Remove colour attributes from body and strip most of the font tags out
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James Gregory
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362 changed files with 1632 additions and 1706 deletions
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14-01.html
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14-01.html
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<!--CHAPTER=14//-->
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<!--PAGES=260-263//-->
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<!--UNASSIGNED1//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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<!--UNASSIGNED2//--></HEAD><body>
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<CENTER>
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<TABLE BORDER>
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@ -36,9 +36,8 @@
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</TABLE>
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</CENTER>
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<P><BR></P>
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<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 14<BR>Boyer-Moore String Searching
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</FONT></H2>
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<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Optimizing a Pretty Optimum Search Algorithm</FONT></H3>
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<H2><A NAME="Heading1"></A>Chapter 14<BR>Boyer-Moore String Searching</H2>
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<H3><A NAME="Heading2"></A>Optimizing a Pretty Optimum Search Algorithm</H3>
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<P>When you seem to be stumped, stop for a minute and <I>think.</I> All the information you need may be right in front of your nose if you just look at things a little differently. Here’s a case in point:</P>
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<P>When I was in college, I used to stay around campus for the summer. Oh, I’d take a course or two, but mostly it was an excuse to hang out and have fun. In that spirit, my girlfriend, Adrian (<I>not</I> my future wife, partly for reasons that will soon become apparent), bussed in to spend a week, sharing a less-than-elegant $150 per month apartment with me and, by necessity, my roommate.</P>
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<P>Our apartment was pretty much standard issue for two male college students; maybe even a cut above. The dishes were usually washed, there was generally food in the refrigerator, and nothing larger than a small dog had taken up permanent residence in the bathroom. However, there was one sticking point (literally): the kitchen floor. This floor—standard tile, with a nice pattern of black lines on an off-white background (or so we thought)—had never been cleaned. By which I mean that I know for a certainty that <I>we</I> had never cleaned it, but I suspect that it had in fact not been cleaned since the Late Jurassic, or possibly earlier. Our feet tended to stick to it; had the apartment suddenly turned upside-down, I think we’d all have been hanging from the ceiling.</P>
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<P>She stamped her foot (really; the only time I’ve ever seen it happen), and said, “No, you jerks! The kitchen floor! Look at the floor! I cleaned it!”</P>
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<P>The floor really did look amazing. It was actually all white; the black lines had been grooves filled with dirt. We assured her that it looked terrific, it just wasn’t that obvious until you knew to look for it; anyone would tell you that it wasn’t the kind of thing that jumped out at you, but it really was great, no kidding. We had almost smoothed things over, when a friend walked in, looked around with a start, and said, “Hey! Did you guys put in a new floor?”</P>
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<P>As I said, sometimes everything you need to know is right in front of your nose. Which brings us to Boyer-Moore string searching.</P>
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<H3><A NAME="Heading3"></A><FONT COLOR="#000077">String Searching Refresher</FONT></H3>
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<H3><A NAME="Heading3"></A>String Searching Refresher</H3>
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<P>I’ve discussed string searching earlier in this book, in Chapters 5 and 9. You may want to refer back to these chapters for some background on string searching in general. I’m also going to use some of the code from that chapter as part of this chapter’s test suite. For further information, you may want to refer to the discussion of string searching in the excellent <I>Algorithms in C,</I> by Robert Sedgewick (Addison-Wesley), which served as the primary reference for this chapter. (If you look at Sedgewick, be aware that in the Boyer-Moore listing on page 288, there is a mistake: “j > 0” in the <B>for</B> loop should be “j >= 0,” unless I’m missing something.)</P>
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<P>String searching is the simple matter of finding the first occurrence of a particular sequence of bytes (the pattern) within another sequence of bytes (the buffer). The obvious, brute-force approach is to try every possible match location, starting at the beginning of the buffer and advancing one position after each mismatch, until either a match is found or the buffer is exhausted. There’s even a nifty string instruction, <B>REPZ CMPS,</B> that’s perfect for comparing the pattern to the contents of the buffer at each location. What could be simpler?</P>
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<P>We have some important information that we’re not yet using, though. Typically, the buffer will contain a wide variety of bytes. Let’s assume that the buffer contains text, in which case there will be dozens of different characters; and although the distribution of characters won’t usually be even, neither will any one character constitute half the buffer, or anything close. A reasonable conclusion is that the first character of the pattern will rarely match the first character of the buffer location currently being checked. This allows us to use the speedy <B>REPNZ SCASB</B> to whiz through the buffer, eliminating most potential match locations with single repetitions of <B>SCASB.</B> Only when that first character does (infrequently) match must we drop back to the slower <B>REPZ CMPS</B> approach.</P>
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<hr width="90%" size="1" noshade>
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<div align="center">
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
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Graphics Programming Black Book © 2001 Michael Abrash
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</div>
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