CLHS: Issue HASH-TABLE-REHASH-SIZE-INTEGER Writeup

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## Issue HASH-TABLE-REHASH-SIZE-INTEGER Writeup

    Issue:		HASH-TABLE-REHASH-SIZE-INTEGER
    Reference:	Draft 8.81, p.19-3
    Category:	CLARIFICATION/CHANGE
    Edit History:	Version 1, 06/16/91, Kim Barrett
    		Version 2, 09/26/91, Steve Haflich, add Franz current practice
    		Version 3, 01/10/91, Steve Haflich, Lucid & Chestnut
    			 current practice

    Problem Description:

     The semantics for the :REHASH-SIZE argument to MAKE-HASH-TABLE are unclear.
     The description in the draft says it can be

       "an integer greater than zero, which is the number of entries to add, or it
        can be a floating-point number greater than 1, which is the ratio of the
        new size to the old size."

     When the :REHASH-SIZE argument is an integer, it is unclear whether it is
     expected to be scaled as the size is increased or if it is supposed to
     indicate an expected additional number of entries to add.

     At issue is whether a programmer can use the type of value provided for the
     :REHASH-SIZE argument to give the implementation a hint as to the expected
     growth rate for the table, with an integer indicating an additive growth rate
     and a float indicating a multiplicative growth rate.

    Proposal:

     Specify that if the :REHASH-SIZE argument is an integer then the
     implementation may assume that the expected growth rate for the table is
     additive, and that if the argument is a float then it may assume that the
     expected growth rate is multiplicative.

     Clarify that the value of the :REHASH-SIZE argument does not constrain the
     implementation to use any particular method for computing the new size when
     the hash table is enlarged.  The actual method for computing the new size is
     implementation dependent and the :REHASH-SIZE argument only provides hints
     from the programmer to the implementation.

    Editorial Impact:

     An isolated change to MAKE-HASH-TABLE and HASH-TABLE-REHASH-SIZE.

    Rationale:

     Provides a means for the programmer to reliably provide to the implementor a
     particular piece of information about the programmer's intent, without
     constraining the implementor to any particular implementation technique.

    Current Practice:

     Symbolics Genera and IIM appear to use the :SIZE and integral :REHASH-SIZE
     arguments to produce a ratio which is then used as the effective rehash size
     in the same way as if a float with the same value had been specified for the
     :REHASH-SIZE.

     Lucid, Franz, and Chestnut conform to the interpretation of
     :REHASH-SIZE here suggested.

    Discussion:

     Pitman:
      Only the application programmer knows whether new elements are expected to
      arrive in an additive or multiplicative way.  All the implementation knows at
      the time growth needs to occur is that there isn't room.  It can't tell how
      many extra elements are coming.  And just because the computation on the size
      is allowed to be slightly fuzzy, that doesn't mean it doesn't matter whether
      the input to that computation should be allowed to be fuzzy.

      Control of memory growth is a frequently cited reason for preferring C over
      Lisp.  In some places, fixing the problems that underlie this is virtually a
      research topic because no one can even figure out what they'd want to write
      down in order to advise the program about what to do.  Controlled growth as
      in vector-push-extend or hash-table-rehash-size is not in that camp.  To the
      extent that we have a linguistic facilities begging for the opportunity to be
      properly expressive, I see no reason to be vague--even if we're going to let
      implementations do something a little different than what was asked for, we
      should still define the language in such a way that the implementation at
      least knows what was asked for.  Both of the possible values (integers and
      floats) are potentially meaningful in distinct ways, and trivial to
      implement, so why blur their intent?

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