142 lines
6 KiB
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142 lines
6 KiB
HTML
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<META name=vsisbn content="1576101746">
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<META name=vstitle content="Michael Abrash's Graphics Programming Black Book, Special Edition">
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<META name=vsauthor content="Michael Abrash">
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<META name=vspublisher content="The Coriolis Group">
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<META name=vspubdate content="07/01/97">
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<META name=vscategory content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development">
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<TITLE>Michael Abrash's Graphics Programming Black Book Special Edition: Boyer-Moore String Searching</TITLE>
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<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
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<!--AUTHOR=Michael Abrash//-->
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<!--PUBLISHER=The Coriolis Group, Inc.//-->
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<!--CHAPTER=14//-->
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<!--PAGES=267-268//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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<CENTER>
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<TABLE BORDER>
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<TR>
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<TD><A HREF="14-02.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<TD><A HREF="14-04.html">Next</A></TD>
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<P><BR></P>
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<P>How fast <I>is</I> Boyer-Moore? Listing 14.1 is a C implementation of Boyer-Moore searching; Listing 14.2 is a test-bed program that searches up to the first 32K of a file for a pattern. Table 14.1 (all times measured with Turbo Profiler on a 20 MHz cached 386, searching a modified version of the text of this chapter) shows that this implementation is generally much slower than <B>REPNZ SCASB,</B> although it does come close when searching for long patterns. Listing 14.1 is designed primarily to make later assembly implemenmore comprehensible, rather than faster; Sedge’s implementation uses arrays rather than pointers, is a great deal more compact and very clever, and may be somewhat faster. Regardless, the far superior performance of <B>REPNZ SCASB</B> clearly indicates that assembly language is in order at this point.</P>
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<TABLE WIDTH="100%"><TR>
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<TD COLSPAN="7"><HR>
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<TR>
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="30%">
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">“g;”
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">“Yogi”
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">“igoY”
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">“Adrian”
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">“Conclusion”
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<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="20%">“You don’t know what you know”
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<TR>
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<TD COLSPAN="7"><HR>
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">Searching approach
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<TD VALIGN="TOP" ALIGN="LEFT">(16K)
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<TD VALIGN="TOP" ALIGN="LEFT">(16K)
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<TD VALIGN="TOP" ALIGN="LEFT">(16K)
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<TD VALIGN="TOP" ALIGN="LEFT">(<1K)
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<TD VALIGN="TOP" ALIGN="LEFT">(16K)
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<TD VALIGN="TOP" ALIGN="LEFT">(16K)
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">REPNZ SCASB on first char a(Listing 9.1)
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<TD VALIGN="TOP" ALIGN="LEFT">8.2
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<TD VALIGN="TOP" ALIGN="LEFT">7.5
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<TD VALIGN="TOP" ALIGN="LEFT">9.7
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<TD VALIGN="TOP" ALIGN="LEFT">0.4
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<TD VALIGN="TOP" ALIGN="LEFT">7.4
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<TD VALIGN="TOP" ALIGN="LEFT">8.1
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">REPNZ SCASB on least common char (Listing 9.2)
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<TD VALIGN="TOP" ALIGN="LEFT">7.6
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<TD VALIGN="TOP" ALIGN="LEFT">7.5
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<TD VALIGN="TOP" ALIGN="LEFT">7.5
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<TD VALIGN="TOP" ALIGN="LEFT">0.5
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<TD VALIGN="TOP" ALIGN="LEFT">7.5
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<TD VALIGN="TOP" ALIGN="LEFT">7.5
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">Boyer-Moore in C (Listing 14.1)
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<TD VALIGN="TOP" ALIGN="LEFT">71.0
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<TD VALIGN="TOP" ALIGN="LEFT">38.4
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<TD VALIGN="TOP" ALIGN="LEFT">37.7
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<TD VALIGN="TOP" ALIGN="LEFT">1.8
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<TD VALIGN="TOP" ALIGN="LEFT">18.2
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<TD VALIGN="TOP" ALIGN="LEFT">9.2
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">Standard Boyer-Moore in ASM(code not shown)
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<TD VALIGN="TOP" ALIGN="LEFT">38.5
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<TD VALIGN="TOP" ALIGN="LEFT">21.0
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<TD VALIGN="TOP" ALIGN="LEFT">20.5
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<TD VALIGN="TOP" ALIGN="LEFT">0.8
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<TD VALIGN="TOP" ALIGN="LEFT">9.4
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<TD VALIGN="TOP" ALIGN="LEFT">4.8
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT">Quick handling of first mismatch Boyer-Moore in ASM(Listing 14.3)
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<TD VALIGN="TOP" ALIGN="LEFT">14.1
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<TD VALIGN="TOP" ALIGN="LEFT">8.9
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<TD VALIGN="TOP" ALIGN="LEFT">7.7
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<TD VALIGN="TOP" ALIGN="LEFT">0.4
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<TD VALIGN="TOP" ALIGN="LEFT">4.0
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<TD VALIGN="TOP" ALIGN="LEFT">2.0
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT"><=255 pattern length + sentinelBoyer-Moore in ASM(Listing 14.4)
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<TD VALIGN="TOP" ALIGN="LEFT">8.1
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<TD VALIGN="TOP" ALIGN="LEFT">5.2
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<TD VALIGN="TOP" ALIGN="LEFT">4.6
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<TD VALIGN="TOP" ALIGN="LEFT">0.3
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<TD VALIGN="TOP" ALIGN="LEFT">2.6
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<TD VALIGN="TOP" ALIGN="LEFT">1.2
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<TR>
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<TD VALIGN="TOP" ALIGN="LEFT" COLSPAN="7"><SMALL>Search pattern (approximate distance searched before match is shown in parentheses).</SMALL><BR><SMALL>Times are in milliseconds; shorter is better.</SMALL>
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<TR>
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<TD COLSPAN="7"><HR>
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<TR>
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<TH COLSPAN="7" CAPTION ALIGN="LEFT">Table 14.1 Comparison of searching techniques.
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<TR>
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<TD COLSPAN="7"><HR>
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</TABLE>
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<P>The entry “Standard Boyer-Moore in ASM” in Table 14.1 refers to straight-forward hand optimization of Listing 14.1, code that is not included in this chapter for the perfectly good reason that it is slower in most cases than <B>REPNZ SCASB.</B> I say this casually now, but not so yesterday, when I had all but concluded that Boyer-Moore was simply inferior on the x86, due to two architectural quirks: the string instructions and slow branch. I had even coined a neat phrase for it: Architecture is destiny. Has a nice ring, doesn’t it?</P><P><BR></P>
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<TD><A HREF="14-02.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
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