Fix default column selector for explode

Author: nikitinas

src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/explode.kt

@@ -10,11 +10,13 @@ import org.jetbrains.kotlinx.dataframe.impl.api.explodeImpl
 import org.jetbrains.kotlinx.dataframe.impl.columns.toColumns
 import kotlin.reflect.KProperty
 
+private val defaultExplodeColumns: ColumnsSelector<*, *> = { dfs { it.isList() || it.isFrameColumn() } }
+
 // region explode DataFrame
 
 public fun <T> DataFrame<T>.explode(
     dropEmpty: Boolean = true,
-    selector: ColumnsSelector<T, *> = { all() }
+    selector: ColumnsSelector<T, *> = defaultExplodeColumns
 ): DataFrame<T> = explodeImpl(dropEmpty, selector)
 
 public fun <T> DataFrame<T>.explode(vararg columns: String, dropEmpty: Boolean = true): DataFrame<T> =
@@ -32,7 +34,7 @@ public fun <T, C> DataFrame<T>.explode(vararg columns: KProperty<C>, dropEmpty:
 
 public fun <T> DataRow<T>.explode(
     dropEmpty: Boolean = true,
-    selector: ColumnsSelector<T, *> = { all() }
+    selector: ColumnsSelector<T, *> = defaultExplodeColumns
 ): DataFrame<T> = toDataFrame().explode(dropEmpty, selector)
 
 public fun <T> DataRow<T>.explode(vararg columns: String, dropEmpty: Boolean = true): DataFrame<T> =

src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/explode.kt

@@ -8,8 +8,24 @@ class ExplodeTests {
     @Test
     fun `explode into`() {
         val df = dataFrameOf("a" to listOf(1), "b" to listOf(listOf(2, 3)))
-        val imploded = df.explode { "b" into "c" }
+        val exploded = df.explode { "b" into "c" }
         val expected = dataFrameOf("a" to listOf(1, 1), "c" to listOf(2, 3))
-        imploded shouldBe expected
+        exploded shouldBe expected
+    }
+
+    @Test
+    fun `explode list and duplicate value`() {
+        val exploded = dataFrameOf("a", "b")(1, listOf(2, 3)).explode()
+        exploded shouldBe dataFrameOf("a", "b")(1, 2, 1, 3)
+    }
+
+    @Test
+    fun `explode list and frame column`() {
+        val exploded = dataFrameOf("a", "b", "c", "d")(1, listOf(2, 3), dataFrameOf("x", "y")(4, 5, 6, 7), listOf(8))
+            .explode().ungroup("c")
+        exploded shouldBe dataFrameOf("a", "b", "x", "y", "d")(
+            1, 2, 4, 5, 8,
+            1, 3, 6, 7, null
+        )
     }
 }