Expression System

/**
 * Expression converter for transforming between expression representations
 */
trait ExpressionConverter {
  
  /**
   * Converts Table API expression to Calcite RexNode representation
   * @param expression Table API expression to convert
   * @return RexNode representation for Calcite optimization
   */
  def convertToRexNode(expression: Expression): RexNode
  
  /**
   * Converts Calcite RexNode back to Table API expression
   * @param rexNode Calcite RexNode to convert
   * @return Table API expression representation
   */
  def convertToExpression(rexNode: RexNode): Expression
  
  /**
   * Converts SQL expression string to RexNode
   * @param sqlExpression SQL expression as string
   * @param inputRowType Input row type for context
   * @return Parsed and validated RexNode
   */
  def convertSqlToRexNode(sqlExpression: String, inputRowType: RelDataType): RexNode
  
  /**
   * Converts expression with type coercion
   * @param expression Expression to convert
   * @param targetType Target type for coercion
   * @return Converted expression with type coercion applied
   */
  def convertWithCoercion(expression: Expression, targetType: DataType): RexNode
}

/**
 * Expression validation utilities
 */
object ExpressionValidator {
  
  /**
   * Validates expression syntax and semantics
   * @param expression Expression to validate
   * @param inputType Input data type context
   * @throws ValidationException if expression is invalid
   */
  def validateExpression(expression: Expression, inputType: RowType): Unit
  
  /**
   * Validates RexNode expression in Calcite context
   * @param rexNode RexNode to validate
   * @param rexBuilder RexBuilder for validation context
   * @throws CalciteException if RexNode is invalid
   */
  def validateRexNode(rexNode: RexNode, rexBuilder: RexBuilder): Unit
  
  /**
   * Validates expression type compatibility
   * @param sourceType Source expression type
   * @param targetType Target expected type
   * @return True if types are compatible, false otherwise
   */
  def isTypeCompatible(sourceType: DataType, targetType: DataType): Boolean
  
  /**
   * Validates function call expression
   * @param functionCall Function call expression
   * @param functionCatalog Function catalog for resolution
   * @throws ValidationException if function call is invalid
   */
  def validateFunctionCall(
    functionCall: CallExpression, 
    functionCatalog: FunctionCatalog
  ): Unit
}

/**
 * Type inference engine for expressions
 */
object TypeInferenceEngine {
  
  /**
   * Infers result type of binary operation
   * @param leftType Left operand type
   * @param rightType Right operand type  
   * @param operator Binary operator
   * @return Inferred result type
   */
  def inferBinaryOperationType(
    leftType: DataType,
    rightType: DataType,
    operator: BinaryOperator
  ): DataType
  
  /**
   * Infers result type of function call
   * @param functionDefinition Function definition
   * @param argumentTypes Argument types
   * @return Inferred result type
   */
  def inferFunctionCallType(
    functionDefinition: FunctionDefinition,
    argumentTypes: List[DataType]
  ): DataType
  
  /**
   * Infers type with nullability consideration
   * @param operandTypes Types of operands
   * @param operation Operation being performed
   * @return Inferred type with proper nullability
   */
  def inferTypeWithNullability(
    operandTypes: List[DataType],
    operation: Operation
  ): DataType
  
  /**
   * Finds common type for multiple expressions (for CASE, UNION, etc.)
   * @param types List of types to find common type for
   * @return Common super type, or failure if no common type exists
   */
  def findCommonType(types: List[DataType]): Option[DataType]
}

/**
 * Expression utility functions
 */
object ExpressionUtils {
  
  /**
   * Extracts referenced field names from expression
   * @param expression Expression to analyze
   * @return Set of field names referenced in expression
   */
  def extractFieldNames(expression: Expression): Set[String]
  
  /**
   * Checks if expression is deterministic (always returns same result for same input)
   * @param expression Expression to check
   * @return True if expression is deterministic
   */
  def isDeterministic(expression: Expression): Boolean
  
  /**
   * Simplifies expression by applying constant folding and other optimizations
   * @param expression Expression to simplify
   * @param rexBuilder RexBuilder for creating simplified expressions
   * @return Simplified expression
   */
  def simplifyExpression(expression: RexNode, rexBuilder: RexBuilder): RexNode
  
  /**
   * Converts expression to conjunctive normal form (CNF)
   * @param expression Boolean expression to convert
   * @return Expression in CNF
   */
  def toCNF(expression: RexNode): RexNode
  
  /**
   * Splits conjunctive expression into individual conjuncts
   * @param expression AND-connected expression
   * @return List of individual conjunct expressions
   */
  def splitConjunction(expression: RexNode): List[RexNode]
}

/**
 * Calcite RexNode utilities for Flink integration
 */
public class FlinkRexUtil {
  
  /**
   * Converts Flink DataType to Calcite RelDataType
   * @param dataType Flink data type
   * @param typeFactory Calcite type factory
   * @return Calcite RelDataType representation
   */
  public static RelDataType toRelDataType(DataType dataType, RelDataTypeFactory typeFactory);
  
  /**
   * Converts Calcite RelDataType to Flink DataType
   * @param relDataType Calcite rel data type
   * @return Flink DataType representation
   */
  public static DataType toDataType(RelDataType relDataType);
  
  /**
   * Creates RexNode for field access
   * @param fieldIndex Index of field to access
   * @param inputRowType Input row type
   * @param rexBuilder RexBuilder for node creation
   * @return RexNode for field access
   */
  public static RexNode createFieldAccess(
    int fieldIndex, 
    RelDataType inputRowType,
    RexBuilder rexBuilder
  );
  
  /**
   * Creates RexNode for literal value
   * @param value Literal value
   * @param dataType Type of the literal
   * @param rexBuilder RexBuilder for node creation
   * @return RexNode for literal value
   */
  public static RexNode createLiteral(Object value, DataType dataType, RexBuilder rexBuilder);
}

/**
 * Expression code generation utilities
 */
object ExpressionCodeGenerator {
  
  /**
   * Generates code for expression evaluation
   * @param expression Expression to generate code for
   * @param inputType Input row type
   * @param config Table configuration
   * @param classLoader Class loader for generated code
   * @return Generated expression evaluation code
   */
  def generateExpression(
    expression: RexNode,
    inputType: RowType,
    config: TableConfig,
    classLoader: ClassLoader
  ): GeneratedExpression
  
  /**
   * Generates code for conditional expression (CASE/IF)
   * @param condition Condition expression
   * @param trueExpr Expression for true case
   * @param falseExpr Expression for false case
   * @return Generated conditional expression code
   */
  def generateConditional(
    condition: GeneratedExpression,
    trueExpr: GeneratedExpression,
    falseExpr: GeneratedExpression
  ): GeneratedExpression
  
  /**
   * Generates code for null-safe expression evaluation
   * @param expression Expression that may produce nulls
   * @param nullDefault Default value for null results
   * @return Generated null-safe expression code
   */
  def generateNullSafeExpression(
    expression: GeneratedExpression,
    nullDefault: GeneratedExpression
  ): GeneratedExpression
}

/**
 * Generated expression representation
 */
case class GeneratedExpression(
  resultTerm: String,          // Generated code term for result
  resultType: DataType,        // Result data type
  nullTerm: String,           // Generated code term for null check
  code: String                // Generated Java code
) {
  /**
   * Deep copy of generated expression with new terms
   * @param newResultTerm New result term
   * @param newNullTerm New null term
   * @return Copied expression with updated terms
   */
  def copy(newResultTerm: String = resultTerm, newNullTerm: String = nullTerm): GeneratedExpression
}

/**
 * Built-in expression definitions
 */
object BuiltInExpressions {
  
  // Arithmetic operators
  val PLUS: BinaryOperator
  val MINUS: BinaryOperator  
  val MULTIPLY: BinaryOperator
  val DIVIDE: BinaryOperator
  val MOD: BinaryOperator
  
  // Comparison operators
  val EQUALS: BinaryOperator
  val NOT_EQUALS: BinaryOperator
  val LESS_THAN: BinaryOperator
  val LESS_THAN_OR_EQUAL: BinaryOperator
  val GREATER_THAN: BinaryOperator
  val GREATER_THAN_OR_EQUAL: BinaryOperator
  
  // Logical operators
  val AND: BinaryOperator
  val OR: BinaryOperator
  val NOT: UnaryOperator
  
  // String operators
  val LIKE: BinaryOperator
  val SIMILAR_TO: BinaryOperator
  val SUBSTRING: FunctionDefinition
  val TRIM: FunctionDefinition
  
  // Null handling
  val IS_NULL: UnaryOperator
  val IS_NOT_NULL: UnaryOperator
  val COALESCE: FunctionDefinition
  val NULLIF: FunctionDefinition
  
  // Case expression
  val CASE: FunctionDefinition
  
  // Type conversion
  val CAST: FunctionDefinition
  val TRY_CAST: FunctionDefinition
}

/**
 * Window function expression utilities
 */
object WindowExpressionUtils {
  
  /**
   * Creates window function call expression
   * @param functionDefinition Window function definition
   * @param arguments Function arguments
   * @param partitionKeys Partition by keys
   * @param orderKeys Order by keys
   * @param windowFrame Window frame specification
   * @return Window function call expression
   */
  def createWindowFunctionCall(
    functionDefinition: FunctionDefinition,
    arguments: List[Expression],
    partitionKeys: List[Expression],
    orderKeys: List[OrderByExpression],
    windowFrame: WindowFrame
  ): CallExpression
}

/**
 * Time and watermark expression utilities
 */
object TimeExpressionUtils {
  
  /**
   * Creates watermark expression for event time processing
   * @param rowtimeExpression Rowtime field expression
   * @param delayExpression Watermark delay expression
   * @return Watermark generation expression
   */
  def createWatermarkExpression(
    rowtimeExpression: Expression,
    delayExpression: Expression
  ): Expression
  
  /**
   * Creates proctime attribute expression
   * @return Processing time attribute expression
   */
  def createProctimeExpression(): Expression
  
  /**
   * Validates time attribute expression
   * @param expression Time attribute expression
   * @return True if valid time attribute
   */
  def isValidTimeAttribute(expression: Expression): Boolean
}

// Common expression validation errors
try {
  ExpressionValidator.validateExpression(expr, inputType)
} catch {
  case e: ValidationException =>
    // Handle validation errors:
    // - Type mismatch
    // - Unknown function reference
    // - Invalid field reference
    // - Null handling issues
    
  case e: CalciteException =>
    // Handle Calcite-specific errors:
    // - SQL parsing errors
    // - RexNode validation failures
    // - Type system inconsistencies
}

// Type inference failures
val resultType = try {
  TypeInferenceEngine.inferFunctionCallType(funcDef, argTypes)
} catch {
  case e: TypeInferenceException =>
    // Handle type inference failures:
    // - No matching function signature
    // - Ambiguous function overloads
    // - Incompatible argument types
    None
}