477 lines
10 KiB
HTML
477 lines
10 KiB
HTML
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<!DOCTYPE HTML>
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<HTML LANG="en-us"
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><HEAD
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><TITLE
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>sxhash | Common Lisp Nova Spec</TITLE
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><META CHARSET="US-ASCII"
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><LINK REL="canonical" HREF="f_sxhash.html"
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><LINK REL="next" HREF="19_Filenames.html" TYPE="text/html" TITLE="19. Filenames"
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><LINK REL="prev" HREF="f_clrhash.html" TYPE="text/html" TITLE="clrhash"
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><LINK REL="up" HREF="18_2_Hash_Tables_Dictionary.html" TYPE="text/html" TITLE="18.2 Hash Tables Dictionary"
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><LINK REL="start" HREF="index.html" TYPE="text/html" TITLE="Common Lisp Nova Spec"
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><META NAME="VIEWPORT" CONTENT="width=device-width, initial-scale=1.0"
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><BODY
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><DIV
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><DIV CLASS="topnav"
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><DIV CLASS="breadcrumb"
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><SPAN CLASS="breadcrumb-item"
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><A HREF="index.html"
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>Common Lisp Nova Spec</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="18_Hash_Tables.html"
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>18. Hash Tables</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="18_2_Hash_Tables_Dictionary.html"
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>18.2 Hash Tables Dictionary</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="f_sxhash.html"
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>sxhash</A
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></SPAN
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></DIV
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><DIV CLASS="apropos"
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><DIV CLASS="apropos-io"
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><A HREF="f_clrhash.html" CLASS="prev"
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>←</A
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><SPAN ID="apropos-label"
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>Apropos </SPAN
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><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
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><A HREF="19_Filenames.html" CLASS="next"
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>→</A
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></DIV
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><DIV ID="apropos-res"
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></DIV
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></DIV
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></DIV
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><DIV CLASS="matter"
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><DIV CLASS="com"
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><DIV CLASS="begincom"
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><HR
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TR
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="sxhash"
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></SPAN
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><B
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>sxhash</B
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Function</I
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></TD
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></TR
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></TABLE
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><HR
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></DIV
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Syntax</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>sxhash</B
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> <SPAN CLASS="cmssi"
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>object</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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> <SPAN CLASS="cmssi"
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>hash-code</SPAN
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></P
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></DD
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><DT
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><B
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>Arguments and Values</B
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></DT
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><DD
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><P CLASS="j"
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><VAR CLASS="param"
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>object</VAR
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> — an <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>hash-code</VAR
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> — a non-negative <A HREF="26_1_Glossary.html#fixnum"
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><EM CLASS="term"
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>fixnum</EM
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></A
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>.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_sxhash.html" CLASS="funref"
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><B
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>sxhash</B
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></A
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> returns a hash code for <VAR CLASS="param"
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>object</VAR
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>. </P
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><P CLASS="j"
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>The manner in which the hash code is computed is <A HREF="26_1_Glossary.html#implementation-dependent"
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><EM CLASS="term"
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>implementation-dependent</EM
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></A
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>, but subject to certain constraints:</P
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><OL
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><LI
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><P CLASS="j"
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><CODE CLASS="f"
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>(equal <VAR CLASS="param"
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>x</VAR
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> <VAR CLASS="param"
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>y</VAR
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>)</CODE
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> implies <CODE CLASS="f"
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>(= (sxhash <VAR CLASS="param"
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>x</VAR
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>) (sxhash <VAR CLASS="param"
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>y</VAR
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>))</CODE
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>.</P
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></LI
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><LI
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><P CLASS="j"
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>For any two <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>objects</EM
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></A
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>, <VAR CLASS="param"
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>x</VAR
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> and <VAR CLASS="param"
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>y</VAR
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>, both of which are <A HREF="26_1_Glossary.html#bit_vector"
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><EM CLASS="term"
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>bit vectors</EM
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></A
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>, <A HREF="26_1_Glossary.html#character"
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><EM CLASS="term"
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>characters</EM
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></A
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>, <A HREF="26_1_Glossary.html#cons"
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><EM CLASS="term"
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>conses</EM
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></A
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>, <A HREF="26_1_Glossary.html#number"
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><EM CLASS="term"
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>numbers</EM
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></A
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>, <A HREF="26_1_Glossary.html#pathname"
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><EM CLASS="term"
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>pathnames</EM
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></A
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>, <A HREF="26_1_Glossary.html#string"
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><EM CLASS="term"
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>strings</EM
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></A
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>, or <A HREF="26_1_Glossary.html#symbol"
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><EM CLASS="term"
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>symbols</EM
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></A
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>, and which are <A HREF="26_1_Glossary.html#similar"
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><EM CLASS="term"
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>similar</EM
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></A
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>, <CODE CLASS="f"
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>(sxhash <VAR CLASS="param"
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>x</VAR
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>)</CODE
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> and <CODE CLASS="f"
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>(sxhash <VAR CLASS="param"
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>y</VAR
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>)</CODE
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> <A HREF="26_1_Glossary.html#yield"
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><EM CLASS="term"
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>yield</EM
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></A
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> the same mathematical value even if <VAR CLASS="param"
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>x</VAR
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> and <VAR CLASS="param"
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>y</VAR
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> exist in different <A HREF="26_1_Glossary.html#Lisp_image"
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><EM CLASS="term"
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>Lisp images</EM
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></A
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> of the same <A HREF="26_1_Glossary.html#implementation"
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><EM CLASS="term"
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>implementation</EM
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></A
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>. See <A HREF="3_2_Compilation.html#sec_3_2_4" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>3.2.4</SPAN
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> <SPAN CLASS="cmr"
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>(Literal</SPAN
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> <SPAN CLASS="cmr"
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>Objects</SPAN
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> <SPAN CLASS="cmr"
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>in</SPAN
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> <SPAN CLASS="cmr"
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>Compiled</SPAN
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> <SPAN CLASS="cmr"
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>Files)</SPAN
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></A
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>.</P
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></LI
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><LI
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><P CLASS="j"
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>The <VAR CLASS="param"
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>hash-code</VAR
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> for an <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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> is always the <A HREF="26_1_Glossary.html#same"
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><EM CLASS="term"
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>same</EM
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></A
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> within a single <A HREF="26_1_Glossary.html#session"
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><EM CLASS="term"
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>session</EM
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></A
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> provided that the <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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> is not visibly modified with regard to the equivalence test <A HREF="f_equal.html" CLASS="funref"
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><B
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>equal</B
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></A
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>. See <A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>18.1.2</SPAN
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> <SPAN CLASS="cmr"
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>(Modifying</SPAN
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> <SPAN CLASS="cmr"
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>Hash</SPAN
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> <SPAN CLASS="cmr"
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>Table</SPAN
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> <SPAN CLASS="cmr"
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>Keys)</SPAN
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></A
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>.</P
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></LI
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><LI
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><P CLASS="j"
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>The <VAR CLASS="param"
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>hash-code</VAR
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> is intended for hashing. This places no verifiable constraint on a <A HREF="26_1_Glossary.html#conforming_implementation"
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><EM CLASS="term"
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>conforming implementation</EM
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></A
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>, but the intent is that an <A HREF="26_1_Glossary.html#implementation"
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><EM CLASS="term"
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>implementation</EM
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></A
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> should make a good-faith effort to produce <VAR CLASS="param"
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>hash-codes</VAR
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> that are well distributed within the range of non-negative <A HREF="26_1_Glossary.html#fixnum"
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><EM CLASS="term"
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>fixnums</EM
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></A
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>.</P
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></LI
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><LI
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><P CLASS="j"
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>Computation of the <VAR CLASS="param"
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>hash-code</VAR
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> must terminate, even if the <VAR CLASS="param"
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>object</VAR
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> contains circularities.</P
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></LI
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></OL
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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> (= (sxhash (list 'list "ab")) (sxhash (list 'list "ab"))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> <A HREF="26_1_Glossary.html#true"
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><EM CLASS="term"
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>true</EM
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></A
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>
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(= (sxhash "a") (sxhash (make-string 1 :initial-element #\a))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> <A HREF="26_1_Glossary.html#true"
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><EM CLASS="term"
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>true</EM
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></A
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>
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(let ((r (make-random-state)))
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(= (sxhash r) (sxhash (make-random-state r))))
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> <A HREF="26_1_Glossary.html#implementation-dependent"
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><EM CLASS="term"
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>implementation-dependent</EM
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></A
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></PRE
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></DD
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><DT
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><B
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>Affected By</B
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></DT
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><DD
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><P CLASS="j"
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>The <A HREF="26_1_Glossary.html#implementation"
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><EM CLASS="term"
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>implementation</EM
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></A
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>.</P
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></DD
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><DT
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><B
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>Notes</B
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></DT
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><DD
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><P CLASS="j"
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>Many common hashing needs are satisfied by <A HREF="f_make-hash-table.html" CLASS="funref"
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><B
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>make-hash-table</B
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></A
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> and the related functions on <A HREF="26_1_Glossary.html#hash_table"
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><EM CLASS="term"
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>hash tables</EM
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></A
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|
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>. <A HREF="f_sxhash.html" CLASS="funref"
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|
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><B
|
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>sxhash</B
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></A
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> is intended for use where the pre-defined abstractions are insufficient. Its main intent is to allow the user a convenient means of implementing more complicated hashing paradigms than are provided through <A HREF="26_1_Glossary.html#hash_table"
|
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|
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><EM CLASS="term"
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>hash tables</EM
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></A
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>. </P
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|
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><P CLASS="j"
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>The hash codes returned by <A HREF="f_sxhash.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>sxhash</B
|
||
|
|
></A
|
||
|
|
> are not necessarily related to any hashing strategy used by any other <A HREF="26_1_Glossary.html#function"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>function</EM
|
||
|
|
></A
|
||
|
|
> in Common Lisp. </P
|
||
|
|
><P CLASS="j"
|
||
|
|
>For <A HREF="26_1_Glossary.html#object"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>objects</EM
|
||
|
|
></A
|
||
|
|
> of <A HREF="26_1_Glossary.html#type"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>types</EM
|
||
|
|
></A
|
||
|
|
> that <A HREF="f_equal.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>equal</B
|
||
|
|
></A
|
||
|
|
> compares with <A HREF="f_eq.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>eq</B
|
||
|
|
></A
|
||
|
|
>, item 3 requires that the <VAR CLASS="param"
|
||
|
|
>hash-code</VAR
|
||
|
|
> be based on some immutable quality of the identity of the object. Another legitimate implementation technique would be to have <A HREF="f_sxhash.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>sxhash</B
|
||
|
|
></A
|
||
|
|
> assign (and cache) a random hash code for these <A HREF="26_1_Glossary.html#object"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>objects</EM
|
||
|
|
></A
|
||
|
|
>, since there is no requirement that <A HREF="26_1_Glossary.html#similar"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>similar</EM
|
||
|
|
></A
|
||
|
|
> but non-<A HREF="f_eq.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>eq</B
|
||
|
|
></A
|
||
|
|
> objects have the same hash code. </P
|
||
|
|
><P CLASS="j"
|
||
|
|
>Although <A HREF="26_1_Glossary.html#similarity"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>similarity</EM
|
||
|
|
></A
|
||
|
|
> is defined for <A HREF="26_1_Glossary.html#symbol"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>symbols</EM
|
||
|
|
></A
|
||
|
|
> in terms of both the <A HREF="26_1_Glossary.html#symbol"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>symbol</EM
|
||
|
|
></A
|
||
|
|
>’s <A HREF="26_1_Glossary.html#name"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>name</EM
|
||
|
|
></A
|
||
|
|
> and the <A HREF="26_1_Glossary.html#package"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>packages</EM
|
||
|
|
></A
|
||
|
|
> in which the <A HREF="26_1_Glossary.html#symbol"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>symbol</EM
|
||
|
|
></A
|
||
|
|
> is <A HREF="26_1_Glossary.html#accessible"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>accessible</EM
|
||
|
|
></A
|
||
|
|
>, item 3 disallows using <A HREF="26_1_Glossary.html#package"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>package</EM
|
||
|
|
></A
|
||
|
|
> information to compute the hash code, since changes to the package status of a symbol are not visible to <VAR CLASS="param"
|
||
|
|
>equal</VAR
|
||
|
|
>.</P
|
||
|
|
></DD
|
||
|
|
></DL
|
||
|
|
></DIV
|
||
|
|
></DIV
|
||
|
|
><DIV CLASS="footer"
|
||
|
|
><DIV CLASS="btmnav"
|
||
|
|
><A HREF="f_clrhash.html" CLASS="prev"
|
||
|
|
>←</A
|
||
|
|
><A HREF="19_Filenames.html" CLASS="next"
|
||
|
|
>→</A
|
||
|
|
></DIV
|
||
|
|
><DIV CLASS="trail"
|
||
|
|
>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
|
||
|
|
></DIV
|
||
|
|
></DIV
|
||
|
|
><SCRIPT
|
||
|
|
>domReady();</SCRIPT
|
||
|
|
></BODY
|
||
|
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></HTML
|
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|
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>
|