feat(isthmus): add dynamic function conversion capabilities (#457)

Author: ZorinAnton

isthmus/src/main/java/io/substrait/isthmus/ExtensionUtils.java

@@ -0,0 +1,49 @@
+package io.substrait.isthmus;
+
+import io.substrait.extension.SimpleExtension;
+import io.substrait.isthmus.expression.FunctionMappings;
+import java.util.List;
+import java.util.Locale;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+public class ExtensionUtils {
+
+  /**
+   * Extracts dynamic extensions from a collection of extensions.
+   *
+   * <p>A <b>dynamic extension</b> is a user-defined function (UDF) that is not part of the standard
+   * Substrait function catalog. These are custom functions that users define and provide at
+   * runtime, extending the built-in function set with domain-specific or application-specific
+   * operations.
+   *
+   * <p>This method filters out all functions that are already known to the Calcite operator table
+   * (the standard/built-in functions) and returns only the custom functions that represent new
+   * capabilities not available in the default function set.
+   *
+   * <p><b>Example:</b> If a user defines a custom UDF "my_hash_function" that computes a
+   * proprietary hash, this would be a dynamic extension since it's not part of the standard
+   * Substrait specification.
+   *
+   * @param extensions the complete collection of extensions (both standard and custom)
+   * @return a new ExtensionCollection containing only the dynamic (custom/user-defined) functions
+   *     that are not present in the standard Substrait function catalog
+   */
+  public static SimpleExtension.ExtensionCollection getDynamicExtensions(
+      SimpleExtension.ExtensionCollection extensions) {
+    Set<String> knownFunctionNames =
+        FunctionMappings.SCALAR_SIGS.stream()
+            .map(FunctionMappings.Sig::name)
+            .collect(Collectors.toSet());
+
+    List<SimpleExtension.ScalarFunctionVariant> customFunctions =
+        extensions.scalarFunctions().stream()
+            .filter(f -> !knownFunctionNames.contains(f.name().toLowerCase(Locale.ROOT)))
+            .collect(Collectors.toList());
+
+    return SimpleExtension.ExtensionCollection.builder()
+        .scalarFunctions(customFunctions)
+        // TODO: handle aggregates and other functions
+        .build();
+  }
+}

isthmus/src/main/java/io/substrait/isthmus/FeatureBoard.java

@@ -17,4 +17,19 @@ public abstract class FeatureBoard {
   public Casing unquotedCasing() {
     return Casing.TO_UPPER;
   }
+
+  /**
+   * Controls whether to support dynamic user-defined functions (UDFs) during SQL to Substrait plan
+   * conversion.
+   *
+   * <p>When enabled, custom functions defined in extension YAML files are available for use in SQL
+   * queries. These functions will be dynamically converted to SQL operators during plan conversion.
+   * This feature must be explicitly enabled by users and is disabled by default.
+   *
+   * @return true if dynamic UDFs should be supported; false otherwise (default)
+   */
+  @Value.Default
+  public boolean allowDynamicUdfs() {
+    return false;
+  }
 }