Re: tonumber for char* data, is it possible?`
- From: Vladimir Dronnikov <dronnikov@xxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Sat, 16 Nov 2013 22:53:08 +0400
On Sat, Nov 16, 2013 at 9:43 PM, Mike Pall <mike-1311@xxxxxxxxxx> wrote:
> Vladimir Dronnikov wrote:
> > Need a fast function to convert a FFI char* data to number.
>
> Well, what kind of number? Integer? Decimal/hex? Floating-point?
>
Decimal floating-point in general here.
>
> > src/lj_strscan.c already contains ready-to-use function:
> > `StrScanFmt lj_strscan_scan(const uint8_t *p, TValue *o, uint32_t opt)`
> >
> > Wonder is it exposed to userspace, or how it can be done?
>
> It's not really exposed, unless you're willing to allocate a
> temporary string, i.e. tonumber(ffi.string(cdata)). Arguably that
> mostly negates the performance benefit.
>
Yes, interning char* string leads to circa 700 times slowdown compared to
plain Lua tonumber on Lua string, in my tests.
>
> The easiest solution would be to call strtod() or strtol() via the
> FFI. Though this means you're at the mercy of the C library wrt.
> accuracy and performance.
>
ffi.C.atof gives circa 650 times slowdown comared to tonumber on Lua
string, ever if I preallocate reusable buffer, sufficient enough to hold
copy of source string plus terminating \0.
>
> If you need to scan a pure integer then a simple multiply-and-add
> loop is probably the fastest. But I recommend to find out first what
the typical range of numbers will be. Sometimes a little unrolling
> can do wonders, e.g. if you mostly scan numbers from 0 to 99.
>
>
I see. Will keep thinking.
Thank you.
> --Mike
>
>
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