presto-main/src/main/java/io/prestosql/operator/scalar/MapFilterFunction.java/MapFilterFunction/generateFilter
class MapFilterFunction:    private static MethodHandle generateFilter(MapType mapType)
    {
        CallSiteBinder binder = new CallSiteBinder();
        Type keyType = mapType.getKeyType();
        Type valueType = mapType.getValueType();
        Class<?> keyJavaType = Primitives.wrap(keyType.getJavaType());
        Class<?> valueJavaType = Primitives.wrap(valueType.getJavaType());

        ClassDefinition definition = new ClassDefinition(
                a(PUBLIC, FINAL),
                makeClassName("MapFilter"),
                type(Object.class));
        definition.declareDefaultConstructor(a(PRIVATE));

        Parameter state = arg("state", Object.class);
        Parameter block = arg("block", Block.class);
        Parameter function = arg("function", BinaryFunctionInterface.class);
        MethodDefinition method = definition.declareMethod(
                a(PUBLIC, STATIC),
                "filter",
                type(Block.class),
                ImmutableList.of(state, block, function));

        BytecodeBlock body = method.getBody();
        Scope scope = method.getScope();
        Variable positionCount = scope.declareVariable(int.class, "positionCount");
        Variable position = scope.declareVariable(int.class, "position");
        Variable pageBuilder = scope.declareVariable(PageBuilder.class, "pageBuilder");
        Variable mapBlockBuilder = scope.declareVariable(BlockBuilder.class, "mapBlockBuilder");
        Variable singleMapBlockWriter = scope.declareVariable(BlockBuilder.class, "singleMapBlockWriter");
        Variable keyElement = scope.declareVariable(keyJavaType, "keyElement");
        Variable valueElement = scope.declareVariable(valueJavaType, "valueElement");
        Variable keep = scope.declareVariable(Boolean.class, "keep");

        // invoke block.getPositionCount()
        body.append(positionCount.set(block.invoke("getPositionCount", int.class)));

        // prepare the single map block builder
        body.append(pageBuilder.set(state.cast(PageBuilder.class)));
        body.append(new IfStatement()
                .condition(pageBuilder.invoke("isFull", boolean.class))
                .ifTrue(pageBuilder.invoke("reset", void.class)));
        body.append(mapBlockBuilder.set(pageBuilder.invoke("getBlockBuilder", BlockBuilder.class, constantInt(0))));
        body.append(singleMapBlockWriter.set(mapBlockBuilder.invoke("beginBlockEntry", BlockBuilder.class)));

        SqlTypeBytecodeExpression keySqlType = constantType(binder, keyType);
        BytecodeNode loadKeyElement;
        if (!keyType.equals(UNKNOWN)) {
            // key element must be non-null
            loadKeyElement = new BytecodeBlock().append(keyElement.set(keySqlType.getValue(block, position).cast(keyJavaType)));
        }
        else {
            loadKeyElement = new BytecodeBlock().append(keyElement.set(constantNull(keyJavaType)));
        }

        SqlTypeBytecodeExpression valueSqlType = constantType(binder, valueType);
        BytecodeNode loadValueElement;
        if (!valueType.equals(UNKNOWN)) {
            loadValueElement = new IfStatement()
                    .condition(block.invoke("isNull", boolean.class, add(position, constantInt(1))))
                    .ifTrue(valueElement.set(constantNull(valueJavaType)))
                    .ifFalse(valueElement.set(valueSqlType.getValue(block, add(position, constantInt(1))).cast(valueJavaType)));
        }
        else {
            loadValueElement = new BytecodeBlock().append(valueElement.set(constantNull(valueJavaType)));
        }

        body.append(new ForLoop()
                .initialize(position.set(constantInt(0)))
                .condition(lessThan(position, positionCount))
                .update(incrementVariable(position, (byte) 2))
                .body(new BytecodeBlock()
                        .append(loadKeyElement)
                        .append(loadValueElement)
                        .append(keep.set(function.invoke("apply", Object.class, keyElement.cast(Object.class), valueElement.cast(Object.class)).cast(Boolean.class)))
                        .append(new IfStatement("if (keep != null && keep) ...")
                                .condition(and(notEqual(keep, constantNull(Boolean.class)), keep.cast(boolean.class)))
                                .ifTrue(new BytecodeBlock()
                                        .append(keySqlType.invoke("appendTo", void.class, block, position, singleMapBlockWriter))
                                        .append(valueSqlType.invoke("appendTo", void.class, block, add(position, constantInt(1)), singleMapBlockWriter))))));

        body.append(mapBlockBuilder
                .invoke("closeEntry", BlockBuilder.class)
                .pop());
        body.append(pageBuilder.invoke("declarePosition", void.class));
        body.append(constantType(binder, mapType)
                .invoke(
                        "getObject",
                        Object.class,
                        mapBlockBuilder.cast(Block.class),
                        subtract(mapBlockBuilder.invoke("getPositionCount", int.class), constantInt(1)))
                .ret());

        Class<?> generatedClass = defineClass(definition, Object.class, binder.getBindings(), MapFilterFunction.class.getClassLoader());
        return methodHandle(generatedClass, "filter", Object.class, Block.class, BinaryFunctionInterface.class);
    }