13 lines
539 B
Common Lisp
13 lines
539 B
Common Lisp
(defun mid3 (n)
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(let ((a (abs n))
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(hmd)) ; how many digits
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(labels ((give (fmt &optional x y) (return-from mid3 (format nil fmt x y)))
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(need (x) (give "Need ~a digits, not ~d." x hmd))
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(nbr (n) (give "~3,'0d" n)))
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(when (zerop n) (give "Zero is 1 digit"))
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(setq hmd (truncate (1+ (log a 10))))
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(cond ((< hmd 3) (need "3+"))
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((= hmd 3) (nbr a))
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((evenp hmd) (need "odd number of"))
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(t (nbr (mod (truncate a (expt 10 (/ (- hmd 3) 2))) 1000)))))))
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