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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/iss190_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss191.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss192_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-TESTS Writeup</H2>
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<PRE><B>Status:</B> Passed, Jan 89 X3J13<P>
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<B>Issue:</B> <A HREF="iss191.htm">HASH-TABLE-TESTS</A><P>
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<P>
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<B>References:</B> CLtL, p382 (third paragraph), and p383<P>
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Issue <A HREF="iss143.htm">EQUAL-STRUCTURE</A><P>
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Requires Issue: <A HREF="iss086.htm">CONTAGION-ON-NUMERICAL-COMPARISONS</A><P>
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<P>
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<B>Category:</B> Addition<P>
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<P>
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<B>Edit history:</B> 26-Sep-88 Version 1 by JonL<P>
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8-Dec-88 Version 2 by Masinter<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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A great many users try to coalesce two equivalent <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>DEFSTRUCT</B></A> instances,<P>
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or two equivalent pointer arrays, using hash tables; but they are rudely<P>
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awakened when they find out that <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> is not an appropriate test for<P>
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this case, and that there is no :test argument to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A> which <P>
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will "hash on non-tree structures".<P>
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<P>
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Proposal: <A HREF="iss191.htm">HASH-TABLE-TESTS:ADD-EQUALP</A><P>
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<P>
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With the advent of the issue <A HREF="iss086.htm">CONTAGION-ON-NUMERICAL-COMPARISONS</A>, we<P>
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can expect <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> to be a true equivalence function, and thus a suitable<P>
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candidate for the :test function to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A>. Hash-tables will <P>
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come in four kinds, the difference being whether the keys are compared <P>
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with <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A>, <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A>, <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A>, or <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>.<P>
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<P>
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<P>
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<B>Examples:<P>
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</B><P>
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> (<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> foo a b c)<P>
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FOO<P>
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> (<A REL=DEFINITION HREF="../Body/s_setq.htm#setq"><B>setq</B></A> x (make-foo :a 1 :b 'b :c '(1 . 2))<P>
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x-copy (make-foo :a 1 :b 'b :c '(1 . 2)))<P>
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#S(FOO A 1 B B C (1 . 2))<P>
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> (<A REL=DEFINITION HREF="../Body/s_setq.htm#setq"><B>setq</B></A> y #(1 B (1 . 2))<P>
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y-copy (<A REL=DEFINITION HREF="../Body/f_cp_seq.htm#copy-seq"><B>copy-seq</B></A> y))<P>
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#(1 B (1 . 2))<P>
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> (<A REL=DEFINITION HREF="../Body/s_setq.htm#setq"><B>setq</B></A> ht-equal (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test 'equal) <P>
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ht-equalp (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test '<A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>equalp</B></A>))<P>
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<P>
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#<Hash-Table BB1F7B><P>
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> (<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>progn</B></A> (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> x ht-equal) t) (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> x ht-equalp) t) <P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> y ht-equal) t) (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> y ht-equalp) t))<P>
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T<P>
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> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> x-copy ht-equal)<P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> x-copy ht-equalp)<P>
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T<P>
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T<P>
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> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> y-copy ht-equal)<P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> (<A REL=DEFINITION HREF="../Body/f_cp_seq.htm#copy-seq"><B>copy-seq</B></A> y) ht-equalp)<P>
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T<P>
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T<P>
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> <P>
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<P>
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<P>
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<B>Rationale: <P>
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</B><P>
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Implementing hash-tables efficiently is not an easy task; it makes more<P>
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sense for this to be standardly available than for individual programmers <P>
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to keep trying to re-invent this obscure part of technology.<P>
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<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Lucid's release 3.0 implements this proposal [some 2.1-level release<P>
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supported it "provisionally"]. Symbolics implementation is reputed<P>
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to be robust enough to implement this proposal trivially.<P>
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<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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Moderate. Implementors have already dealt with EQUAL; the only tricky <P>
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part will be ensuring the implication:<P>
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"If 'a' is <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> to 'b', then 'a' and 'b' must lie in the<P>
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same collision chain in any given <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> hash table"<P>
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It has been suggested that merely linear searching a table is an acceptable<P>
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implementation technique for CL's hash-tables [although no serious <P>
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implementation limits itself thus] and that such tables have no "collision <P>
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chains"; but in fact, this is the degenerate case wherein all entries are <P>
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in the same collision chain, so the implication is trivially satisfied.<P>
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<P>
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Some persons prefer to say that the "reprobe sequence will be the same for<P>
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the two items", rather than using the term "collision chain"; the meaning <P>
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is the same. <P>
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<P>
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<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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None. This is an entirely upwards-compatible addition.<P>
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<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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Continuing bug reports from users about why "hashing <P>
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doesn't work" when said user tries entering pointer-containing objects<P>
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other than cons cells into hash tables. Continuing delay in same<P>
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user's work until they figure out a new strategy for identifying<P>
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equivalent structures. More difficulty in debugging their alternatives.<P>
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<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Addresses one aspect of the difficult equivalence problem. Makes<P>
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hash tables more useful. Permits case-insensitive hashing<P>
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on strings [tables of type <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> are case-sensitive on strings]; <P>
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another use is to allow = comparison for numbers<P>
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[tables of type <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> use <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> on numbers].<P>
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<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Reduces the discontinuity between basic equivalence functions and those<P>
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usable as equivalence relations in hash-tables.<P>
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<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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With the rejection of all the issues related to <A HREF="iss143.htm">EQUAL-STRUCTURE</A>, there is <P>
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little or no hope that <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> will be "beefed up" to meet the expectations<P>
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of so many of the user community on compound structures. If one wants<P>
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a <A REL=DEFINITION HREF="../Body/t_hash_t.htm#hash-table"><B>hash-table</B></A> with a :test function that has fewer equivalence classes <P>
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(i.e., does more "coalescing"), then there is no alternative now except <P>
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to use the function <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>.<P>
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<P>
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It would also be possible to extend hash tables to allow = or<P>
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<A REL=DEFINITION HREF="../Body/f_stgeq_.htm#stringEQ"><B>STRING=</B></A>, but those are not being proposed at this time.<P>
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</PRE>
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