CLHS: Issue HASH-TABLE-SIZE Writeup

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## Issue HASH-TABLE-SIZE Writeup

    Status: Passed, as amended here, Jun 89 X3J13
    Issue:         HASH-TABLE-SIZE
    References:    CLtL p.283
    Category:      CLARIFICATION
    Edit history:  Version 1, 20-Mar-89, by Moon
    		Version 2, 1-Jul-89, by Masinter (per Jun89 X3J13 amendments)

    Problem description:

      CLtL contradicts itself on the meaning of the :SIZE argument to
      MAKE-HASH-TABLE.  At the top of p.283, it says that the size is "the
      maximum number of entries it can hold.  Usually the actual capacity of
      the table is somewhat less."  At the bottom of the page it says "this
      argument serves as a hint to the implementation of approximately how
      many entries you intend to store."  So does the :SIZE intended to be the
      actual capacity of the table, or the amount of storage allocated to hold
      the table.  For example, if the implementation of hash tables is
      designed for a loading of 65%, and the user specifies :SIZE 100, does
      the table returned have space allocated for 100 entries, so that it
      overflows and becomes bigger when 65 entries are inserted, or does the
      table have space allocated for 154 entries, so that it overflows and
      becomes bigger when 100 entries are inserted?

    Proposal (HASH-TABLE-SIZE:INTENDED-ENTRIES):

      Believe the bottom of p.283 rather than the top.  The :SIZE argument
      is approximately the number of entries that can be inserted without
      the table having to grow.

      Keep the first and last sentences of the definition of :REHASH-THRESHOLD
      (CLtL p. 284.) Replace the rest with:
      "This can be a REAL between 0 and 1, inclusive, which is a hint to the
      implementation of the maximum desired hash table occupancy level. An
      implementation is permitted to ignore this argument."

    Editorial consequences:
      Remove the second complete paragraph of CLtL p. 283. It's too implementa-
      tion-dependent. Clarify the implementation is not necessarily an actual
      hash table of any particular hashing technique.

    Rationale:

      The bottom of p.283 is user-oriented, the top is implementation-oriented.
      User-oriented seems more appropriate.

    Current practice:

      Symbolics Genera 7.4 adheres to HASH-TABLE-SIZE:INTENDED-ENTRIES.
      Other implementations were not surveyed.

    Cost to Implementors:

      At worst adding a multiplication to MAKE-HASH-TABLE.

    Cost to Users:

      Probably none, but it is hard to predict.

    Cost of non-adoption:

      Implementations will probably vary in which of the two interpretations
      they believe.  The language standard will not be self-consistent.

    Performance impact:

      None of any significance.

    Benefits/Esthetics:

      More self-consistent language.

    Discussion:

      None.

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