The german Ruby forum recognized a strange behaviour of the Fixnum class.
In exponentation Fixnum is much slower than Bignum, which lets us assume, that some kind of a problem occurs
in the implementation. Example:
Code (Ruby) >>>>>
require 'benchmark'
N = 10**6
B = 65891264863465298234965902602612457060348499377
F = 4628
Benchmark.bmbm(10) do |bm|
bm.report('Bignum') do N.times do
B ** 0.5
end end
bm.report('Fixnum') do N.times do
F ** 0.5
end end
end
Result >>>>>
user system total real
Bignum 3.856000 0.000000 3.856000 ( 3.946000)
Fixnum 7.861000 0.020000 7.881000 ( 8.081000)
End of Example >>>>>
Further investigations leads to the responsible C code:
Code (C) >>>>>
static VALUE
fix_pow(x, y)
VALUE x, y;
{
if (FIXNUM_P(y)) {
long a, b;
b = FIX2LONG(y);
if (b == 0) return INT2FIX(1);
if (b == 1) return x;
a = FIX2LONG(x);
if (b > 0) {
return rb_big_pow(rb_int2big(a), y);
}
return rb_float_new(pow((double)a, (double)b));
}
return rb_num_coerce_bin(x, y); // <-- das dauert
}
/* vs. */
VALUE
rb_big_pow(x, y)
VALUE x, y;
{
double d;
long yy;
if (y == INT2FIX(0)) return INT2FIX(1);
switch (TYPE(y)) {
case T_FLOAT: // <-- das geht schnell
d = RFLOAT(y)->value;
break;
case T_BIGNUM:
...
case T_FIXNUM:
...
default:
return rb_num_coerce_bin(x, y);
}
return rb_float_new(pow(rb_big2dbl(x), d));
}
End of Code >>>>>
Is it possible to put this Mail to right place for "Ruby Problem Reports". I didn't find out the right place.
Best regards, Wolfgang Nádasi-Donner