[GauravPawar101]

Closes #1108

kmerge_impl.rs and k_smallest.rs each had their own heap code. This moves
heapify and sift_down into src/heap.rs and uses them in both places.

PS

I kept the sift_down from kmerge_impl.rs since it was written to be
branchless. It's a min-heap, so k_smallest_general now passes a "greater
than" predicate to get the largest element on top, like before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (6814180) to head (1a6e281).
⚠️ Report is 229 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1139      +/-   ##
==========================================
- Coverage   94.38%   93.71%   -0.68%     
==========================================
  Files          48       53       +5     
  Lines        6665     6458     -207     
==========================================
- Hits         6291     6052     -239     
- Misses        374      406      +32     

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[GauravPawar101]

None of them is mine except fmt.

[phimuemue]

Thank you.

[phimuemue]

Thanks for this.

I always find it confusing to pass a greater_than into a function that expects a less_than (such as heapify or sift_down), and then commenting at the call site what you're doing.

It is a matter of taste, but what I usually do for these is this:

• make the functions heapify and sift_down take a is_better instead of less_than.
• annotate the callers this way heapify(/*is_better*/greater_than).

If you want, open a follow-up PR with these changes.