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• Base64-impl

B64-encode-last-group-str

String-based version of b64-encode-last-group.

Signature

(b64-encode-last-group-str x n xl) → (mv * * * *)

Arguments

x — Guard (stringp x).

n — Guard (natp n).

xl — Guard (eql xl (length x)).

Definitions and Theorems

Function: b64-encode-last-group-str

(defun b64-encode-last-group-str (x n xl)
  (declare (xargs :guard (and (stringp x)
                              (natp n)
                              (eql xl (length x)))))
  (declare (type (integer 0 *) n xl)
           (type string x))
  (declare (xargs :guard (< n xl)))
  (let ((acl2::__function__ 'b64-encode-last-group-str))
    (declare (ignorable acl2::__function__))
    (b* (((the (integer 0 *) left)
          (mbe :logic (nfix (- (nfix xl) (nfix n)))
               :exec (- xl n)))
         ((when (int= left 1))
          (b* (((mv c1 c2) (b64-enc1 (char x n))))
            (mv c1 c2 #\= #\=)))
         ((when (int= left 2))
          (b* (((mv c1 c2 c3)
                (b64-enc2 (char x n) (char x (+ 1 n)))))
            (mv c1 c2 c3 #\=))))
      (b64-enc3 (char x n)
                (char x (+ 1 n))
                (char x (+ 2 n))))))

Theorem: b64-encode-last-group-str-correct

(defthm b64-encode-last-group-str-correct
  (implies (and (force (stringp x))
                (force (natp n))
                (force (equal xl (length x)))
                (force (< n xl)))
           (equal (b64-encode-last-group-str x n xl)
                  (b64-encode-last-group (nthcdr n (explode x))))))