perf: replace stats::stepfun with stepfun2 in expected_event

[yihui]

Summary

• Replace 4 stats::stepfun() calls with stepfun2() in expected_event()
• Improve stepfun2() itself: use a for-loop of vectorized comparisons (x >= b) for small breakpoint vectors (n <= 10), falling back to findInterval for larger ones

Why

stats::stepfun() is 57x slower to create than stepfun2() due to S3 class setup, environment construction, and method registration. In expected_event, step functions are created fresh on every call and evaluated once on small vectors (3-5 elements) — creation cost dominates.

The loop approach avoids findInterval's dispatch overhead and binary search, which are overkill for 3-4 breakpoints. Each iteration is a single vectorized primitive (x >= b), with no allocation.

Benchmark (the 4-stepfun create+eval pattern in expected_event, 50k calls)

Implementation	Time	Speedup vs stats::stepfun
stats::stepfun	10.8s	1x
stepfun2 (old, findInterval)	1.0s	11x
stepfun2 (new, loop for n<=10)	0.5s	21x

Test plan

• test-developer-expected_event.R passes
• test-independent-expected_event.R passes
• Verified stepfun2 produces identical results to stats::stepfun for both right = FALSE and right = TRUE

🤖 Generated with Claude Code

[yihui]

@LittleBeannie I'm very confident that the new stepfun2() can safely replace stats::stepfun(), but please feel free to close the PR if you are not comfortable with changes in expected_event().

[LittleBeannie]

Thank you so much for your work on optimizing this. I really appreciate the effort and the improvements you've proposed.

The expected_event() function is a very low-level and complex component with many interconnected formulas. Because of its critical nature, our current policy is to only merge changes that have been scoped and suggested by Keaven, who is the primary architect of this module.

Therefore, I will be closing this PR for now. This is purely a process decision to maintain stability and is not a reflection on the quality of your work, which is excellent.

We can definitely re-open this or a similar PR in the future when the development roadmap for this module is updated.

Thank you again for your contribution!