zeroshade commented on PR #1256:
URL: https://github.com/apache/arrow-go/pull/1256#issuecomment-5457879484
Rebased onto current `main`, which now carries both #1172 and #1220.
#1220 added an early return to `spacedExpand` once the decoded prefix is
already aligned. I mirrored it in `spacedExpandSwap` — at that point every
remaining swap is a self-swap, so the same reasoning applies and the two
functions stay consistent:
```go
idxDecode -= run.Length
// Once the decoded prefix is already aligned every remaining swap is a
// self-swap, so there is nothing left to do. Mirrors spacedExpand.
if idxDecode == run.Pos {
return numValues
}
```
Re-verified on the new base: the randomized differential test now compares
against the early-returning `spacedExpand` and still agrees at every valid
position while never leaving duplicates; the decoder-level regression test
still fails at all six widths without the fix, and the `pqarrow` round-trip
still fails without it. `parquet/...` passes in full and `golangci-lint`
reports 0 issues.
The earlier measurement stands: with #1172 now on `main`, `DecodeSpaced` is
0 allocs/op in steady state with this fix applied.
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