[Gregory P. Smith <g...@krypto.org>]
> The reason for digits being a multiple of 5 bits should be revisited vs
> its original intent
I added that. The only intent was to make it easier to implement
bigint exponentiation easily while viewing the exponent as being in
base 32 (so as to chew up 5 bits at a time).. Since the number of
digit bits _was_ 15 at the time, and it was 'obvious enough" that 30
bits would work fine when 64-bit boxes became important enough to care
about, it was easy to settle on "multiple of 5" as a quick, pragmatic
tradeoff. The number controls how much space and time is needed to
precompute a lookup table for a "large" (many bits) power - and powers
can be very large indeed in the modular exponentiation case.
I doubt Python will move beyond 30-bit digits. While 64-bit support
became increasingly and steadily more widespread (both SW and HW) in
the 32-bit world. I see very much less enthusiasm to move beyond 64
bits. Yes, 64x64 -> 128 multiply is spreading, but that's about it.
Some applications can make very good use of that, but, in the grand
scheme of things, they remain niche. Whole universes of apps are
getting real benefit by moving from 32 to 64 bits. That's needed just
to get integers big enough to represent physical memory addresses in
many consumer desktops now. But we'll likely never need to go beyond
64 bits for that.
In any case, "multiple of 5" is a shallow constraint. and getting rid
of it shouldn't require more than rewriting longobject.c's
long_pow()'s
for (j = PyLong_SHIFT - 5; j >= 0; j -= 5) {
loop in a more convoluted way to cross digit boundaries when trying to
suck up "the next" 5 bits.
In fact, there are good reasons to make that loop more convoluted
anyway, but I never got around to it (it "should" instead be aiming to
find "the next" 5-bit slice that ends with a 1-bit, which would allow
cutting the aforementioned precomputed table in half).
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