Editing WoW:HOWTO: Speed up string match lookups

When you have large number of patterns (dozens) to scan to find out which pattern is matching a given string, there's a few things you can do to speed up the job.

If the patterns are hard coded, there is of course any number of ways that you can be clever. But if you do not know what the patterns look like beforehand, which is the case when you're trying to match input strings against patterns in [{{wdnlink|FrameXML/GlobalStrings.lua}} GlobalStrings.lua] using a formatstring-to-regex utility like [http://www.wowinterface.com/downloads/fileinfo.php?id=4809 BabbleLib's] Deformat() function.


The approach below works by making lists of words used by patterns, and then looking at words in the input strings to determine which list(s) to look for matches in.

Actually, the process is 2-pass. The first pass figures out the LEAST commonly used words, and then just uses those.


* Note: The example contains a ''very'' simplistic "MyDeformatterFunc()" for converting "%s" to "(.*)". It will not work for other locales than english. Do not use it in the real world, please.


<div style="max-width: 80em; margin-right: 2em; height: 30em; overflow: scroll;">
 
 -- Functions that we want called for different string matches
 function RoughPokeFunc(v1,v2) print("RoughPokeFunc "..v1.." "..v2); end
 function SoftPokeFunc(v1,v2) print("SoftPokeFunc "..v1.." "..v2); end
 function SoftNudgeFunc(v1,v2) print("SoftNudgeFunc "..v1.." "..v2); end
 function ChickenFunc(v1,v2) print("ChickenFunc "..v1.." "..v2); end
 
 
 -- Strings to match mapped to functions that we want called
 MatchStrings = {
   ["%s roughly pokes %s"] = RoughPokeFunc,
   ["%s softly pokes %s"] = SoftPokeFunc,
   ["%s softly nudges %s"] = SoftNudgeFunc,
   ["%s gets nudged by %s and runs away screaming"] = ChickenFunc,
 }
 
 -- VERY simplistic deformatter function. 
 -- You probably want a real deformatting library for this.
 function MyDeformatterFunc(str)	
   return (string.gsub(str, "%%s", "(.*)"));
 end
 
 
 -- First run: count how many occurences there are of each word
 WordCounts = {}
 for str,func in MatchStrings do
   for word in string.gfind(str, "[^ ]+") do
   	if(string.find(word, "^%%")) then
   		-- ignore format strings
   	else
   		WordCounts[word] = (WordCounts[word] or 0) + 1;
   	end
   end
 end
 
 -- Second run: for each string, pick the least common word and place string in that hash bucket
 MatchStringsHash = {}
 for str,func in MatchStrings do
   local bestword, num;
   for word in string.gfind(str, "[^ ]+") do
   	if(string.find(word, "^%%")) then
   		-- ignore format strings
   	else
   		if(not num or WordCounts[word] &lt; num) then
   			num = WordCounts[word];
   			bestword = word;
   		end
   	end
   end
   
   assert(bestword);
   
   if(not MatchStringsHash[bestword]) then MatchStringsHash[bestword] = {}; end
   MatchStringsHash[bestword][MyDeformatterFunc(str)] = func;
 end
 
 WordCounts = nil; -- now we don't need the counts anymore
 
 
 -- Dump our MatchStringsHash on-screen so we can see what it looks like!
 print "Examining hash buckets"
 print "----------------------"
 for word,strings in MatchStringsHash do
   print("  "..word..":");
   for str,func in strings do
   	print("    \""..str.."\"");
   end
 end
 
 
 
 
 
 -- Function that scans for matches and calls the resulting function
 function ScanForMatch(str)
   local bDone = false;
   local nCompares = 0;
   
   for word in string.gfind(str, "[^ ]+") do
   	if(MatchStringsHash[word]) then
   		for pattern,func in MatchStringsHash[word] do
   			nCompares = nCompares + 1;
   			local success,_,v1,v2,v3,v4 = string.find(str, pattern);
   			if(success) then
   				func(v1,v2,v3,v4);
   				bDone=true;
   				break;
   			end
   		end
   	end
   	
   	if(bDone) then break; end
   end
 
   print("  \""..str.."\": "..nCompares.." string.finds actually executed\n");
 end
 
 print("");
 print("Executing!");
 print("----------");
 
 
 ScanForMatch("Alice roughly pokes Bob");
 ScanForMatch("Bob softly pokes Charles");
 ScanForMatch("Charles softly nudges Denise");
 ScanForMatch("Denise gets nudged by Eve and runs away screaming");
 ScanForMatch("This string does not exist");
</div>


Running the above produces the following output:
<div style="margin-right: 2em;">
 
 Examining hash buckets
 ----------------------
   roughly:
     "(.*) roughly pokes (.*)"
   nudges:
     "(.*) softly nudges (.*)"
   gets:
     "(.*) gets nudged by (.*) and runs away screaming"
   softly:
     "(.*) softly pokes (.*)"
 
 Executing!
 ----------
 RoughPokeFunc Alice Bob
   "Alice roughly pokes Bob": 1 string.finds actually executed
 
 SoftPokeFunc Bob Charles
   "Bob softly pokes Charles": 1 string.finds actually executed
 
 SoftNudgeFunc Charles Denise
   "Charles softly nudges Denise": 2 string.finds actually executed
 
 ChickenFunc Denise Eve
   "Denise gets nudged by Eve and runs away screaming": 1 string.finds actually executed
 
   "This string does not exist": 0 string.finds actually executed
 
</div>


== Problems with this approach ==

There is no guarantee as to which order the string matches will be attempted.

For example, assume these two patterns:
#"%s hits %s."
#"%s hits %s hard."

Now, given the input string "Alice hits Bob.", only #1 will match, and all is good.

But with the input string "Alice hits Bob hard.", there is NO guarantee which string will match. You can get #1 with the arguments "Alice", "Bob hard". Or you can get #2 with the arguments "Alice", "Bob".


[[Category:HOWTOs|Speed up string match lookups]]