Added convenience method to get root of an operator tree.

vitrivr-engine-core/src/main/kotlin/org/vitrivr/engine/core/operators/Operator.kt

@@ -7,9 +7,13 @@ import kotlinx.coroutines.flow.Flow
  * A basic implementation of a pipeline-able operator.
  *
  * @author Ralph Gasser
- * @version 1.0.0
+ * @version 1.1.0
  */
 sealed interface Operator<O> {
+    /**
+     * Returns root of this [Unary].
+     */
+    fun root(): Operator<*>
 
     /**
      * Converts this [Operator] to a [Flow] of type [O].
@@ -22,14 +26,24 @@ sealed interface Operator<O> {
     /**
      * A nullary operator (usually a source).
      */
-    interface Nullary<O>: Operator<O>
+    interface Nullary<O>: Operator<O> {
+        /**
+         * Returns root of this [Nullary] (which is this).
+         */
+        override fun root(): Operator<*> = this
+    }
 
     /**
      * A unary [Operator] with one input [Operator] and one output [Operator].
      */
     interface Unary<I,O>: Operator<O> {
         /** The input [Operator]. */
         val input: Operator<I>
+
+        /**
+         * Returns root of this [Unary].
+         */
+        override fun root(): Operator<*> = this.input.root()
     }
 
     /**
@@ -41,6 +55,11 @@ sealed interface Operator<O> {
 
         /** The input [Operator]. */
         val input2: Operator<I>
+
+        /**
+         * Returns root of this [Unary], which is the left-hand operator (by definition).
+         */
+        override fun root(): Operator<*> = this.input1.root()
     }
 
     /**
@@ -49,6 +68,11 @@ sealed interface Operator<O> {
     interface NAry<I,O>: Operator<O> {
         /** The input [Operator]s. */
         val inputs: List<Operator<I>>
+
+        /**
+         * Returns root of this [Unary], which is the left-hand operator (by definition).
+         */
+        override fun root(): Operator<*> = this.inputs.first().root()
     }
 
     /**
@@ -57,5 +81,10 @@ sealed interface Operator<O> {
     interface Sink<I>: Operator<Unit> {
         /** The input [Operator]. */
         val input: Operator<I>
+
+        /**
+         * Returns root of this [Unary], which is the left-hand operator (by definition).
+         */
+        override fun root(): Operator<*> = this.input.root()
     }
 }