zeroshade commented on code in PR #1034:
URL: https://github.com/apache/arrow-go/pull/1034#discussion_r3660790285
##########
parquet/pqarrow/schema.go:
##########
@@ -246,7 +246,7 @@ func DecimalSize(precision int32) int32 {
if precision <= 76 {
return byteblock[precision]
}
- return int32(math.Ceil(float64(precision)/8.0)*math.Log2(10) + 1)
+ return int32(math.Ceil((float64(precision)*math.Log2(10) + 1) / 8))
Review Comment:
This now matches the generator comment at line 238 verbatim, which is the
strongest evidence it's the intended formula.
Confirmed exactly minimal against big-integer arithmetic for p=77..3000, and
continuous with `byteblock` at the p=76 boundary (new formula reproduces all 76
table entries). The previous expression applied `ceil` to `precision/8` before
multiplying by `log2(10)`, which mixed units and produced a step function —
constant 34 across p=74..80 — and under-allocated from p=248 onward.
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