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 Post subject: Binary Search for 65C816
PostPosted: Fri Jun 27, 2014 7:21 pm 
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Joined: Thu May 28, 2009 9:46 pm
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Location: Midwestern USA
In the process of getting from one place to another, I needed to concoct a binary search algorithm for the 65C816 running in native mode. The job was to look up display control "mnemonics" (actually parameterless macros) embedded in text strings and then send an escape sequence to the display device to produce the desired result. I chose the binary search because quite a few of these mnemonics could be used, making rapid performance essential.

Anyhow, I subsequently generalized my code so that it can be used to search through an ordered list of character strings of varying lengths, e.g, a list of names, using a 16 bit pointer array to index the strings. All working storage is on the stack. Code is attached, and has been submitted to Mike for inclusion in the code library. There's plenty of commentary so you can see what's going on.
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File comment: Binary Search for 65C816
bsrch816.zip [199.42 KiB]
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PostPosted: Thu Oct 02, 2014 12:31 pm 
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Joined: Fri Sep 28, 2012 12:27 pm
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I do not know if the mentionned algorithm could be applied in this context but there is a 6502 Assembly sample code at the URL http://www.txbobsc.com/aal/1985/aal8506.html that describes how a "Boyers Moore Morris" search algorithm could be implemented (looking up occurences of a single string pattern in a large text body). I understand that what you are looking for is to extract multiple patterns as soon as possible from a character stream.

HTHATS,
Benoît


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PostPosted: Thu Oct 02, 2014 3:33 pm 
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Benoit0123 wrote:
I do not know if the mentionned algorithm could be applied in this context but there is a 6502 Assembly sample code at the URL http://www.txbobsc.com/aal/1985/aal8506.html that describes how a "Boyers Moore Morris" search algorithm could be implemented (looking up occurences of a single string pattern in a large text body). I understand that what you are looking for is to extract multiple patterns as soon as possible from a character stream.

HTHATS,
Benoît

Firstly, it's the "Boyer-Moore" algorithm.

Fundamentally different problems are being solved.  Boyer-Moore addresses the case of searching through an arbitrary range of memory looking for a specific pattern.  A binary search works through an ordered list looking for the pattern.  Hence only log2N comparisons are required to determine whether or not the search pattern is present.  With Boyer-Moore, performance rapidly degrades as more of the range being searched consists of alike characters, e.g., long strings of $00, peppered with an occasional $01 in a case where $01 $01 is being sought.  In such a case, Boyer-Moore becomes more like the brute force method in performance, but actually worse, because of the overhead needed to maintain the incidence table.

An aspect of Boyer-Moore that could be considered a negative in some cases is the need for the incidence table to be equal in size to all possible byte values.  In some 6502 systems, having to devote a full page of RAM in support of a search may be a lot to ask.  I gave that some thought when I wrote the search (H) function in Supermon 816.  During testing, I concluded that the performance of the brute force method was more than acceptable, so I did not use Boyer-Moore.

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