User-Defined Functions

/**
 * Base class for user-defined scalar functions
 */
public abstract class ScalarFunction extends UserDefinedFunction {
    /**
     * Evaluation method that must be implemented for the function logic.
     * Method can be overloaded for different parameter combinations.
     * @param args Input arguments (types must match function signature)
     * @return Computed result value
     */
    public abstract Object eval(Object... args);
    
    /**
     * Optional method to specify the result type when it cannot be inferred
     * @param signature Array of argument types
     * @return DataType of the function result
     */
    public DataType getResultType(DataType[] signature);
    
    /**
     * Optional method to specify type inference for parameters
     * @return TypeInference specification
     */
    public TypeInference getTypeInference();
}

// Custom hash function
public class HashFunction extends ScalarFunction {
    public String eval(String input) {
        if (input == null) {
            return null;
        }
        return Integer.toHexString(input.hashCode());
    }
    
    // Overloaded version for multiple inputs
    public String eval(String input1, String input2) {
        if (input1 == null || input2 == null) {
            return null;
        }
        return Integer.toHexString((input1 + input2).hashCode());
    }
}

// Register and use the function
tEnv.createTemporarySystemFunction("hash", HashFunction.class);

Table result = orders
    .select($("order_id"), 
            call("hash", $("customer_email")).as("customer_hash"),
            call("hash", $("order_id").cast(DataTypes.STRING()), 
                        $("customer_email")).as("order_hash"));

/**
 * Base class for user-defined table functions
 * @param <T> Type of the output rows
 */
public abstract class TableFunction<T> extends UserDefinedFunction {
    /**
     * Evaluation method that must be implemented for the function logic.
     * Use collect() method to emit output rows.
     * @param args Input arguments (types must match function signature)
     */
    public abstract void eval(Object... args);
    
    /**
     * Emit an output row from the table function
     * @param result Row to emit
     */
    protected void collect(T result);
    
    /**
     * Optional method to specify the result type when it cannot be inferred
     * @param signature Array of argument types
     * @return DataType of the function result rows
     */
    public DataType getResultType(DataType[] signature);
}

// Function to split comma-separated values into rows
@FunctionHint(output = @DataTypeHint("STRING"))
public class SplitFunction extends TableFunction<String> {
    public void eval(String str) {
        if (str != null) {
            for (String s : str.split(",")) {
                collect(s.trim());
            }
        }
    }
}

// Function returning structured rows
@FunctionHint(output = @DataTypeHint("ROW<word STRING, length INT>"))
public class WordAnalyzer extends TableFunction<Row> {
    public void eval(String sentence) {
        if (sentence != null) {
            for (String word : sentence.split("\\s+")) {
                collect(Row.of(word, word.length()));
            }
        }
    }
}

// Register and use table functions
tEnv.createTemporarySystemFunction("split", SplitFunction.class);
tEnv.createTemporarySystemFunction("analyze_words", WordAnalyzer.class);

// Use with LATERAL JOIN
Table result = orders
    .joinLateral(call("split", $("product_tags")).as("tag"))
    .select($("order_id"), $("tag"));

// Use with LEFT JOIN LATERAL for optional results
Table analysis = documents
    .leftOuterJoinLateral(call("analyze_words", $("title")).as("word", "length"))
    .select($("document_id"), $("word"), $("length"));

/**
 * Base class for user-defined aggregate functions
 * @param <T> Type of the aggregation result
 * @param <ACC> Type of the accumulator used during aggregation
 */
public abstract class AggregateFunction<T, ACC> extends UserDefinedFunction {
    /**
     * Create and initialize a new accumulator
     * @return New accumulator instance
     */
    public abstract ACC createAccumulator();
    
    /**
     * Accumulate input values into the accumulator
     * @param accumulator Current accumulator state
     * @param args Input values to accumulate
     */
    public abstract void accumulate(ACC accumulator, Object... args);
    
    /**
     * Extract the final result from the accumulator
     * @param accumulator Final accumulator state
     * @return Aggregated result
     */
    public abstract T getValue(ACC accumulator);
    
    /**
     * Retract input values from the accumulator (for streaming updates)
     * @param accumulator Current accumulator state
     * @param args Input values to retract
     */
    public void retract(ACC accumulator, Object... args) {
        // Optional: implement for streaming scenarios with retractions
    }
    
    /**
     * Merge two accumulators (for distributed aggregation)
     * @param accumulator Target accumulator
     * @param others Accumulators to merge from
     */
    public void merge(ACC accumulator, Iterable<ACC> others) {
        // Optional: implement for distributed scenarios
    }
    
    /**
     * Reset the accumulator to initial state
     * @param accumulator Accumulator to reset
     */
    public void resetAccumulator(ACC accumulator) {
        // Optional: implement for reusing accumulators
    }
}

// Custom weighted average function
public class WeightedAvg extends AggregateFunction<Double, WeightedAvgAccumulator> {
    
    // Accumulator class
    public static class WeightedAvgAccumulator {
        public double sum = 0;
        public double weightSum = 0;
    }
    
    @Override
    public WeightedAvgAccumulator createAccumulator() {
        return new WeightedAvgAccumulator();
    }
    
    @Override
    public void accumulate(WeightedAvgAccumulator acc, Double value, Double weight) {
        if (value != null && weight != null) {
            acc.sum += value * weight;
            acc.weightSum += weight;
        }
    }
    
    @Override
    public Double getValue(WeightedAvgAccumulator acc) {
        if (acc.weightSum == 0) {
            return null;
        }
        return acc.sum / acc.weightSum;
    }
    
    @Override
    public void retract(WeightedAvgAccumulator acc, Double value, Double weight) {
        if (value != null && weight != null) {
            acc.sum -= value * weight;
            acc.weightSum -= weight;
        }
    }
    
    @Override
    public void merge(WeightedAvgAccumulator acc, Iterable<WeightedAvgAccumulator> others) {
        for (WeightedAvgAccumulator other : others) {
            acc.sum += other.sum;
            acc.weightSum += other.weightSum;
        }
    }
}

// Register and use aggregate function
tEnv.createTemporarySystemFunction("weighted_avg", WeightedAvg.class);

Table result = sales
    .groupBy($("product_category"))
    .select($("product_category"),
            call("weighted_avg", $("price"), $("quantity")).as("weighted_avg_price"));

/**
 * Base class for asynchronous user-defined scalar functions
 */
public abstract class AsyncScalarFunction extends UserDefinedFunction {
    /**
     * Asynchronous evaluation method
     * @param resultFuture CompletableFuture to complete with the result
     * @param args Input arguments
     */
    public abstract void eval(CompletableFuture<Object> resultFuture, Object... args);
    
    /**
     * Optional method to specify timeout for async operations
     * @return Timeout duration in milliseconds
     */
    public long getTimeout() {
        return 60000; // Default 60 seconds
    }
}

// Async function for external service lookup
public class AsyncEnrichFunction extends AsyncScalarFunction {
    private transient HttpClient httpClient;
    
    @Override
    public void open(FunctionContext context) throws Exception {
        httpClient = HttpClient.newHttpClient();
    }
    
    public void eval(CompletableFuture<String> resultFuture, String userId) {
        if (userId == null) {
            resultFuture.complete(null);
            return;
        }
        
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.example.com/users/" + userId))
            .build();
            
        httpClient.sendAsync(request, HttpResponse.BodyHandlers.ofString())
            .thenApply(HttpResponse::body)
            .thenAccept(resultFuture::complete)
            .exceptionally(throwable -> {
                resultFuture.complete(null); // Handle errors gracefully
                return null;
            });
    }
    
    @Override
    public void close() throws Exception {
        if (httpClient != null) {
            // Cleanup resources
        }
    }
}

/**
 * Base class for asynchronous user-defined table functions
 * @param <T> Type of the output rows
 */
public abstract class AsyncTableFunction<T> extends UserDefinedFunction {
    /**
     * Asynchronous evaluation method
     * @param resultFuture CompletableFuture to complete with collection of results
     * @param args Input arguments
     */
    public abstract void eval(CompletableFuture<Collection<T>> resultFuture, Object... args);
}

/**
 * Base class for process table functions with advanced capabilities
 * @param <T> Type of the output rows
 */
public abstract class ProcessTableFunction<T> extends UserDefinedFunction {
    /**
     * Process method with access to context
     * @param ctx Processing context with state and timer access
     * @param args Input arguments
     */
    public abstract void eval(ProcessContext ctx, Object... args) throws Exception;
    
    /**
     * Processing context interface
     */
    public interface ProcessContext {
        /**
         * Get keyed state for maintaining function state
         * @param stateDescriptor State descriptor
         * @return State instance
         */
        <S extends State> S getState(StateDescriptor<S, ?> stateDescriptor);
        
        /**
         * Emit an output row
         * @param result Row to emit
         */
        void collect(T result);
        
        /**
         * Get current processing time
         * @return Current processing time timestamp
         */
        long currentProcessingTime();
        
        /**
         * Get current watermark
         * @return Current event time watermark
         */
        long currentWatermark();
    }
}

/**
 * Register a function class as a temporary system function
 * @param name Function name for SQL usage
 * @param functionClass Function implementation class
 */
public void createTemporarySystemFunction(String name, Class<? extends UserDefinedFunction> functionClass);

/**
 * Register a function instance as a temporary system function
 * @param name Function name for SQL usage
 * @param functionInstance Function implementation instance
 */
public void createTemporarySystemFunction(String name, UserDefinedFunction functionInstance);

/**
 * Register a function in a specific catalog and database
 * @param path Full path to the function (catalog.database.function)
 * @param functionClass Function implementation class
 */
public void createFunction(String path, Class<? extends UserDefinedFunction> functionClass);

/**
 * Drop a temporary system function
 * @param name Function name to drop
 * @return true if function existed and was dropped
 */
public boolean dropTemporarySystemFunction(String name);

/**
 * Annotation for providing function-level type hints
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface FunctionHint {
    /**
     * Hint for function input parameters
     */
    DataTypeHint[] input() default {};
    
    /**
     * Hint for function output type
     */
    DataTypeHint output() default @DataTypeHint();
    
    /**
     * Whether function is deterministic
     */
    boolean isDeterministic() default true;
}

/**
 * Annotation for providing data type hints
 */
@Target({ElementType.PARAMETER, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface DataTypeHint {
    /**
     * Data type specification string
     */
    String value() default "";
    
    /**
     * Whether the type is nullable
     */
    DefaultBoolean allowRawGlobally() default DefaultBoolean.TRUE;
}

@FunctionHint(
    input = {@DataTypeHint("STRING"), @DataTypeHint("INT")},
    output = @DataTypeHint("ARRAY<STRING>")
)
public class RepeatString extends ScalarFunction {
    public String[] eval(String str, Integer count) {
        if (str == null || count == null || count <= 0) {
            return new String[0];
        }
        String[] result = new String[count];
        Arrays.fill(result, str);
        return result;
    }
}

// Complex type hint for nested structure
@FunctionHint(output = @DataTypeHint("ROW<name STRING, stats ROW<count BIGINT, avg DOUBLE>>"))
public class AnalyzeData extends TableFunction<Row> {
    public void eval(String data) {
        // Implementation that emits Row objects matching the hint
    }
}

/**
 * Base class providing lifecycle methods for all user-defined functions
 */
public abstract class UserDefinedFunction {
    /**
     * Initialize function resources when function is first used
     * @param context Function context with configuration and metrics
     */
    public void open(FunctionContext context) throws Exception {
        // Override to initialize resources
    }
    
    /**
     * Clean up function resources when function is no longer needed
     */
    public void close() throws Exception {
        // Override to clean up resources
    }
    
    /**
     * Check if function calls are deterministic
     * @return true if function is deterministic (same input = same output)
     */
    public boolean isDeterministic() {
        return true;
    }
}

/**
 * Function context providing access to configuration and metrics
 */
public interface FunctionContext {
    /**
     * Get metric group for function metrics
     * @return MetricGroup for registering custom metrics
     */
    MetricGroup getMetricGroup();
    
    /**
     * Get function configuration
     * @return Configuration object with job and function parameters
     */
    Configuration getJobParameter();
}

public class DatabaseLookupFunction extends AsyncScalarFunction {
    private transient Connection connection;
    private transient Counter lookupCounter;
    
    @Override
    public void open(FunctionContext context) throws Exception {
        // Initialize database connection
        String jdbcUrl = context.getJobParameter().getString("database.url");
        connection = DriverManager.getConnection(jdbcUrl);
        
        // Register metrics
        lookupCounter = context.getMetricGroup().counter("lookup_count");
    }
    
    public void eval(CompletableFuture<String> resultFuture, String key) {
        lookupCounter.inc();
        
        CompletableFuture.supplyAsync(() -> {
            try (PreparedStatement stmt = connection.prepareStatement("SELECT value FROM lookup WHERE key = ?")) {
                stmt.setString(1, key);
                ResultSet rs = stmt.executeQuery();
                return rs.next() ? rs.getString("value") : null;
            } catch (SQLException e) {
                throw new RuntimeException(e);
            }
        }).thenAccept(resultFuture::complete);
    }
    
    @Override
    public void close() throws Exception {
        if (connection != null) {
            connection.close();
        }
    }
}