Added nullability support for the describe() (#384)

* Added fix and test

* Generated files

Author: zaleslaw

core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/describe.kt

@@ -64,7 +64,7 @@ internal fun describeImpl(cols: List<AnyCol>): DataFrame<ColumnDescription> {
         if (hasLongPaths) {
             ColumnDescription::path from { it.path() }
         }
-        ColumnDescription::type from { it.type.jvmErasure.simpleName }
+        ColumnDescription::type from { buildTypeName(it) }
         ColumnDescription::count from { it.size }
         ColumnDescription::unique from { it.countDistinct() }
         ColumnDescription::nulls from { it.values.count { it == null } }
@@ -91,3 +91,12 @@ internal fun describeImpl(cols: List<AnyCol>): DataFrame<ColumnDescription> {
 
     return df.cast()
 }
+
+private fun buildTypeName(it: AnyCol): String {
+    val rawJavaType = it.type.jvmErasure.simpleName.toString()
+    return if (it.type.isMarkedNullable) {
+        "$rawJavaType?"
+    } else {
+        rawJavaType
+    }
+}

core/generated-sources/src/test/kotlin/org/jetbrains/kotlinx/dataframe/testSets/person/DataFrameTests.kt

@@ -2170,6 +2170,18 @@ class DataFrameTests : BaseTest() {
     fun describe() {
         val desc = typed.group { age and weight }.into("info").groupBy { city }.toDataFrame().describe()
         desc.nrow shouldBe typed.ncol + 1
+        desc["type"][0] shouldBe "String?"
+        desc["path"][1] shouldBe listOf("group", "name")
+        desc["name"][0] shouldBe "city"
+        desc["count"][3] shouldBe 7
+        desc["unique"][4] shouldBe 6
+        desc["nulls"][3] shouldBe 2
+        desc["top"][0] shouldBe "London"
+        desc["mean"][2] shouldBe 28.571428571428573
+        desc["std"][2] shouldBe 11.073348527841414
+        desc["min"][2] shouldBe 15
+        desc["median"][2] shouldBe 30
+        desc["max"][2] shouldBe 45
         desc.print()
     }
 

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

@@ -64,7 +64,7 @@ internal fun describeImpl(cols: List<AnyCol>): DataFrame<ColumnDescription> {
         if (hasLongPaths) {
             ColumnDescription::path from { it.path() }
         }
-        ColumnDescription::type from { it.type.jvmErasure.simpleName }
+        ColumnDescription::type from { buildTypeName(it) }
         ColumnDescription::count from { it.size }
         ColumnDescription::unique from { it.countDistinct() }
         ColumnDescription::nulls from { it.values.count { it == null } }
@@ -91,3 +91,12 @@ internal fun describeImpl(cols: List<AnyCol>): DataFrame<ColumnDescription> {
 
     return df.cast()
 }
+
+private fun buildTypeName(it: AnyCol): String {
+    val rawJavaType = it.type.jvmErasure.simpleName.toString()
+    return if (it.type.isMarkedNullable) {
+        "$rawJavaType?"
+    } else {
+        rawJavaType
+    }
+}

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/testSets/person/DataFrameTests.kt

@@ -2170,6 +2170,18 @@ class DataFrameTests : BaseTest() {
     fun describe() {
         val desc = typed.group { age and weight }.into("info").groupBy { city }.toDataFrame().describe()
         desc.nrow shouldBe typed.ncol + 1
+        desc["type"][0] shouldBe "String?"
+        desc["path"][1] shouldBe listOf("group", "name")
+        desc["name"][0] shouldBe "city"
+        desc["count"][3] shouldBe 7
+        desc["unique"][4] shouldBe 6
+        desc["nulls"][3] shouldBe 2
+        desc["top"][0] shouldBe "London"
+        desc["mean"][2] shouldBe 28.571428571428573
+        desc["std"][2] shouldBe 11.073348527841414
+        desc["min"][2] shouldBe 15
+        desc["median"][2] shouldBe 30
+        desc["max"][2] shouldBe 45
         desc.print()
     }