Guard BitVectorHelper.getNullCount in StructVector.nullMaskOrNull

[koperagen]

Noticed that Total runtime of reading a bunch of parquet datasets goes from 55 sec in rc01 to 70 sec in dev.
Main difference comes from BitVectorHelper.getNullCount usage in new code. It actually goes over entire vector to count, even though no nulls are expected in StructVector according to schema
Extra check on StructVector nullability seems to achieve the same result, assuming we can trust file schema to not mislead us and not have sneaky nulls:

val nullMask = if (field.isNullable) vector.nullMaskOrNull(range) else null

then nullCount > 0 should not be needed then and reading time goes back to 55s.

[koperagen]

Index: dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt b/dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt
--- a/dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt	(revision b9bc70bc1576555c309929c033cdfb238478e5a9)
+++ b/dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt	(date 1788465604628)
@@ -355,8 +355,11 @@
  * child vectors — or `null` when this slice has no null-parent rows, so required groups are read as before.
  */
 private fun StructVector.nullMaskOrNull(range: IntRange): BooleanArray? =
-    if (nullCount > 0) {
-        BooleanArray(range.count()) { isNull(range.first + it) }.takeIf { mask -> mask.any { it } }
+    if (field.isNullable) {
+        val from = range.first
+        val size = range.last - from + 1
+        if (size <= 0) return null
+        BooleanArray(size) { isNull(range.first + it) }.takeIf { mask -> mask.any { it } }
     } else {
         null
     }
@@ -375,7 +378,7 @@
             // children instead. When the struct has null rows we also read its children with Widening: their
             // physically-null cells at those rows are about to be nulled anyway and must not trip
             // NullabilityOptions.Checking. Required groups (no struct-level nulls) are read exactly as before.
-            val nullMask = vector.nullMaskOrNull(range)
+            val nullMask = if (field.isNullable) vector.nullMaskOrNull(range) else null
             val childNullability = if (nullMask != null) NullabilityOptions.Widening else nullability
             val columns = field.children.map { childField ->
                 val child = readField(vector.getChild(childField.name), childField, childNullability, range)
@@ -476,6 +479,7 @@
         val frames = range.map { i ->
             val start = accessor.getElementStartIndex(i)
             val end = accessor.getElementEndIndex(i)
+            val nullMask = if (structField.isNullable) dataVector.nullMaskOrNull(start until end) else null
             val columns = structField.children.map { childField ->
                 readField(
                     dataVector.getChild(childField.name),
@@ -484,7 +488,6 @@
                     start until end,
                 )
             }
-            val nullMask = dataVector.nullMaskOrNull(start until end)
             val elementColumns = if (nullMask != null) columns.map { it.injectNullsAt(nullMask) } else columns
             elementColumns.toDataFrame()
         }