ndigits with negative base: error out instead of giving incorrect result #29148
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Because of an unchecked conversion via `signed` (instead of `Signed`), we were giving wrong results, e.g. we had `ndigits(typemax(UInt), base=-2) != ndigits(big(typemax(UInt)), base=-2)`
|
What label would fit here? I would say "math" would be the closest, but not quite... |
|
Could we just give the correct answer here? Maybe by widening to a larger type? It may also be possible to tweak this computation for negative bases somehow instead. |
|
So, I think this definition might work: ndigits0znb(x::Unsigned, b::Integer) = ndigits0znb(-signed(fld(x, -b)), b) + (x != 0)It relies on the fact that |
|
Oh I was too lazy, but your idea is clever! I pushed it here to launch CI, but please re-commit in your name! |
rfourquet
force-pushed
the
rf/ndigit0zsnb-unsigned
branch
from
c67ca34 to
d2c92b8
Compare
KristofferC
pushed a commit
that referenced
this pull request
Sep 17, 2018
It relies on the fact that `cld(x, b) == -fld(x, -b)` but does the conversion from unsigned to signed before negating the unsigned quotient; since `-b ≥ 2` the quotient always fits in the signed type. (cherry picked from commit 77ec1ec)
Merged
KristofferC
pushed a commit
that referenced
this pull request
Feb 11, 2019
It relies on the fact that `cld(x, b) == -fld(x, -b)` but does the conversion from unsigned to signed before negating the unsigned quotient; since `-b ≥ 2` the quotient always fits in the signed type. (cherry picked from commit 77ec1ec)
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Because of an unchecked conversion via
signed(instead ofSigned),we were giving wrong results, e.g. we had
ndigits(typemax(UInt), base=-2) != ndigits(big(typemax(UInt)), base=-2)