Re: [Rd] shash in unique.c

From: Matthew Dowle <mdowle_at_mdowle.plus.com>
Date: Fri, 05 Mar 2010 11:37:04 +0000

I was hoping for a 'yes', 'no', 'maybe' or 'bad idea because ...'. No response resulted in a retry() after a Sys.sleep(10 days).

If its a "yes" or "maybe" then I could proceed to try it, test it, and present the test results and timings to you along with the patch. It would be on 32bit Ubuntu first, and I would need to either buy, rent time on, or borrow a 64bit machine to be able to then test there, owing to the nature of the suggestion.

If its "no", "bad idea because..." or "we were already working on it, or better", then I won't spend any more time on it.

Matthew

"Matthew Dowle" <mdowle_at_mdowle.plus.com> wrote in message news:hlu4qh$l7s$1_at_dough.gmane.org...
>
> Looking at shash in unique.c, from R-2.10.1 I'm wondering if it makes
> sense to hash the pointer itself rather than the string it points to?
> In other words could the SEXP pointer be cast to unsigned int and the
> usual scatter be called on that as if it were integer?
>
> shash would look like a slightly modified version of ihash like this :
>
> static int shash(SEXP x, int indx, HashData *d)
> {
> if (STRING_ELT(x,indx) == NA_STRING) return 0;
> return scatter((unsigned int) (STRING_ELT(x,indx), d);
> }
>
> rather than its current form which appears to hash the string it points to
> :
>
> static int shash(SEXP x, int indx, HashData *d)
> {
> unsigned int k;
> const char *p;
> if(d->useUTF8)
> p = translateCharUTF8(STRING_ELT(x, indx));
> else
> p = translateChar(STRING_ELT(x, indx));
> k = 0;
> while (*p++)
> k = 11 * k + *p; /* was 8 but 11 isn't a power of 2 */
> return scatter(k, d);
> }
>
> Looking at sequal, below, and reading its comments, if the pointers are
> equal it doesn't look at the strings they point to, which lead to the
> question above.
>
> static int sequal(SEXP x, int i, SEXP y, int j)
> {
> if (i < 0 || j < 0) return 0;
> /* Two strings which have the same address must be the same,
> so avoid looking at the contents */
> if (STRING_ELT(x, i) == STRING_ELT(y, j)) return 1;
> /* Then if either is NA the other cannot be */
> /* Once all CHARSXPs are cached, Seql will handle this */
> if (STRING_ELT(x, i) == NA_STRING || STRING_ELT(y, j) == NA_STRING)
> return 0;
> return Seql(STRING_ELT(x, i), STRING_ELT(y, j));
> }
>
> Matthew
>



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