tiny fix for sum and null-filled columns

Author: Jolanrensen

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

@@ -6,6 +6,7 @@ import java.math.BigDecimal
 import java.math.BigInteger
 import kotlin.reflect.KClass
 import kotlin.reflect.KType
+import kotlin.reflect.full.isSubtypeOf
 import kotlin.reflect.full.withNullability
 import kotlin.reflect.typeOf
 
@@ -224,10 +225,24 @@ internal fun Sequence<Number?>.convertToUnifiedNumberType(
                 "Cannot find unified number type of types: ${types.joinToString { renderType(it) }}",
             )
     }
-    val converter = createConverter(typeOf<Number>(), commonNumberType)!! as (Number) -> Number?
-    return map {
-        if (it == null) return@map null
-        converter(it) ?: error("Can not convert $it to $commonNumberType")
+    require(commonNumberType.isSubtypeOf(typeOf<Number?>())) {
+        "Cannot convert numbers to $commonNumberType; it is not a subtype of Number?"
+    }
+    return when (commonNumberType) {
+        nothingType -> {
+            require(null !in this) { "Cannot unify numbers to Nothing; it contains nulls" }
+            this
+        }
+
+        nullableNothingType -> this
+
+        else -> {
+            val converter = createConverter(typeOf<Number>(), commonNumberType)!! as (Number) -> Number?
+            this.map {
+                if (it == null) return@map null
+                converter(it) ?: error("Can not convert $it to $commonNumberType")
+            }
+        }
     }
 }
 

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/statistics/sum.kt

@@ -5,6 +5,8 @@ import io.kotest.matchers.doubles.shouldBeNaN
 import io.kotest.matchers.floats.shouldBeNaN
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.string.shouldContain
+import org.jetbrains.kotlinx.dataframe.DataColumn
+import org.jetbrains.kotlinx.dataframe.api.cast
 import org.jetbrains.kotlinx.dataframe.api.columnOf
 import org.jetbrains.kotlinx.dataframe.api.dataFrameOf
 import org.jetbrains.kotlinx.dataframe.api.isEmpty
@@ -14,7 +16,9 @@ import org.jetbrains.kotlinx.dataframe.api.sum
 import org.jetbrains.kotlinx.dataframe.api.sumFor
 import org.jetbrains.kotlinx.dataframe.api.sumOf
 import org.jetbrains.kotlinx.dataframe.api.toDataFrame
+import org.jetbrains.kotlinx.dataframe.impl.nullableNothingType
 import org.junit.Test
+import kotlin.reflect.typeOf
 
 class SumTests {
 
@@ -46,6 +50,28 @@ class SumTests {
         df.sumOf { value() } shouldBe expected
     }
 
+    @Test
+    fun `empty column with types`() {
+        val emptyIntCol by columnOf<Int?>(null, null)
+        emptyIntCol.sum() shouldBe 0
+
+        // empty column with Number type
+        val emptyNumberColumn = DataColumn.createValueColumn<Number?>(
+            "emptyNumberColumn",
+            listOf(null, null),
+            typeOf<Number?>(),
+        )
+        emptyNumberColumn.sum() shouldBe 0.0
+
+        // empty column with nullable Nothing type
+        val emptyNothingColumn = DataColumn.createValueColumn(
+            "emptyNothingColumn",
+            listOf(null, null),
+            nullableNothingType,
+        )
+        emptyNothingColumn.cast<Number?>().sum() shouldBe 0.0
+    }
+
     @Test
     fun `test multiple columns`() {
         val value1 by columnOf(1, 2, 3)