Creating a way to allow Kandy to create a custom GroupBy class by the GroupByInternal interface

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/groupBy.kt

@@ -11,7 +11,7 @@ import org.jetbrains.kotlinx.dataframe.annotations.Interpretable
 import org.jetbrains.kotlinx.dataframe.annotations.Refine
 import org.jetbrains.kotlinx.dataframe.columns.FrameColumn
 import org.jetbrains.kotlinx.dataframe.columns.toColumnSet
-import org.jetbrains.kotlinx.dataframe.impl.GroupByImpl
+import org.jetbrains.kotlinx.dataframe.impl.GroupByInternal
 import org.jetbrains.kotlinx.dataframe.impl.aggregation.PivotImpl
 import org.jetbrains.kotlinx.dataframe.impl.api.getPivotColumnPaths
 import org.jetbrains.kotlinx.dataframe.impl.api.groupByImpl
@@ -101,7 +101,7 @@ public fun <T, G> GroupBy<T, G>.toDataFrame(groupedColumnName: String? = null):
         internal().df.rename(groups).into(groupedColumnName)
     }
 
-internal fun <T, G> GroupBy<T, G>.internal(): GroupByImpl<T, G> = this as GroupByImpl<T, G>
+internal fun <T, G> GroupBy<T, G>.internal(): GroupByInternal<T, G> = this as GroupByInternal<T, G>
 
 public interface Grouped<out T> : Aggregatable<T>
 

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

@@ -16,6 +16,7 @@ import org.jetbrains.kotlinx.dataframe.api.getColumnsWithPaths
 import org.jetbrains.kotlinx.dataframe.api.isColumnGroup
 import org.jetbrains.kotlinx.dataframe.api.minus
 import org.jetbrains.kotlinx.dataframe.api.pathOf
+import org.jetbrains.kotlinx.dataframe.api.toDataFrame
 import org.jetbrains.kotlinx.dataframe.columns.FrameColumn
 import org.jetbrains.kotlinx.dataframe.impl.aggregation.AggregatableInternal
 import org.jetbrains.kotlinx.dataframe.impl.aggregation.GroupByReceiverImpl
@@ -29,14 +30,26 @@ import org.jetbrains.kotlinx.dataframe.ncol
 import org.jetbrains.kotlinx.dataframe.nrow
 import org.jetbrains.kotlinx.dataframe.values
 
+/**
+ * Internal implementation interface for [GroupBy].
+ *
+ * While [df] should be hidden in the [GroupBy] DSL, it must be accessible when running
+ * [GroupBy.toDataFrame].
+ *
+ * This interface is public to allow Kandy to implement it.
+ */
+public interface GroupByInternal<T, G> : GroupBy<T, G> {
+    public val df: DataFrame<T>
+}
+
 /**
  * @property df DataFrame containing [groups] column and key columns. Represents GroupBy.
  */
 internal class GroupByImpl<T, G>(
-    val df: DataFrame<T>,
+    override val df: DataFrame<T>,
     override val groups: FrameColumn<G>,
     internal val keyColumnsInGroups: ColumnsSelector<G, *>,
-) : GroupBy<T, G>,
+) : GroupByInternal<T, G>,
     AggregatableInternal<G> {
 
     override val keys by lazy { df - groups }