#15820: Implement sequences of bounded integers
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Reporter: SimonKing | Owner:
Type: enhancement | Status: needs_work
Priority: major | Milestone: sage-6.3
Component: algebra | Resolution:
Keywords: sequence bounded | Merged in:
integer | Reviewers:
Authors: Simon King | Work issues: Fix flakyness in
Report Upstream: N/A | adding sequences
Branch: | Commit:
u/SimonKing/ticket/15820 | 735939e593c9c302ff42c4973cbfe2e48eb22644
Dependencies: | Stopgaps:
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Comment (by SimonKing):
Now I can explain what goes wrong in above example.
GMP has its limbs, and I have my sequence items. In the example, each item
consumes 5 bit, but each limb comprises 32 bit. As it happens, the last
bit of my last item is 0, and the last limb ''only'' stores this single
zero bit. Hence, when I ask GMP to do high-level operations (which makes
sense for concatenation, I wouldn't like to do it low-level), GMP tries to
be clever and drops the last limb, since it is zero. But when I transform
the result to a list, I want to access the last bit. But since the last
bit is not in a limb any longer, it belongs to non-allocated memory.
Hence, randomly, the last bit can be 0 or 1, which explains why in the
example above the difference between the correct and the wrong last item
is 16.
So, what I should do: When transforming a sequence to a string or list (or
when iterating), I should check whether I am running outside of GMP's
limbs. Alternatively, I should provide a bit 1 right behind of my last
item, so that GMP will not drop the last limb (as I forced it to be non-
zero).
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Ticket URL: <http://trac.sagemath.org/ticket/15820#comment:74>
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