[repo-assist] Perf: eliminate per-element allocation in distinctUntilChangedWithAsync - #342
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Track the previous element with a hasPrev flag and mutable field instead of wrapping it in 'T option, matching the pattern already used by pairwise. Removes a per-element heap allocation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
Optimises
AsyncSeq.distinctUntilChangedWithAsync(and its callersdistinctUntilChangedWith/distinctUntilChanged) to avoid a per-element heap allocation.Previously the previous element was tracked as
prev : 'T option, reassigned viaprev <- Some von every iteration — each call allocated a newSomebox. The new implementation uses ahasPrev: boolflag plus a directmutable prev: 'Tfield, matching the same optimisation already applied topairwisein a prior PR (seeAsyncSeq.fspairwisefor precedent).No public API or behavioral changes — output sequence and semantics are identical for all inputs, including empty sequences and single-element sequences.
Test Status
dotnet build FSharp.Control.AsyncSeq.sln -c Release— succeeded, 0 errors.dotnet test tests/FSharp.Control.AsyncSeq.Tests -c Release— 486/486 tests passed (no test changes needed; existingdistinctUntilChanged/distinctUntilChangedWithtests continue to pass unchanged, confirming behavioral equivalence).🤖 This PR was created by Repo Assist, an automated AI assistant, as part of scheduled performance-improvement maintenance (Task 8). A human maintainer should review before merging.
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