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<TITLE>CLHS: Issue HASH-TABLE-SIZE Writeup</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss189_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss190.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss191_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue HASH-TABLE-SIZE Writeup</H2>
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<PRE><B>Status:</B> Passed, as amended here, Jun 89 X3J13<P>
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<B>Issue:</B> <A REL=DEFINITION HREF="../Body/f_hash_4.htm#hash-table-size"><B>HASH-TABLE-SIZE</B></A><P>
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<B>References:</B> CLtL p.283<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> Version 1, 20-Mar-89, by Moon<P>
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Version 2, 1-Jul-89, by Masinter (per Jun89 X3J13 amendments)<P>
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<P>
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<B>Problem description:<P>
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</B> <P>
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CLtL contradicts itself on the meaning of the :SIZE argument to<P>
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<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A>. At the top of p.283, it says that the size is "the<P>
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maximum number of entries it can hold. Usually the actual capacity of<P>
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the table is somewhat less." At the bottom of the page it says "this<P>
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argument serves as a hint to the implementation of approximately how<P>
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many entries you intend to store." So does the :SIZE intended to be the<P>
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actual capacity of the table, or the amount of storage allocated to hold<P>
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the table. For example, if the implementation of hash tables is<P>
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designed for a loading of 65%, and the user specifies :SIZE 100, does<P>
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the table returned have space allocated for 100 entries, so that it<P>
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overflows and becomes bigger when 65 entries are inserted, or does the<P>
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table have space allocated for 154 entries, so that it overflows and<P>
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becomes bigger when 100 entries are inserted?<P>
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<P>
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<B>Proposal (HASH-TABLE-SIZE:INTENDED-ENTRIES):<P>
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</B><P>
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Believe the bottom of p.283 rather than the top. The :SIZE argument<P>
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is approximately the number of entries that can be inserted without<P>
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the table having to grow.<P>
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<P>
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Keep the first and last sentences of the definition of :REHASH-THRESHOLD<P>
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(CLtL p. 284.) Replace the rest with:<P>
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"This can be a <A REL=DEFINITION HREF="../Body/t_real.htm#real"><B>REAL</B></A> between 0 and 1, inclusive, which is a hint to the<P>
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implementation of the maximum desired hash table occupancy level. An<P>
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implementation is permitted to ignore this argument."<P>
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<P>
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<B>Editorial consequences:<P>
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</B> Remove the second complete paragraph of CLtL p. 283. It's too implementa-<P>
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tion-dependent. Clarify the implementation is not necessarily an actual<P>
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hash table of any particular hashing technique.<P>
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<P>
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<B>Rationale:<P>
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</B> <P>
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The bottom of p.283 is user-oriented, the top is implementation-oriented.<P>
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User-oriented seems more appropriate.<P>
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<P>
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<B>Current practice:<P>
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</B> <P>
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Symbolics Genera 7.4 adheres to <A HREF="iss190.htm">HASH-TABLE-SIZE:INTENDED-ENTRIES</A>.<P>
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Other implementations were not surveyed.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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At worst adding a multiplication to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A>.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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Probably none, but it is hard to predict.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B> <P>
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Implementations will probably vary in which of the two interpretations<P>
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they believe. The language <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> will not be self-consistent.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None of any significance.<P>
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<P>
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<B>Benefits/Esthetics:<P>
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</B><P>
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More self-consistent language.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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None.<P>
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</PRE>
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