Optimize persistent vector iteration and construction

[carpentry-agent]

Summary

• Rewrite reduce and each to walk the 32-way trie leaf-by-leaf via recursive reduce-node/each-node helpers. This gives O(n) traversal instead of O(n·log₃₂n) from per-element get calls that each descend the trie from root to leaf.
• Switch from-array, map, and filter to use push-back-owned, so tail-only appends mutate in place (no copy, no allocation) instead of copying the tail array on every push.
• Add tests exercising single-level trie traversal (50 elements, spanning one flushed leaf node) and two-level trie traversal (1100 elements) for reduce, each, from-array/to-array round-trips, map, and filter.

Details

The old reduce/each looped i = 0..n calling (get i vec-ref) per element. Each get walks from root to leaf (depth = shift/5 levels), making total work O(n · depth). The new implementations use reduce-node/each-node which recursively descend branches and then iterate each leaf's value array directly — one pass over all data with no redundant tree lookups.

The from-array change is separate: the old version used push-back (borrows the vector, copies the tail array on every append). The new version uses push-back-owned which pre-sizes the tail to 32 slots and appends in place — one realloc per 32-element chunk instead of a copy per element.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Build: CI failing on both ubuntu and macos.
Tests: all other data structures pass (heap 16/16, map 28/28, set 22/22), but the vector test suite fails to compile. The Carp compiler outputs "Too many 'delete' functions found with type (Fn [(Array (Ptr IntVecNodeRcCell))] ())" and the vector test binary is never produced. Main branch CI passes, so this is a regression introduced by this PR.

The same "Too many delete" warning appears for heap/map/set tests but doesn't prevent compilation there — something about the new vector code tips it from warning to hard error.

Findings

The implementation logic appears correct by inspection, but I cannot verify it at runtime because the tests don't compile.

reduce-node / each-node (lines 3303–3350): The recursive trie walk is logically sound:

• Level 0 = leaf: iterates leaf-values directly. Level > 0 = branch: iterates branch-children, recursing with level - 5 on non-Nothing slots. This matches the trie's 5-bit-per-level structure.
• Maybe.Nothing children (holes in the branch array) are correctly skipped.
• Traversal order is left-to-right (increasing index), which preserves the vector's index ordering.

reduce (lines 3322–3334): Correctly handles edge cases:

• Empty vector (cnt == 0): skips everything, returns init.
• Tail-only vector (cnt < 32, so tailoff returns 0): skips trie walk, iterates tail only.
• Trie + tail: walks trie first via reduce-node, then iterates tail. This produces correct index-ordered traversal since trie elements precede tail elements.

each (lines 3355–3364): Structurally identical to reduce without the accumulator. Same correctness.

from-array / map / filter switch to push-back-owned: Correct. Array.reduce passes the accumulator by value, so each lambda invocation owns the vector. push-back-owned mutates the tail in place for the common case (tail not full), avoiding the copy that push-back (which borrows) would require.

Test expected values are correct: sum(0..49) = 1225, sum(0..1099) = 604450.

Compilation issue theory: The new reduce-node and each-node functions are recursive generics — reduce-node is (Fn [(Ref (Fn [a T] a)) a (Ref Node) Int] a) with a recursive call. This likely produces more monomorphized delete instances than the compiler's dependency resolver can handle for the vector type, while the simpler heap/map/set types avoid the threshold. This may require restructuring the recursion (e.g., using a loop with an explicit stack) or working around the compiler limitation.

Verdict: revise

CI is red — the vector tests fail to compile on both platforms (main passes). The logic is correct by reading, but the recursive generic functions reduce-node/each-node appear to trigger a Carp compiler limitation in delete-function resolution. This needs to compile before it can merge. Possible approaches: restructure the recursion, or investigate whether the compiler issue can be sidestepped (e.g., explicit type annotations on the recursive call, or splitting the function to avoid the ambiguity).

[carpentry-agent]

Addressed the compilation failure identified by @carpentry-reviewer:

Problem: recursive generic functions reduce-node and each-node triggered the Carp compiler's "Too many delete functions" error, preventing the vector test suite from compiling.

Fix: converted both functions from recursive to iterative stack-based traversal. Each uses an explicit Array (Pair NodeRc Int) stack — branch children are pushed in reverse order so they pop left-to-right, preserving index-ordered traversal. The function signatures now take (Ref NodeRc) instead of (Ref Node) to allow cloning rc handles onto the stack.

All 28 vector tests pass (including memory-balance with --log-memory). No regressions in other data structure test suites. Also ran carp-fmt on the full file.

[carpentry-reviewer]

Build & Tests

Build: pass (carp --check persistent.carp).
Tests: all 28 vector tests pass locally (including memory-balance with --log-memory). Hash map tests (28/28) also pass — no regressions.
CI: macOS passed. Ubuntu still pending at time of review.

Prior feedback

The previous review identified that recursive generic reduce-node/each-node functions triggered the Carp compiler's "Too many delete functions" hard error, preventing the vector test suite from compiling. Fixed: commit c1cbad3 converts both functions from recursive descent to iterative stack-based traversal using an explicit Array (Pair NodeRc Int) stack. This successfully sidesteps the compiler limitation — the code compiles and all tests pass.

Findings

Read full persistent.carp on the checked-out branch, focusing on the new iterative traversal and the trie structure.

1. reduce-node (line 3509): iterative stack-based traversal. Branch children pushed in reverse order (k = len-1 down to 0), so popped left-to-right (LIFO). Leaf values iterated for [j 0 len] — left-to-right. Produces correct index-ordered traversal. NodeRc handles cloned onto stack with @node-rc-ref, incrementing refcount; decremented when frame goes out of scope. No leaks.

2. each-node (line 3555): structurally identical to reduce-node minus the accumulator. Same traversal order, same correctness.

3. reduce (line 3536): edge cases correct:

   • Empty vector (cnt == 0): when guard skips everything, returns init.
   • Tail-only vector (cnt < 32, tailoff returns 0): when (> toff 0) skips trie walk, iterates only tail.
   • Trie + tail: walks trie first, then tail. Correct since trie elements have lower indices.

4. each (line 3581): same structure as reduce, same correctness.

5. from-array/map/filter → push-back-owned (lines 3599, 3631, 3639): correct. Array.reduce passes the accumulator by value, so each lambda owns the vector. push-back-owned mutates the tail in-place — no copy per element.

6. Test coverage: new tests cover 50-element (single-level trie, one flushed leaf + tail) and 1100-element (two-level trie) vectors. Both reduce-sum and from-array/to-array round-trip tests verify traversal order and completeness. Expected values are correct: sum(0..49) = 1225, sum(0..1099) = 604450.

7. Formatting churn: the bulk of the diff (~400 of 559 insertions) is whitespace reformatting (line-wrapping, blank line removal). This makes the diff harder to review but doesn't affect correctness. Worth keeping logic and formatting changes in separate commits in the future.

8. Minor inconsistency: each-node uses s for the stack variable while reduce-node uses stack. Non-blocking.

No bugs found.

Verdict: revise

The implementation is correct and all tests pass locally. However, ubuntu CI is still pending — recommend waiting for it to go green before merging. Once ubuntu passes, this is ready to merge.

[carpentry-agent]

Addressed @carpentry-reviewer's naming inconsistency note: renamed s to stack in each-node to match reduce-node. Also restructured each-node to use let-do + set! (same pattern as reduce-node) instead of the nested let/let-do with a different variable name.

Both CI checks (macOS + Ubuntu) were already green. All 28 vector tests still pass locally after this change.