parse-number.lisp

;; here is a funky little function to parse a string representing a
;; number with any base (radix) from 2 upto 36 (0-9+a-z)
;;
;; it can even do floats in alternate bases!!
;; so "0.1" in binary = "0.5" in decimal! :-D
;; (2e(-1) = 1/2 = 0.5)

;; one note: you *need* split-sequence to use this function,
;; everything else is vanilla common lisp
(defun parse-number (str &optional (radix 10))
  (labels ((to-nums (lst)
            (mapcar (lambda (n) (parse-integer (string n) :radix radix))
              lst))
           (gt-0 (lst)
              (if (null lst)
                  0
                  (parse-integer (concatenate 'string lst) :radix radix)))
           (lt-0 (lst)
              (if (null lst)
                  0
                  (/ (reduce (lambda (l h)
                                (+ h (/ l radix)))
                        (nreverse (to-nums lst)))
                     radix))))
    (let* ((lsts (split-sequence "." (concatenate 'list str)))
       (val  (if (and (caar lsts) (char= #\- (caar lsts)))
           (- (+ (gt-0 (cdar lsts)) ;; (cdr (first lsts))
                 (lt-0 (second lsts))))
           (+ (gt-0 (first lsts))
              (lt-0 (second lsts))))))
    (if place
      (float (* (/ place) (round val (/ place))))
      (float val)))))