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William D Clinger wrote:
Alex Shinn quoting me: > > I acknowledged that the algorithm will still work. My point > > is that its asymptotic complexity may be degraded. > > I'm sorry, perhaps I'm just misunderstanding, but if the exact > same algorithm, in fact the exact same code, can be used, > how is the asymptotic complexity affected? The different representations for the string being searched (UTF-8 vs UTF-32) change the problem. The main difference is that UTF-32 admits random access of characters, while UTF-8 does not. UTF-8 admits random access of bytes, but there is no way to convert a byte value obtained by random access into a UTF-8 string into the character offset on which the Boyer-Moore algorithm depends,
I don't claim to understand Boyer-Moore, beyond what I gather is the key insight: if you're searching for "ABC" and character N is "D" there is no point in checking characters N+1 or N+2. (Hence the non-obvious part of building the appropriate delta tables before you start.) But I'm puzzled, since I think Alex's point is this: A Boyer-Moore implementation that works on 8-bits characters (e.g. Latin-1) will work unchanged on UTF-8 characters. Naively, one would think it would have the same performance characteristics. I guess that statistics of multi-byte characters might throw off the "delta" tables so they delta will tend to be smaller. (However, don't go into details for my sake: I suspect t would take me too much effort to delve into it.)
As Per Bothner agreed, that can degrade the asympototic complexity of the algorithm from O(n/m) to O(n).
I did? -- --Per Bothner per@xxxxxxxxxxx http://per.bothner.com/