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Re^6: Alternative to bytes::length() (7% solution)

by creamygoodness (Curate)
on Dec 23, 2009 at 16:09 UTC ( #814121=note: print w/ replies, xml ) Need Help??


in reply to Re^5: Alternative to bytes::length() (7% solution)
in thread Alternative to bytes::length()

A parser which consumes the string would be an example application.

I agree that in many cases the caching is going to hide or eliminate the problem, and in fact, it was a little tricky to compose the original example.

However, in code that doesn't need to know the actual length of the string in characters, it's poor practice to use length(), whose cost scales with the size of the string on SVf_UTF8 scalars. The advantage of bytes::length() is that it's basically a lookup on a member of the SV struct. There's a little bit of extra math to deal with offsets, but it's still O(1) rather than O(n).

The ne "" idiom is also O(1) because it's just checking to see whether the total byte size of the string is 0, not counting characters and seeing if the count is 0. Which makes it an effective replacement.


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Re^7: Alternative to bytes::length() (7% solution)
by BrowserUk (Pope) on Dec 23, 2009 at 16:58 UTC
    A parser which consumes the string would be an example application.

    Then It would be better to test for the success of the operation that consumes the string, than perform that operation and then test to see if there is any string left to consume. Simplistically:

    cmpthese -1, { a=>q[ 1 while chop( $sa ) ], b=>q[ chop( $sb ) while $sb ne '' ] };; Rate b a b 8537447/s -- -59% a 20591583/s 141% --

    But most other operations that alter (reduce) the length of a scalar can be tested in the same way.

    That said, if there is any reason why the reducing operation itself cannot be used as the controlling condition, just testing the string for truth achieves the desired effect even more efficiently:

    [0] Perl> cmpthese -1, { a=>q[ 1 while chop( $sa ) ], b=>q[ chop( $sb ) while $sb ne '' ], c=>q[ chop( $sc ) while $sc ] };; Rate b a c b 8686428/s -- -2% -20% a 8821000/s 2% -- -19% c 10841356/s 25% 23% --

    Though I realise that can fail under some conditions.


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