migrated Column DataCollector to use typeOf instead of ::class, and added redundancy checks for nothing types in column creation

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/ColumnDataCollector.kt

@@ -3,16 +3,18 @@ package org.jetbrains.kotlinx.dataframe.impl
 import org.jetbrains.kotlinx.dataframe.AnyFrame
 import org.jetbrains.kotlinx.dataframe.AnyRow
 import org.jetbrains.kotlinx.dataframe.DataColumn
-import org.jetbrains.kotlinx.dataframe.DataFrame
-import org.jetbrains.kotlinx.dataframe.DataRow
+import org.jetbrains.kotlinx.dataframe.api.asDataColumn
+import org.jetbrains.kotlinx.dataframe.api.cast
 import org.jetbrains.kotlinx.dataframe.api.concat
 import org.jetbrains.kotlinx.dataframe.api.toDataFrame
 import org.jetbrains.kotlinx.dataframe.impl.columns.createColumnGuessingType
 import kotlin.reflect.KClass
 import kotlin.reflect.KType
 import kotlin.reflect.full.isSubclassOf
+import kotlin.reflect.full.isSubtypeOf
 import kotlin.reflect.full.withNullability
 import kotlin.reflect.jvm.jvmErasure
+import kotlin.reflect.typeOf
 
 public interface DataCollector<T> {
 
@@ -38,17 +40,29 @@ internal abstract class DataCollectorBase<T>(initCapacity: Int) : DataCollector<
         data.add(value)
     }
 
-    protected fun createColumn(name: String, type: KType): DataColumn<T> {
-        val classifier = type.classifier as KClass<*>
-        if (classifier.isSubclassOf(DataFrame::class) && !hasNulls) {
-            return DataColumn.createFrameColumn(name, data as List<AnyFrame>) as DataColumn<T>
-        }
-        if (classifier.isSubclassOf(DataRow::class) && !hasNulls) {
-            val mergedDf = (data as List<AnyRow>).map { it.toDataFrame() }.concat()
-            return DataColumn.createColumnGroup(name, mergedDf) as DataColumn<T>
-        }
-        return DataColumn.createValueColumn(name, data, type.withNullability(hasNulls)) as DataColumn<T>
-    }
+    @Suppress("UNCHECKED_CAST")
+    protected fun createColumn(name: String, type: KType): DataColumn<T> =
+        when {
+            type == nothingType -> {
+                require(values.isEmpty()) { "Cannot create non-empty DataColumn of type Nothing" }
+                DataColumn.empty(name)
+            }
+
+            type == nullableNothingType -> {
+                require(values.all { it == null }) { "Cannot create DataColumn of type Nothing? with non-null values" }
+                DataColumn.createValueColumn(name, values, nullableNothingType)
+            }
+
+            type.isSubtypeOf(typeOf<AnyFrame?>()) && !hasNulls ->
+                DataColumn.createFrameColumn(name, data as List<AnyFrame>)
+
+            type.isSubtypeOf(typeOf<AnyRow?>()) && !hasNulls -> {
+                val mergedDf = (data as List<AnyRow>).map { it.toDataFrame() }.concat()
+                DataColumn.createColumnGroup(name, mergedDf).asDataColumn()
+            }
+
+            else -> DataColumn.createValueColumn(name, data, type.withNullability(hasNulls))
+        }.cast()
 }
 
 internal open class ColumnDataCollector(initCapacity: Int = 0, val typeOf: (KClass<*>) -> KType) :