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basp1/last-set-bit.lisp

Created May 11, 2016 19:07
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last-set-bit.lisp
(defun last-set-bit (value)
(if (zerop value)
-1
(let ((shifted-value (ash value -32)))
(if (not (zerop shifted-value))
(+ 32 (last-set-bit shifted-value))
(flet ((stepf (x b)
(if (>= value (ash 1 x))
(progn
(setf value (ash value (- x)))
(+ b x))
b)))
(stepf 1 (stepf 2 (stepf 4 (stepf 8 (stepf 16 0))))))))))
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