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egallesio / STklos

STklos Scheme
http://stklos.net
GNU General Public License v2.0
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Make `log` return exact result when possible #609

Closed jpellegrini closed 1 year ago

jpellegrini commented 1 year ago

If the arguments to log are exact and the result is an integer, return it as an exact int.

(log 16   2) => 4
(log 16.0 2) => 4.0

(log 1/4 2  ) => -2
(log 1/4 2.0) => -2.0
jpellegrini commented 1 year ago

Not sure if this is correct. Would we possibly give an exact result when it's not really supposed to be exact? STklos uses the C log function... Does it ever return an integer wrongly?

jpellegrini commented 1 year ago

Nope... Here are two different numbers with the same log, using this patch:

(log 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858368 2)
1024
stklos> (log 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858367 2)
1024

So this would only work if both the first argument and the result are less than the first non-representable integer. I'll work on this later.

jpellegrini commented 1 year ago

We'd actually need to do an exponentiation to check the result, and I think that'd be slow. Closing this... Sorry about the noise!