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• Strpos

Strpos-fast

Fast implementation of strpos.

Definitions and Theorems

Function: strpos-fast

(defun strpos-fast (x y n xl yl)
  (declare (type string x)
           (type string y)
           (type integer n)
           (type integer xl)
           (type integer yl)
           (xargs :guard (and (stringp x)
                              (stringp y)
                              (natp xl)
                              (natp yl)
                              (natp n)
                              (<= n (length y))
                              (= xl (length x))
                              (= yl (length y)))))
  (cond ((mbe :logic (prefixp (explode x)
                              (nthcdr n (explode y)))
              :exec (strprefixp-impl (the string x)
                                     (the string y)
                                     (the integer 0)
                                     (the integer n)
                                     (the integer xl)
                                     (the integer yl)))
         (lnfix n))
        ((mbe :logic (zp (- (nfix yl) (nfix n)))
              :exec (int= n yl))
         nil)
        (t (strpos-fast (the string x)
                        (the string y)
                        (+ 1 (lnfix n))
                        (the integer xl)
                        (the integer yl)))))

Theorem: strpos-fast-removal

(defthm strpos-fast-removal
  (implies (and (force (stringp x))
                (force (stringp y))
                (force (natp n))
                (force (equal xl (length x)))
                (force (equal yl (length y))))
           (equal (strpos-fast x y n xl yl)
                  (let ((idx (listpos (explode x)
                                      (nthcdr n (explode y)))))
                    (and idx (+ n idx))))))