Re: [Haskell-cafe] Substring replacements

From: Daniel Fischer To: "Branimir Maksimovic" CC: Haskell-Cafe@haskell.org Subject: Re: [Haskell-cafe] Substring replacements Date: Mon, 12 Dec 2005 16:15:46 +0100

Earlier today:

...

Sorry, but Prelude SearchRep> searchReplace "abaaba" "##" "abababaaba" "abababaaba"

I haven't analyzed the algorithm, so I don't know why exactly this fails. I'll take a look sometime soon.

I found the problem (one at least). Say the pattern to be replaced begins with 'a' and we have a sufficiently long match with the pattern starting at the first 'a' in the String. Upon encountering the second 'a', while the first pattern still matches, you start pushing onto the rollback-stack. But that isn't inspected anymore, so if the actual occurence of the pattern starts at the third (or fourth, n-th) occurence of 'a' and that is already pushed onto the rollback, you miss it.

I've corrected this with adjusting rollback position. if rollBack is null then search for rollback starts at second character if not starts at same as searhed character because I skip what was searched. That's all. Though I'm not so sure now when I read this.

So the question is, can we find a cheap test to decide whether to use KMP or Bulat's version?

In real world situation your KMP will always be fastest on average. I like that we are not using C arrays as then we have advantage of lazyness and save on memory usage. C++ program will be faster on shorter strings but on this large strings will loose due memory latency. and with your test, both programs are very fast. Greetings, Bane. _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/