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@eriknw eriknw commented Nov 4, 2023

CC @michelp who requested this.

When n=0, I decided to create a complete diagonal matrix using the identity of the binaryop if the identity exists (and raise if it doesn't).

We want the following equality to hold:

I = A.power(0, semiring)
semiring(I @ A @ I).isequal(A)

We could try to make the diagonal more sparse to better support hypersparse matrices, but this would need to determine both nonempty rows and nonempty columns, because I needs to be able to be the left-identity and the right-identity. Creating the complete diagonal matrix is much simpler.

CC @jim22k. When we added Matrix.power in #483, I believe we discussed n=0, and we decided to punt until it was needed. As we discussed, n=0 is a little awkward b/c the identity value isn't always available (since it ought to come from the binaryop--not the monoid--of the semiring), and we weren't sure whether we should try to return a sparse diagonal or if this is what the user would want and expect. I think the behavior in this PR is the best approach.

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codecov bot commented Nov 4, 2023

Codecov Report

Merging #518 (5bddc8c) into main (7935e50) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   99.20%   99.24%   +0.04%     
==========================================
  Files          95       95              
  Lines       21291    21305      +14     
  Branches     3999     4001       +2     
==========================================
+ Hits        21121    21144      +23     
+ Misses        124      112      -12     
- Partials       46       49       +3     

@eriknw eriknw merged commit 0bfcb66 into python-graphblas:main Dec 12, 2023
@eriknw eriknw added the enhancement Improve existing functionality or make things work better label Dec 13, 2023
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