[mercury-users] a question on efficient parsing of a file

[Peter Wang]

On 2010-05-19, Ralph Becket <rafe at csse.unimelb.edu.au> wrote:
> Vladimir Gubarkov, Tuesday, 18 May 2010:
> >    And third - it operates over string and current position, and my
> >    experiments seem to show that this way is not as fast as operating on
> >    list of chars.
> 
> I'm very surprised to hear that.  parsing_utils reads chars directly
> from the string source, which is just a bounds test and a memory
> reference (see string.index).  List creation and manipulation is
> typically much slower.

On C backends, string.index may incur a strlen() on every call.  You
should use string.unsafe_index unless you want to rely on loop invariant
hoisting.

Also if the parse_state is as before, you may end up create a new
structure per character read.  This could be another reason that
traversing a list is faster -- but that is atypical.

*Not recommended*
For efficiency you can write C with Mercury syntax.  Use a cursor type
that points directly at a character within the string.  Advancing the
cursor is just a pointer increment.  You must be careful to hold a
reference to the string, somewhere, otherwise the string may be garbage
collected.  This only works for C backends.

Peter

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