fixup! adding tests and comments, missing types for median, expanding cumSum

Author: Jolanrensen

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

@@ -29,19 +29,19 @@ public fun <T : Number?> DataColumn<T>.cumSum(skipNA: Boolean = defaultCumSumSki
 
         typeOf<Int>() -> cast<Int>().cumSum().cast()
 
-        // TODO cumSum for Byte returns Int but is cast back to T: Byte
-        typeOf<Byte>() -> cast<Byte>().cumSum().cast()
+        // TODO cumSum for Byte returns Int but is converted back to T: Byte, Issue #558
+        typeOf<Byte>() -> cast<Byte>().cumSum().map { it.toByte() }.cast()
 
-        // TODO cumSum for Short returns Int but is cast back to T: Short
-        typeOf<Short>() -> cast<Short>().cumSum().cast()
+        // TODO cumSum for Short returns Int but is converted back to T: Short, Issue #558
+        typeOf<Short>() -> cast<Short>().cumSum().map { it.toShort() }.cast()
 
         typeOf<Int?>() -> cast<Int?>().cumSum(skipNA).cast()
 
-        // TODO cumSum for Byte? returns Int? but is cast back to T: Byte?
-        typeOf<Byte?>() -> cast<Byte?>().cumSum(skipNA).cast()
+        // TODO cumSum for Byte? returns Int? but is converted back to T: Byte?, Issue #558
+        typeOf<Byte?>() -> cast<Byte?>().cumSum(skipNA).map { it?.toByte() }.cast()
 
-        // TODO cumSum for Short? returns Int? but is cast back to T: Short?
-        typeOf<Short?>() -> cast<Short?>().cumSum(skipNA).cast()
+        // TODO cumSum for Short? returns Int? but is converted back to T: Short?, Issue #558
+        typeOf<Short?>() -> cast<Short?>().cumSum(skipNA).map { it?.toShort() }.cast()
 
         typeOf<Long>() -> cast<Long>().cumSum().cast()
 

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/math/sum.kt

@@ -13,16 +13,16 @@ internal fun <T, R : Number> Iterable<T>.sumOf(type: KType, selector: (T) -> R?)
     return when (type.classifier) {
         Double::class -> sumOf(selector as ((T) -> Double)) as R
 
-        // careful, conversion to Double to Float occurs! TODO
+        // careful, conversion to Double to Float occurs! TODO, Issue #558
         Float::class -> sumOf { (selector as ((T) -> Float))(it).toDouble() }.toFloat() as R
 
         Int::class -> sumOf(selector as ((T) -> Int)) as R
 
-        // careful, cast to Int occurs! TODO
-        Short::class -> sumOf { (selector as ((T) -> Short))(it).toInt() } as R
+        // careful, conversion to Int occurs! TODO, Issue #558
+        Short::class -> sumOf { (selector as ((T) -> Short))(it).toInt() }.toShort() as R
 
-        // careful, cast to Int occurs! TODO
-        Byte::class -> sumOf { (selector as ((T) -> Byte))(it).toInt() } as R
+        // careful, conversion to Int occurs! TODO, Issue #558
+        Byte::class -> sumOf { (selector as ((T) -> Byte))(it).toInt() }.toByte() as R
 
         Long::class -> sumOf(selector as ((T) -> Long)) as R