Implemented recursive schema checker

Enables nested structures to be compared regardless of order.

src/main/scala/com/github/mrpowers/spark/daria/sql/DataFrameSchemaChecker.scala

@@ -1,13 +1,42 @@
 package com.github.mrpowers.spark.daria.sql
 
 import org.apache.spark.sql.DataFrame
-import org.apache.spark.sql.types.StructType
+import org.apache.spark.sql.types.{StructField, StructType}
+
+import scala.annotation.tailrec
+import scala.util.{Failure, Success, Try}
 
 case class InvalidDataFrameSchemaException(smth: String) extends Exception(smth)
 
 private[sql] class DataFrameSchemaChecker(df: DataFrame, requiredSchema: StructType) {
+  private def diff(required: Seq[StructField], schema: StructType): Seq[StructField] = {
+    required.filterNot(isPresentIn(schema))
+  }
+
+  private def isPresentIn(schema: StructType)(reqField: StructField): Boolean = {
+    Try(schema(reqField.name)) match {
+      case Success(namedField) =>
+        val basicMatch =
+          namedField.name == reqField.name &&
+            namedField.nullable == reqField.nullable &&
+            namedField.metadata == reqField.metadata
+
+        val contentMatch = reqField.dataType match {
+          case reqSchema: StructType =>
+            namedField.dataType match {
+              case fieldSchema: StructType =>
+                diff(reqSchema, fieldSchema).isEmpty
+              case _ => false
+            }
+          case _ => reqField == namedField
+        }
+
+        basicMatch && contentMatch
+      case Failure(_) => false
+    }
+  }
 
-  val missingStructFields = requiredSchema.diff(df.schema)
+  val missingStructFields: Seq[StructField] = diff(requiredSchema, df.schema)
 
   def missingStructFieldsMessage(): String = {
     s"The [${missingStructFields.mkString(", ")}] StructFields are not included in the DataFrame with the following StructFields [${df.schema.toString()}]"