[phobos] Optimizing std.conv.parse for float conversions

Andrei Alexandrescu andrei at erdani.com
Tue Feb 16 19:12:49 PST 2010

Heh, incidentally I was looking over the same issue recently.

I think we need for a much more radical approach. We need to be able to 
parse any stream of characters (that means lookahead == 1) and stop as 
soon as the meaningful input has ended.

Walter wrote a while back some solid floating point parsing routines for 
dmc. I think we should start with those and adapt them to work with 
input streams instead of strings.

This applies to std.format too, which is in dire need of an overhaul to 
make it work with all streams.


David Simcha wrote:
> I looked into creating D implementations of equivalent functions to 
> strtold, etc. to address bug 3758 
> (http://d.puremagic.com/issues/show_bug.cgi?id=3758).  It seems like 
> converting strings to floats is actually pretty hard to do right, i.e. 
> without any loss of precision.  However, the inefficiency of requiring 
> several heap allocations per conversions hurts when reading in large 
> files.  From some measurements I did, it seems like the heap allocation 
> and associated garbage collection is the bigger source of overhead than 
> the string copying.  Is everyone ok with me optimizing std.conv.parse to 
> place the zero-terminated version of the string on the stack if it's 
> reasonably small as a temporary fix?
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