 80 | + (cooley-tukey-tailrec data (* 2 stride))))) |
| 81 | + |
| 82 | +(defun cooley-tukey-iter (data) |
| 83 | + "Performs the DFT using the iterative Cooley-Tukey method." |
| 84 | + (cooley-tukey-tailrec (bit-shuffle-indices data) 1)) |
| 85 | + |
| 86 | +(defun approx-eql (list1 list2) |
| 87 | + (let ((diffs (mapcar #'(lambda (e1 e2) (abs (- e1 e2))) |
| 88 | + list1 |
| 89 | + list2))) |
| 90 | + (loop for d in diffs always (< d 1e-9)))) |
| 91 | + |
| 92 | +(defun test-fft (data) |
| 93 | + (let ((dft-result (dft data)) |
| 94 | + (rec-result (cooley-tukey-rec data)) |
| 95 | + (iter-result (cooley-tukey-iter data))) |
| 96 | + (format T "~&DFT and recursive Cooley-Tukey approx. equal: ~a" |
| 97 | + (approx-eql dft-result rec-result)) |
| 98 | + (format T "~&DFT and iterative Cooley-Tukey approx. equal: ~a" |
| 99 | + (approx-eql dft-result iter-result)) |
| 100 | + (format T "~&Recursive Cooley-Tukey and iterative Cooley-Tukey approx. equal: ~a" |
| 101 | + (approx-eql rec-result iter-result)))) |
| 102 | + |
| 103 | +(test-fft '(0.0 0.25 0.5 0.75 0.0 -0.25 -0.5 -0.75)) |
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