[cmucl-ticket] #5: Hash table entry with key and value of :EMPTY is treated as an empty entry.

[cmucl]

#5: Hash table entry with key and value of :EMPTY is treated as an empty entry.
--------------------+-------------------------------------------------------
 Reporter:  rtoy    |       Owner:  somebody
     Type:  defect  |      Status:  new     
 Priority:  minor   |   Milestone:          
Component:  Core    |     Version:  19c     
 Keywords:          |  
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 {{{
 (defvar *h* (make-hash-table))
 (setf (gethash :empty *h*) :empty)
 (maphash #'(lambda (k v) (format t "~A -> ~A~%" k v)) *h*)
 }}}

 produces no output, but printing *h* indicates the hash table has one
 entry in it.

 This is a bug in how the hash tables indicate an empty slot.  The key-
 value vector uses a key and value of :EMPTY to indicate an empty slot.

 A possible solution:  The second slot in the key-value vector is the
 symbol :EMPTY, but is otherwise not used for anything.  So, instead of
 putting :EMPTY there, we can put a gensym'ed symbol there instead, and
 initialize all the remaining slots of the kv vector to be this symbol.  I
 don't think there's any impact on GC, because if the GC code needs the
 empty symbol, it uses kv_vector![1] to get it.  The Lisp code needs to
 change to initialize the kv-vector appropriately.  maphash and with-hash-
 table-iterator need to use kv_vector![1] instead of :EMPTY to determine if
 a slot is empty or not.

 I think this should work.

-- 
Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/5>
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