;; Copyright 2016-2024
;; Kaz Kylheku <kaz@kylheku.com>
;; Vancouver, Canada
;; All rights reserved.
;;
;; Redistribution and use in source and binary forms, with or without
;; modification, are permitted provided that the following conditions are met:
;;
;; 1. Redistributions of source code must retain the above copyright notice,
;;    this list of conditions and the following disclaimer.
;;
;; 2. Redistributions in binary form must reproduce the above copyright notice,
;;    this list of conditions and the following disclaimer in the documentation
;;    and/or other materials provided with the distribution.
;;
;; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
;; AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
;; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
;; ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
;; LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
;; CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
;; SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
;; INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
;; CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
;; ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
;; POSSIBILITY OF SUCH DAMAGE.

(defmacro tagbody (. forms)
  (when forms
    (let* ((tb-id (gensym "tb-id-"))
           (next-var (gensym "next-"))
           (bblocks (partition forms (op where [orf symbolp integerp chrp])))
           (start-lbl (if bblocks [[orf symbolp integerp chrp] (caar bblocks)])))
      (unless start-lbl
        (push (gensym "entry-") (car bblocks)))
      (if (and (not start-lbl) (not (cdr bblocks)))
        ^(progn nil ,*forms nil)
        (let* ((lbls [mapcar car bblocks])
               (forms [mapcar cdr bblocks])
               ;; This trickery transform the individually labeled form
               ;; blocks into branches, such that each branch falls through
               ;; to the next one thanks to substructure sharing.
               (threaded-1 (mapcar (op member-if true) (conses forms)))
               (threaded-2 [apply nconc forms]) ;; important side effect
               (codes [mapcar car threaded-1]))
          (ignore threaded-2)
          (unless (eql (length (uniq lbls)) (length lbls))
            (throwf 'eval-error "~s: duplicate labels occur" 'tagbody))
          (let* ((basic-code ^(let ((,tb-id (gensym "tb-dyn-id-"))
                                    (,next-var 0))
                                (sys:for-op ()
                                            (,next-var)
                                            ((set ,next-var
                                                  (block* ,tb-id
                                                    (sys:switch ,next-var #(,*codes))
                                                    nil)))))))
            ^(macrolet ((go (:form form label)
                          (let ((index (posql label ',lbls)))
                            (if index ^(return* ,',tb-id ,index) form))))
               ,basic-code)))))))

(defmacro go (label)
  (if [[orf symbolp integerp chrp] label]
    (throwf 'eval-error "~s: no ~s label visible" 'go label)
    (throwf 'eval-error "~s: ~s isn't a symbol, integer or character" 'go label)))


(defmacro prog (vars . body)
  ^(block nil
     (let ,vars (tagbody ,*body))))

(defmacro prog* (vars . body)
  ^(block nil
     (let* ,vars (tagbody ,*body))))