Function Integration

/**
 * Utilities for user-defined function management and validation
 */
object UserDefinedFunctionUtils {
  
  /**
   * Validates that a class can be instantiated for UDF usage
   * @param clazz Class to check for instantiation capability
   * @throws ValidationException if class cannot be instantiated
   */
  def checkForInstantiation(clazz: Class[_]): Unit
  
  /**
   * Checks whether the given class is not a Scala singleton object
   * Prevents concurrent risks with TableFunction implementations
   * @param clazz Class to check for singleton pattern
   * @throws ValidationException if class is a Scala object
   */
  def checkNotSingleton(clazz: Class[_]): Unit
  
  /**
   * Gets the eval method signature for a user-defined function
   * @param function User-defined function instance
   * @param expectedTypes Expected parameter types
   * @return Method signature for eval method
   */
  def getEvalMethodSignature(
    function: UserDefinedFunction,
    expectedTypes: Array[LogicalType]
  ): Method
  
  /**
   * Checks if a specified method exists in the function
   * @param method Method name to check
   * @param function User-defined function instance
   * @return true if method exists, false otherwise
   */
  def ifMethodExistInFunction(method: String, function: UserDefinedFunction): Boolean
  
  /**
   * Extracts method signatures from UDF class for function inference
   * @param clazz UDF class to analyze
   * @param methodName Name of the method to extract (e.g., "eval", "accumulate")
   * @return Array of method signatures found
   */
  def getMethodSignatures(clazz: Class[_], methodName: String): Array[MethodSignature]
  
  /**
   * Determines result type information from UDF method signatures
   * @param signatures Array of method signatures
   * @param inputTypes Input argument types
   * @return Inferred result type information
   */
  def getResultTypeFromSignatures(
    signatures: Array[MethodSignature], 
    inputTypes: Array[DataType]
  ): TypeInformation[_]
}

import org.apache.flink.table.planner.functions.utils.UserDefinedFunctionUtils
import org.apache.flink.table.functions.ScalarFunction

// Custom scalar function
class MyUpperFunction extends ScalarFunction {
  def eval(str: String): String = str.toUpperCase
}

// Validate function before registration
val myFunction = new MyUpperFunction()
UserDefinedFunctionUtils.validateScalarFunction(myFunction)
UserDefinedFunctionUtils.checkForInstantiation(myFunction.getClass)

// Function can now be registered with table environment
tableEnv.createTemporarySystemFunction("MY_UPPER", myFunction)

/**
 * SQL integration for scalar functions
 */
class ScalarSqlFunction(
  identifier: String,
  displayName: String,
  scalarFunction: ScalarFunction,
  typeFactory: FlinkTypeFactory
) extends SqlFunction {
  
  /**
   * Returns function kind for SQL integration
   * @return SqlKind.OTHER_FUNCTION for scalar functions
   */
  def getKind: SqlKind = SqlKind.OTHER_FUNCTION
  
  /**
   * Returns SQL identifier for this function
   * @return Function identifier used in SQL
   */
  def getName: String = identifier
  
  /**
   * Infers return type based on operand types
   * @param opBinding Operand binding with argument types
   * @return Inferred return type for SQL query
   */
  def inferReturnType(opBinding: SqlOperatorBinding): RelDataType
  
  /**
   * Validates function call with given operands
   * @param callBinding Call binding with argument information
   * @return True if call is valid, false otherwise
   */
  def checkOperandTypes(callBinding: SqlCallBinding): Boolean
}

/**
 * SQL integration for aggregate functions
 */
class AggSqlFunction(
  identifier: String,
  displayName: String,
  aggregateFunction: AggregateFunction[_, _],
  resultType: RelDataType,
  accType: RelDataType,
  typeFactory: FlinkTypeFactory
) extends SqlAggFunction {
  
  /**
   * Returns function kind for SQL integration
   * @return SqlKind.OTHER_FUNCTION for aggregate functions
   */
  def getKind: SqlKind = SqlKind.OTHER_FUNCTION
  
  /**
   * Returns aggregate function name
   * @return Function identifier used in SQL
   */
  def getName: String = identifier
  
  /**
   * Infers return type for aggregate result
   * @param opBinding Operand binding information
   * @return Return type of aggregation result
   */
  def inferReturnType(opBinding: SqlOperatorBinding): RelDataType = resultType
  
  /**
   * Returns accumulator type information
   * @param typeFactory Type factory for type creation
   * @return Accumulator type information
   */
  def getAccumulatorType(typeFactory: RelDataTypeFactory): RelDataType = accType
  
  /**
   * Validates aggregate function call
   * @param callBinding Call binding with operand information
   * @return True if call is valid
   */
  def checkOperandTypes(callBinding: SqlCallBinding): Boolean
}

/**
 * SQL integration for table functions (UDTFs)
 */
class TableSqlFunction(
  identifier: String,
  displayName: String, 
  tableFunction: TableFunction[_],
  resultType: RelDataType,
  typeFactory: FlinkTypeFactory
) extends SqlFunction {
  
  /**
   * Returns function kind for table functions
   * @return SqlKind.OTHER_FUNCTION for table functions
   */
  def getKind: SqlKind = SqlKind.OTHER_FUNCTION
  
  /**
   * Returns table function name
   * @return Function identifier used in SQL
   */
  def getName: String = identifier
  
  /**
   * Infers return type for table function
   * @param opBinding Operand binding information
   * @return Row type returned by table function
   */
  def inferReturnType(opBinding: SqlOperatorBinding): RelDataType = resultType
  
  /**
   * Validates table function call
   * @param callBinding Call binding with argument types
   * @return True if call is valid
   */
  def checkOperandTypes(callBinding: SqlCallBinding): Boolean
  
  /**
   * Returns table function definition for query planning
   * @return TableFunction instance
   */
  def getTableFunction: TableFunction[_] = tableFunction
}

/**
 * Code generation for function calls
 */
object FunctionCallCodeGenerator {
  
  /**
   * Generates code for scalar function calls
   * @param scalarFunction Scalar function to generate code for
   * @param operands Operand expressions
   * @param resultType Expected result type
   * @param config Table configuration
   * @return Generated code expression
   */
  def generateScalarFunctionCall(
    scalarFunction: ScalarFunction,
    operands: Seq[GeneratedExpression],
    resultType: DataType,
    config: TableConfig
  ): GeneratedExpression
  
  /**
   * Generates code for aggregate function calls
   * @param aggregateFunction Aggregate function
   * @param operands Input operand expressions
   * @param accType Accumulator type
   * @param resultType Result type
   * @return Generated aggregate handler code
   */
  def generateAggregateFunctionCall(
    aggregateFunction: AggregateFunction[_, _],
    operands: Seq[GeneratedExpression],
    accType: DataType,
    resultType: DataType
  ): GeneratedAggregateHandler
}

/**
 * Built-in function definitions and utilities
 */
object BuiltInFunctionDefinitions {
  
  // String functions
  val UPPER: FunctionDefinition
  val LOWER: FunctionDefinition
  val SUBSTRING: FunctionDefinition
  val TRIM: FunctionDefinition
  val CONCAT: FunctionDefinition
  
  // Mathematical functions
  val ABS: FunctionDefinition
  val CEIL: FunctionDefinition
  val FLOOR: FunctionDefinition
  val ROUND: FunctionDefinition
  val SIN: FunctionDefinition
  val COS: FunctionDefinition
  
  // Date/time functions
  val CURRENT_TIMESTAMP: FunctionDefinition
  val DATE_FORMAT: FunctionDefinition
  val EXTRACT: FunctionDefinition
  
  // Aggregate functions
  val COUNT: FunctionDefinition
  val SUM: FunctionDefinition
  val AVG: FunctionDefinition
  val MIN: FunctionDefinition
  val MAX: FunctionDefinition
  
  // Window functions
  val ROW_NUMBER: FunctionDefinition
  val RANK: FunctionDefinition
  val DENSE_RANK: FunctionDefinition
  val LAG: FunctionDefinition
  val LEAD: FunctionDefinition
}

/**
 * Function catalog integration (from flink-table-api)
 * Functions are registered and resolved through FunctionCatalog
 */
public interface FunctionCatalog {
  /**
   * Registers a temporary system function
   * @param name Function name
   * @param functionDefinition Function definition
   */
  void registerTemporarySystemFunction(String name, FunctionDefinition functionDefinition);
  
  /**
   * Registers a temporary catalog function
   * @param objectIdentifier Function identifier with catalog/database/name
   * @param functionDefinition Function definition
   * @param ignoreIfExists Whether to ignore if function already exists
   */
  void registerTemporaryCatalogFunction(
    ObjectIdentifier objectIdentifier,
    FunctionDefinition functionDefinition, 
    boolean ignoreIfExists
  );
  
  /**
   * Looks up function by identifier
   * @param objectIdentifier Function identifier
   * @return Optional function lookup result
   */
  Optional<FunctionLookup.Result> lookupFunction(ObjectIdentifier objectIdentifier);
}

// Common validation scenarios
try {
  UserDefinedFunctionUtils.validateScalarFunction(myFunction)
} catch {
  case e: ValidationException => 
    // Handle validation errors:
    // - Missing eval() method
    // - Invalid method signatures  
    // - Unsupported parameter types
    // - Missing result type information
}

// Function instantiation validation
try {
  UserDefinedFunctionUtils.checkForInstantiation(functionClass)
} catch {
  case e: ValidationException =>
    // Handle instantiation errors:
    // - No default constructor
    // - Constructor throws exceptions
    // - Class is abstract or interface
}

// Generic function example (handled automatically by utilities)
class GenericFunction[T] extends ScalarFunction {
  def eval(input: T): T = input
}

// Type resolution is handled during function registration
val genericFunc = new GenericFunction[String]()
UserDefinedFunctionUtils.validateScalarFunction(genericFunc)

public class MyDeterministicFunction extends ScalarFunction {
  @Override
  public boolean isDeterministic() {
    return true;  // Enables constant folding and other optimizations
  }
  
  public String eval(String input) {
    return input.toUpperCase();
  }
}