perf(sorted_set): O(n+m) co-iteration for set operations

[bobzhang]

Summary

Replace O(n*log m) implementations of subset, disjoint, intersection, difference, and symmetric_difference with O(n+m) co-iteration over both sorted iterators.

Before

Each operation iterated self and called contains() on src for each element — O(log m) per lookup, O(n*log m) total:

pub fn subset(self, src) -> Bool {
  self.iter().all(x => src.contains(x))  // O(n * log m)
}
pub fn intersection(self, src) -> SortedSet {
  let ret = new()
  self.each(x => if src.contains(x) { ret.add(x) })  // O(n * log m)
  ret
}

symmetric_difference was even worse — it created two intermediate sets via difference() then called union() (with a TODO acknowledging this).

After

Co-iterate both sorted iterators in lockstep using compare(), advancing the smaller side — O(n+m) total, zero allocations on the input sets:

pub fn subset(self, src) -> Bool {
  let it1 = self.iter()
  let it2 = src.iter()
  // advance in lockstep, O(n+m)
}

Complexity comparison

Operation	Before	After
subset	O(n·log m)	O(n+m)
disjoint	O(n·log m)	O(n+m)
intersection	O(n·log m)	O(n+m)
difference	O(n·log m)	O(n+m)
symmetric_difference	O(n·log m + m·log n + n+m)	O(n+m)

Codex review

I did not find a correctness issue in the co-iteration logic. The iterator advancement rules for subset, disjoint, intersection, difference, and symmetric_difference match the expected merge-style behavior on two sorted unique streams, and moon test sorted_set passes after the change.

Test plan

• All 48 sorted_set tests pass

🤖 Generated with Claude Code

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[coveralls]

Pull Request Test Coverage Report for Build 3103

Details

• 61 of 71 (85.92%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 95.636%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sorted_set/set.mbt	61	71	85.92%

Totals
Change from base Build 3102:	-0.05%
Covered Lines:	13850
Relevant Lines:	14482

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💛 - Coveralls

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.