Raymond Hettinger <[email protected]> added the comment:
Here's variant (minimally tested) that has a fast path for the common case (no
overflow or underflow), that uses a list instead of a deque, that keeps memory
use small (only storing pending values that can't be combined without
triggering an overflow or underflow).
----------------------------
from math import fabs, prod, isfinite
def product(seq, start=1.0):
total = start
s = [] # values that would overflow the total
s_side = False # true if s_i increase the magnitude of the product
for x in seq:
old_total = total
total *= x
underflow = not total and old_total and not x
if isfinite(total) and not underflow:
continue # fast-path for multiplies that doesn't
overflow/underflow
total = old_total
side = fabs(x) > 1.0
if not s or side == s_side:
s.append(x)
s_side = side
continue
t = [x, total]
while s and t: # opposite sides: matter and antimatter
x = s.pop() * t.pop()
side = fabs(x) > 1.0
s.append(x) if side == s_side else t.append(y)
if t:
s = t
s_side = not s_side
total = s.pop()
return prod(s, start=total)
----------
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Python tracker <[email protected]>
<https://bugs.python.org/issue41458>
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