/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.jexl3.internal;
  
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.Objects;
  import java.util.Queue;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  import org.apache.commons.jexl3.JexlArithmetic;
  import org.apache.commons.jexl3.JexlContext;
  import org.apache.commons.jexl3.JexlContext.NamespaceFunctor;
  import org.apache.commons.jexl3.JexlEngine;
  import org.apache.commons.jexl3.JexlException;
  import org.apache.commons.jexl3.JexlException.VariableIssue;
  import org.apache.commons.jexl3.JexlOperator;
  import org.apache.commons.jexl3.JexlOptions;
  import org.apache.commons.jexl3.introspection.JexlMethod;
  import org.apache.commons.jexl3.introspection.JexlPropertyGet;
  import org.apache.commons.jexl3.introspection.JexlPropertySet;
  import org.apache.commons.jexl3.introspection.JexlUberspect;
  import org.apache.commons.jexl3.parser.ASTArrayAccess;
  import org.apache.commons.jexl3.parser.ASTAssignment;
  import org.apache.commons.jexl3.parser.ASTFunctionNode;
  import org.apache.commons.jexl3.parser.ASTIdentifier;
  import org.apache.commons.jexl3.parser.ASTIdentifierAccess;
  import org.apache.commons.jexl3.parser.ASTMethodNode;
  import org.apache.commons.jexl3.parser.ASTNullpNode;
  import org.apache.commons.jexl3.parser.ASTReference;
  import org.apache.commons.jexl3.parser.ASTTernaryNode;
  import org.apache.commons.jexl3.parser.ASTVar;
  import org.apache.commons.jexl3.parser.JexlNode;
  import org.apache.commons.jexl3.parser.ParserVisitor;
  import org.apache.commons.logging.Log;
  
  /**
   * The helper base of an interpreter of JEXL syntax.
   * @since 3.0
   */
  public abstract class InterpreterBase extends ParserVisitor {
      /**
       * Helping dispatch function calls.
       */
      protected class CallDispatcher {
          /** The syntactic node. */
          final JexlNode node;
          /** Whether solution is cacheable. */
          final boolean cacheable;
          /** Whether arguments have been narrowed.  */
          boolean narrow;
          /** The method to call. */
          JexlMethod vm;
          /** The method invocation target. */
          Object target;
          /** The actual arguments. */
          Object[] argv;
          /** The cacheable funcall if any. */
          Funcall funcall;
  
          /**
           * Dispatcher ctor.
           *
           * @param anode the syntactic node.
           * @param acacheable whether resolution can be cached
           */
          CallDispatcher(final JexlNode anode, final boolean acacheable) {
              this.node = anode;
              this.cacheable = acacheable;
          }
  
          /**
           * Evaluates the method previously dispatched.
           *
           * @param methodName the method name
           * @return the method invocation result
           * @throws Exception when invocation fails
           */
          protected Object eval(final String methodName) throws Exception {
              // we have either evaluated and returned or might have found a method
              if (vm != null) {
                  // vm cannot be null if xjexl is null
                  final Object eval = vm.invoke(target, argv);
                  // cache executor in volatile JexlNode.value
                 if (funcall != null) {
                     node.jjtSetValue(funcall);
                 }
                 return eval;
             }
             return unsolvableMethod(node, methodName, argv);
         }
 
         /**
          * Whether the method is an arithmetic method.
          *
          * @param methodName the method name
          * @param arguments the method arguments
          * @return true if arithmetic, false otherwise
          */
         protected boolean isArithmeticMethod(final String methodName, final Object[] arguments) {
             vm = uberspect.getMethod(arithmetic, methodName, arguments);
             if (vm != null) {
                 argv = arguments;
                 target = arithmetic;
                 if (cacheable && vm.isCacheable()) {
                     funcall = new ArithmeticFuncall(vm, narrow);
                 }
                 return true;
             }
             return false;
         }
 
         /**
          * Whether the method is a context method.
          *
          * @param methodName the method name
          * @param arguments the method arguments
          * @return true if arithmetic, false otherwise
          */
         protected boolean isContextMethod(final String methodName, final Object[] arguments) {
             vm = uberspect.getMethod(context, methodName, arguments);
             if (vm != null) {
                 argv = arguments;
                 target = context;
                 if (cacheable && vm.isCacheable()) {
                     funcall = new ContextFuncall(vm, narrow);
                 }
                 return true;
             }
             return false;
         }
 
         /**
          * Whether the method is a target method.
          *
          * @param ntarget the target instance
          * @param methodName the method name
          * @param arguments the method arguments
          * @return true if arithmetic, false otherwise
          */
         protected boolean isTargetMethod(final Object ntarget, final String methodName, final Object[] arguments) {
             // try a method
             vm = uberspect.getMethod(ntarget, methodName, arguments);
             if (vm != null) {
                 argv = arguments;
                 target = ntarget;
                 if (cacheable && vm.isCacheable()) {
                     funcall = new Funcall(vm, narrow);
                 }
                 return true;
             }
             return false;
         }
 
         /**
          * Attempt to reuse last funcall cached in volatile JexlNode.value (if
          * it was cacheable).
          *
          * @param ntarget the target instance
          * @param methodName the method name
          * @param arguments the method arguments
          * @return TRY_FAILED if invocation was not possible or failed, the
          * result otherwise
          */
         protected Object tryEval(final Object ntarget, final String methodName, final Object[] arguments) {
             // do we have  a method/function name ?
             // attempt to reuse last funcall cached in volatile JexlNode.value (if it was not a variable)
             if (methodName != null && cacheable && ntarget != null) {
                 final Object cached = node.jjtGetValue();
                 if (cached instanceof Funcall) {
                     return ((Funcall) cached).tryInvoke(InterpreterBase.this, methodName, ntarget, arguments);
                 }
             }
             return JexlEngine.TRY_FAILED;
         }
     }
 
     /**
      * Cached arithmetic function call.
      */
     protected static class ArithmeticFuncall extends Funcall {
         /**
          * Constructs a new instance.
          * @param jme  the method
          * @param flag the narrow flag
          */
         protected ArithmeticFuncall(final JexlMethod jme, final boolean flag) {
             super(jme, flag);
         }
 
         @Override
         protected Object tryInvoke(final InterpreterBase ii, final String name, final Object target, final Object[] args) {
             return me.tryInvoke(name, ii.arithmetic, ii.functionArguments(target, narrow, args));
         }
     }
 
     /**
      * Cached context function call.
      */
     protected static class ContextFuncall extends Funcall {
         /**
          * Constructs a new instance.
          * @param jme  the method
          * @param flag the narrow flag
          */
         protected ContextFuncall(final JexlMethod jme, final boolean flag) {
             super(jme, flag);
         }
 
         @Override
         protected Object tryInvoke(final InterpreterBase ii, final String name, final Object target, final Object[] args) {
             return me.tryInvoke(name, ii.context, ii.functionArguments(target, narrow, args));
         }
     }
     /**
      * A ctor that needs a context as 1st argument.
      */
     protected static class ContextualCtor extends Funcall {
         /**
          * Constructs a new instance.
          * @param jme the method
          * @param flag the narrow flag
          */
         protected ContextualCtor(final JexlMethod jme, final boolean flag) {
             super(jme, flag);
         }
 
         @Override
         protected Object tryInvoke(final InterpreterBase ii, final String name, final Object target, final Object[] args) {
             return me.tryInvoke(name, target, ii.callArguments(ii.context, narrow, args));
         }
     }
     /**
      * Cached function call.
      */
     protected static class Funcall implements JexlNode.Funcall {
         /** Whether narrow should be applied to arguments. */
         protected final boolean narrow;
         /** The JexlMethod to delegate the call to. */
         protected final JexlMethod me;
         /**
          * Constructs a new instance.
          * @param jme  the method
          * @param flag the narrow flag
          */
         protected Funcall(final JexlMethod jme, final boolean flag) {
             this.me = jme;
             this.narrow = flag;
         }
 
         /**
          * Try invocation.
          * @param ii     the interpreter
          * @param name   the method name
          * @param target the method target
          * @param args   the method arguments
          * @return the method invocation result (or JexlEngine.TRY_FAILED)
          */
         protected Object tryInvoke(final InterpreterBase ii, final String name, final Object target, final Object[] args) {
             return me.tryInvoke(name, target, ii.functionArguments(null, narrow, args));
         }
     }
 
     /** Empty parameters for method matching. */
     protected static final Object[] EMPTY_PARAMS = {};
 
     /**
      * Pretty-prints a failing property value (de)reference.
      * <p>Used by calls to unsolvableProperty(...).</p>
      * @param node the property node
      * @return the (pretty) string value
      */
     protected static String stringifyPropertyValue(final JexlNode node) {
         return node != null ? new Debugger().depth(1).data(node) : "???";
     }
 
     /** The JEXL engine. */
     protected final Engine jexl;
     /** The logger. */
     protected final Log logger;
     /** The uberspect. */
     protected final JexlUberspect uberspect;
     /** The arithmetic handler. */
     protected final JexlArithmetic arithmetic;
     /** The context to store/retrieve variables. */
     protected final JexlContext context;
     /** The options. */
     protected final JexlOptions options;
     /** Cache executors. */
     protected final boolean cache;
     /** Cancellation support. */
     protected final AtomicBoolean cancelled;
     /** The namespace resolver. */
     protected final JexlContext.NamespaceResolver ns;
     /** The class name resolver. */
     protected final JexlUberspect.ClassNameResolver fqcnSolver;
     /** The operators evaluation delegate. */
     protected final JexlOperator.Uberspect operators;
     /** The map of 'prefix:function' to object resolving as namespaces. */
     protected final Map<String, Object> functions;
     /** The map of dynamically created namespaces, NamespaceFunctor or duck-types of those. */
     protected Map<String, Object> functors;
 
     /**
      * Creates an interpreter base.
      * @param engine   the engine creating this interpreter
      * @param opts     the evaluation options
      * @param aContext the evaluation context
      */
     protected InterpreterBase(final Engine engine, final JexlOptions opts, final JexlContext aContext) {
         this.jexl = engine;
         this.logger = jexl.logger;
         this.uberspect = jexl.uberspect;
         this.context = aContext != null ? aContext : JexlEngine.EMPTY_CONTEXT;
         this.cache = engine.cache != null;
         final JexlArithmetic jexla = jexl.arithmetic;
         this.options = opts == null ? engine.evalOptions(aContext) : opts;
         this.arithmetic = jexla.options(options);
         if (arithmetic != jexla && !arithmetic.getClass().equals(jexla.getClass()) && logger.isWarnEnabled()) {
             logger.warn("expected arithmetic to be " + jexla.getClass().getSimpleName()
                     + ", got " + arithmetic.getClass().getSimpleName()
             );
         }
         if (this.context instanceof JexlContext.NamespaceResolver) {
             ns = (JexlContext.NamespaceResolver) context;
         } else {
             ns = JexlEngine.EMPTY_NS;
         }
         AtomicBoolean acancel = null;
         if (this.context instanceof JexlContext.CancellationHandle) {
             acancel = ((JexlContext.CancellationHandle) context).getCancellation();
         }
         this.cancelled = acancel != null ? acancel : new AtomicBoolean();
         this.functions = options.getNamespaces();
         this.functors = null;
         JexlOperator.Uberspect ops = uberspect.getOperator(arithmetic);
         if (ops == null) {
             ops = new Operator(uberspect, arithmetic);
         }
         this.operators = ops;
         // the import package facility
         this.fqcnSolver = engine.createConstantResolver(options.getImports());
     }
 
     /**
      * Copy constructor.
      * @param ii the base to copy
      * @param jexla the arithmetic instance to use (or null)
      */
     protected InterpreterBase(final InterpreterBase ii, final JexlArithmetic jexla) {
         jexl = ii.jexl;
         logger = ii.logger;
         uberspect = ii.uberspect;
         arithmetic = jexla;
         context = ii.context;
         options = ii.options.copy();
         cache = ii.cache;
         ns = ii.ns;
         operators = ii.operators;
         cancelled = ii.cancelled;
         functions = ii.functions;
         functors = ii.functors;
         fqcnSolver = ii.fqcnSolver;
     }
 
     /**
      * Triggered when an annotation processing fails.
      * @param node     the node where the error originated from
      * @param annotation the annotation name
      * @param cause    the cause of error (if any)
      * @return throws a JexlException if strict and not silent, null otherwise
      */
     protected Object annotationError(final JexlNode node, final String annotation, final Throwable cause) {
         if (isStrictEngine()) {
             throw new JexlException.Annotation(node, annotation, cause);
         }
         if (logger.isDebugEnabled()) {
             logger.debug(JexlException.annotationError(node, annotation), cause);
         }
         return null;
     }
 
     /**
      * Concatenate arguments in call(...).
      * @param target the pseudo-method owner, first to-be argument
      * @param narrow whether we should attempt to narrow number arguments
      * @param args   the other (non-null) arguments
      * @return the arguments array
      */
     protected Object[] callArguments(final Object target, final boolean narrow, final Object[] args) {
         // makes target 1st args, copy others - optionally narrow numbers
         final Object[] nargv = new Object[args.length + 1];
         if (narrow) {
             nargv[0] = functionArgument(true, target);
             for (int a = 1; a <= args.length; ++a) {
                 nargv[a] = functionArgument(true, args[a - 1]);
             }
         } else {
             nargv[0] = target;
             System.arraycopy(args, 0, nargv, 1, args.length);
         }
         return nargv;
     }
 
     /**
      * Cancels this evaluation, setting the cancel flag that will result in a JexlException.Cancel to be thrown.
      * @return false if already cancelled, true otherwise
      */
     protected  boolean cancel() {
         return cancelled.compareAndSet(false, true);
     }
 
     /**
      * Throws a JexlException.Cancel if script execution was cancelled.
      * @param node the node being evaluated
      */
     protected void cancelCheck(final JexlNode node) {
         if (isCancelled()) {
             throw new JexlException.Cancel(node);
         }
     }
 
     /**
      * Attempt to call close() if supported.
      * <p>This is used when dealing with auto-closeable (duck-like) objects
      * @param closeable the object we'd like to close
      */
     protected void closeIfSupported(final Object closeable) {
         if (closeable != null) {
             final JexlMethod mclose = uberspect.getMethod(closeable, "close", EMPTY_PARAMS);
             if (mclose != null) {
                 try {
                     mclose.invoke(closeable, EMPTY_PARAMS);
                 } catch (final Exception xignore) {
                     logger.warn(xignore);
                 }
             }
         }
     }
 
     /**
      * Attempt to call close() if supported.
      * <p>This is used when dealing with auto-closeable (duck-like) objects
      * @param closeables the object queue we'd like to close
      */
     protected void closeIfSupported(final Queue<Object> closeables) {
         for(final Object closeable : closeables) {
             closeIfSupported(closeable);
         }
     }
 
     /**
      * Triggered when a captured variable is const and assignment is attempted.
      * @param node  the node where the error originated from
      * @param variable   the variable name
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object constVariable(final JexlNode node, final String variable) {
         return variableError(node, variable, VariableIssue.CONST);
     }
 
     /**
      * Defines a variable.
      * @param variable the variable to define
      * @param frame the frame in which it will be defined
      * @return true if definition succeeded, false otherwise
      */
     protected boolean defineVariable(final ASTVar variable, final LexicalFrame frame) {
         final int symbol = variable.getSymbol();
         if (symbol < 0) {
             return false;
         }
         if (variable.isRedefined()) {
             return false;
         }
         return frame.defineSymbol(symbol, variable.isCaptured());
     }
 
     /**
      * Finds the node causing a NPE for diadic operators.
      * @param node  the parent node
      * @param left  the left argument
      * @param right the right argument
      * @return the left, right or parent node
      */
     protected JexlNode findNullOperand(final JexlNode node, final Object left, final Object right) {
         if (left == null) {
             return node.jjtGetChild(0);
         }
         if (right == null) {
             return node.jjtGetChild(1);
         }
         return node;
     }
 
     /**
      * @deprecated
      */
     @Deprecated
     protected JexlNode findNullOperand(final RuntimeException xrt, final JexlNode node, final Object left, final Object right) {
         return findNullOperand(node, left, right);
     }
 
     /**
      * Optionally narrows an argument for a function call.
      * @param narrow whether narrowing should occur
      * @param arg    the argument
      * @return the narrowed argument
      */
     protected Object functionArgument(final boolean narrow, final Object arg) {
         return narrow && arg instanceof Number ? arithmetic.narrow((Number) arg) : arg;
     }
 
     /**
      * Concatenate arguments in call(...).
      * <p>When target == context, we are dealing with a global namespace function call
      * @param target the pseudo-method owner, first to-be argument
      * @param narrow whether we should attempt to narrow number arguments
      * @param args   the other (non-null) arguments
      * @return the arguments array
      */
     protected Object[] functionArguments(final Object target, final boolean narrow, final Object[] args) {
         // when target == context, we are dealing with the null namespace
         if (target == null || target == context) {
             if (narrow) {
                 arithmetic.narrowArguments(args);
             }
             return args;
         }
         // makes target 1st args, copy others - optionally narrow numbers
         final Object[] nargv = new Object[args.length + 1];
         if (narrow) {
             nargv[0] = functionArgument(true, target);
             for (int a = 1; a <= args.length; ++a) {
                 nargv[a] = functionArgument(true, args[a - 1]);
             }
         } else {
             nargv[0] = target;
             System.arraycopy(args, 0, nargv, 1, args.length);
         }
         return nargv;
     }
 
     /**
      * Gets an attribute of an object.
      *
      * @param object    to retrieve value from
      * @param attribute the attribute of the object, e.g. an index (1, 0, 2) or key for a map
      * @param node      the node that evaluated as the object
      * @return the attribute value
      */
     protected Object getAttribute(final Object object, final Object attribute, final JexlNode node) {
         if (object == null) {
             throw new JexlException(node, "object is null");
         }
         cancelCheck(node);
         final JexlOperator operator = node != null && node.jjtGetParent() instanceof ASTArrayAccess
                 ? JexlOperator.ARRAY_GET : JexlOperator.PROPERTY_GET;
         final Object result = operators.tryOverload(node, operator, object, attribute);
         if (result != JexlEngine.TRY_FAILED) {
             return result;
         }
         Exception xcause = null;
         try {
             // attempt to reuse last executor cached in volatile JexlNode.value
             if (node != null && cache) {
                 final Object cached = node.jjtGetValue();
                 if (cached instanceof JexlPropertyGet) {
                     final JexlPropertyGet vg = (JexlPropertyGet) cached;
                     final Object value = vg.tryInvoke(object, attribute);
                     if (!vg.tryFailed(value)) {
                         return value;
                     }
                 }
             }
             // resolve that property
             final List<JexlUberspect.PropertyResolver> resolvers = uberspect.getResolvers(operator, object);
             final JexlPropertyGet vg = uberspect.getPropertyGet(resolvers, object, attribute);
             if (vg != null) {
                 final Object value = vg.invoke(object);
                 // cache executor in volatile JexlNode.value
                 if (node != null && cache && vg.isCacheable()) {
                     node.jjtSetValue(vg);
                 }
                 return value;
             }
         } catch (final Exception xany) {
             xcause = xany;
         }
         // lets fail
         if (node == null) {
             // direct call
             final String error = "unable to get object property"
                     + ", class: " + object.getClass().getName()
                     + ", property: " + attribute;
             throw new UnsupportedOperationException(error, xcause);
         }
         final boolean safe = node instanceof ASTIdentifierAccess && ((ASTIdentifierAccess) node).isSafe();
         if (safe) {
             return null;
         }
         final String attrStr = Objects.toString(attribute, null);
         return unsolvableProperty(node, attrStr, true, xcause);
     }
 
     /**
      * Gets a value of a defined local variable or from the context.
      * @param frame the local frame
      * @param block the lexical block if any
      * @param identifier the variable node
      * @return the value
      */
     protected Object getVariable(final Frame frame, final LexicalScope block, final ASTIdentifier identifier) {
         final int symbol = identifier.getSymbol();
         final String name = identifier.getName();
         // if we have a symbol, we have a scope thus a frame
         if ((options.isLexicalShade() || identifier.isLexical()) && identifier.isShaded()) {
             return undefinedVariable(identifier, name);
         }
         // a local var ?
         if (symbol >= 0 && frame.has(symbol)) {
             final Object value = frame.get(symbol);
             // not out of scope with no lexical shade ?
             if (value != Scope.UNDEFINED) {
                 // null operand of an arithmetic operator ?
                 if (value == null && isStrictOperand(identifier)) {
                     return unsolvableVariable(identifier, name, false); // defined but null
                 }
                 return value;
             }
         }
         // consider global
         final Object value = context.get(name);
         // is it null ?
         if (value == null) {
             // is it defined ?
             if (!context.has(name)) {
                 // not defined, ignore in some cases...
                 final boolean ignore = identifier.jjtGetParent() instanceof ASTReference
                         || isSafe() && (symbol >= 0 || identifier.jjtGetParent() instanceof ASTAssignment);
                 if (!ignore) {
                     return undefinedVariable(identifier, name); // undefined
                 }
             } else if (isStrictOperand(identifier)) {
                 return unsolvableVariable(identifier, name, false); // defined but null
             }
         }
         return value;
     }
     /**
      * Triggered when method, function or constructor invocation fails with an exception.
      * @param node       the node triggering the exception
      * @param methodName the method/function name
      * @param xany       the cause
      * @return a JexlException that will be thrown
      */
     protected JexlException invocationException(final JexlNode node, final String methodName, final Throwable xany) {
         final Throwable cause = xany.getCause();
         if (cause instanceof JexlException) {
             return (JexlException) cause;
         }
         if (cause instanceof InterruptedException) {
             return new JexlException.Cancel(node);
         }
         return new JexlException(node, methodName, xany);
     }
 
     /**
      * @return true if interrupt throws a JexlException.Cancel.
      */
     protected boolean isCancellable() {
         return options.isCancellable();
     }
 
     /**
      * Checks whether this interpreter execution was cancelled due to thread interruption.
      * @return true if cancelled, false otherwise
      */
     protected boolean isCancelled() {
         return cancelled.get() || Thread.currentThread().isInterrupted();
     }
 
     /**
      * Whether this interpreter ignores null in navigation expression as errors.
      * @return true if safe, false otherwise
      */
     protected boolean isSafe() {
         return options.isSafe();
     }
 
     /**
      * Whether this interpreter is currently evaluating with a silent mode.
      * @return true if silent, false otherwise
      */
     protected boolean isSilent() {
         return options.isSilent();
     }
 
     /**
      * Whether this interpreter is currently evaluating with a strict engine flag.
      * @return true if strict engine, false otherwise
      */
     protected boolean isStrictEngine() {
         return options.isStrict();
     }
 
     /**
      * @param node the operand node
      * @return true if this node is an operand of a strict operator, false otherwise
      */
     protected boolean isStrictOperand(final JexlNode node) {
        return node.jjtGetParent().isStrictOperator(arithmetic);
     }
 
     /**
      * Check if a null evaluated expression is protected by a ternary expression.
      * <p>
      * The rationale is that the ternary / elvis expressions are meant for the user to explicitly take control
      * over the error generation; ie, ternaries can return null even if the engine in strict mode
      * would normally throw an exception.
      * </p>
      * @return true if nullable variable, false otherwise
      */
     protected boolean isTernaryProtected(final JexlNode startNode) {
         JexlNode node = startNode;
         for (JexlNode walk = node.jjtGetParent(); walk != null; walk = walk.jjtGetParent()) {
             // protect only the condition part of the ternary
             if (walk instanceof ASTTernaryNode
                     || walk instanceof ASTNullpNode) {
                 return node == walk.jjtGetChild(0);
             }
             if (!(walk instanceof ASTReference || walk instanceof ASTArrayAccess)) {
                 break;
             }
             node = walk;
         }
         return false;
     }
 
     /**
      * Checks whether a variable is defined.
      * <p>The var may be either a local variable declared in the frame and
      * visible from the block or defined in the context.
      * @param frame the frame
      * @param block the block
      * @param name the variable name
      * @return true if variable is defined, false otherwise
      */
     protected boolean isVariableDefined(final Frame frame, final LexicalScope block, final String name) {
         if (frame != null && block != null) {
             final Integer ref = frame.getScope().getSymbol(name);
             final int symbol = ref != null ? ref : -1;
             if (symbol >= 0  && block.hasSymbol(symbol)) {
                 final Object value = frame.get(symbol);
                 return value != Scope.UNDEFINED && value != Scope.UNDECLARED;
             }
         }
         return context.has(name);
     }
 
     /**
      * Triggered when an operator fails.
      * @param node     the node where the error originated from
      * @param operator the operator symbol
      * @param cause    the cause of error (if any)
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object operatorError(final JexlNode node, final JexlOperator operator, final Throwable cause) {
         if (isStrictEngine()) {
             throw new JexlException.Operator(node, operator.getOperatorSymbol(), cause);
         }
         if (logger.isDebugEnabled()) {
             logger.debug(JexlException.operatorError(node, operator.getOperatorSymbol()), cause);
         }
         return null;
     }
 
     /**
      * Triggered when a variable is lexically known as being redefined.
      * @param node  the node where the error originated from
      * @param variable   the variable name
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object redefinedVariable(final JexlNode node, final String variable) {
         return variableError(node, variable, VariableIssue.REDEFINED);
     }
 
     /**
      * Resolves a namespace, eventually allocating an instance using context as constructor argument.
      * <p>
      * The lifetime of such instances span the current expression or script evaluation.</p>
      * @param prefix the prefix name (can be null for global namespace)
      * @param node   the AST node
      * @return the namespace instance
      */
     protected Object resolveNamespace(final String prefix, final JexlNode node) {
         Object namespace;
         // check whether this namespace is a functor
         synchronized (this) {
             if (functors != null) {
                 namespace = functors.get(prefix);
                 if (namespace != null) {
                     return namespace;
                 }
             }
         }
         // check if namespace is a resolver
         namespace = ns.resolveNamespace(prefix);
         if (namespace == null) {
             namespace = functions.get(prefix);
             if (namespace == null) {
                 namespace = jexl.getNamespace(prefix);
             }
             if (prefix != null && namespace == null) {
                 throw new JexlException(node, "no such function namespace " + prefix, null);
             }
         }
         Object functor = null;
         // class or string (*1)
         if (namespace instanceof Class<?> || namespace instanceof String) {
             // the namespace(d) identifier
             final ASTIdentifier nsNode = (ASTIdentifier) node.jjtGetChild(0);
             final boolean cacheable = cache && prefix != null;
             final Object cached = cacheable ? nsNode.jjtGetValue() : null;
             // we know the class is used as namespace of static methods, no functor
             if (cached instanceof Class<?>) {
                 return cached;
             }
             // attempt to reuse last cached constructor
             if (cached instanceof JexlContext.NamespaceFunctor) {
                 final Object eval = ((JexlContext.NamespaceFunctor) cached).createFunctor(context);
                 if (JexlEngine.TRY_FAILED != eval) {
                     functor = eval;
                     namespace = cached;
                 }
             }
             if (functor == null) {
                 // find a constructor with that context as argument or without
                 for (int tried = 0; tried < 2; ++tried) {
                     final boolean withContext = tried == 0;
                     final JexlMethod ctor = withContext
                             ? uberspect.getConstructor(namespace, context)
                             : uberspect.getConstructor(namespace);
                     if (ctor != null) {
                         try {
                             functor = withContext
                                     ? ctor.invoke(namespace, context)
                                     : ctor.invoke(namespace);
                             // defensive
                             if (functor != null) {
                                 // wrap the namespace in a NamespaceFunctor to shield us from the actual
                                 // number of arguments to call it with.
                                 final Object nsFinal = namespace;
                                 // make it a class (not a lambda!) so instanceof (see *2) will catch it
                                 namespace = (NamespaceFunctor) context -> withContext
                                         ? ctor.tryInvoke(null, nsFinal, context)
                                         : ctor.tryInvoke(null, nsFinal);
                                 if (cacheable && ctor.isCacheable()) {
                                     nsNode.jjtSetValue(namespace);
                                 }
                                 break; // we found a constructor that did create a functor
                             }
                         } catch (final Exception xinst) {
                             throw new JexlException(node, "unable to instantiate namespace " + prefix, xinst);
                         }
                     }
                 }
                 // did not, will not create a functor instance; use a class, namespace of static methods
                 if (functor == null) {
                     try {
                         // try to find a class with that name
                         if (namespace instanceof String) {
                             namespace = uberspect.getClassLoader().loadClass((String) namespace);
                         }
                         // we know it's a class in all cases (see *1)
                         if (cacheable) {
                             nsNode.jjtSetValue(namespace);
                         }
                     } catch (final ClassNotFoundException e) {
                         // not a class
                         throw new JexlException(node, "no such class namespace " + prefix, e);
                     }
                 }
             }
         }
         // if a namespace functor, instantiate the functor (if not done already) and store it (*2)
         if (functor == null && namespace instanceof JexlContext.NamespaceFunctor) {
             functor = ((JexlContext.NamespaceFunctor) namespace).createFunctor(context);
         }
         // got a functor, store it and return it
         if (functor != null) {
             synchronized (this) {
                 if (functors == null) {
                     functors = new HashMap<>();
                 }
                 functors.put(prefix, functor);
             }
             return functor;
         }
         return namespace;
     }
 
     /**
      * Sets an attribute of an object.
      *
      * @param object    to set the value to
      * @param attribute the attribute of the object, e.g. an index (1, 0, 2) or key for a map
      * @param value     the value to assign to the object's attribute
      * @param node      the node that evaluated as the object
      */
     protected void setAttribute(final Object object, final Object attribute, final Object value, final JexlNode node) {
         cancelCheck(node);
         final JexlOperator operator = node != null && node.jjtGetParent() instanceof ASTArrayAccess
                                       ? JexlOperator.ARRAY_SET : JexlOperator.PROPERTY_SET;
         final Object result = operators.tryOverload(node, operator, object, attribute, value);
         if (result != JexlEngine.TRY_FAILED) {
             return;
         }
         Exception xcause = null;
         try {
             // attempt to reuse last executor cached in volatile JexlNode.value
             if (node != null && cache) {
                 final Object cached = node.jjtGetValue();
                 if (cached instanceof JexlPropertySet) {
                     final JexlPropertySet setter = (JexlPropertySet) cached;
                     final Object eval = setter.tryInvoke(object, attribute, value);
                     if (!setter.tryFailed(eval)) {
                         return;
                     }
                 }
             }
             final List<JexlUberspect.PropertyResolver> resolvers = uberspect.getResolvers(operator, object);
             JexlPropertySet vs = uberspect.getPropertySet(resolvers, object, attribute, value);
             // if we can't find an exact match, narrow the value argument and try again
             if (vs == null) {
                 // replace all numbers with the smallest type that will fit
                 final Object[] narrow = {value};
                 if (arithmetic.narrowArguments(narrow)) {
                     vs = uberspect.getPropertySet(resolvers, object, attribute, narrow[0]);
                 }
             }
             if (vs != null) {
                 // cache executor in volatile JexlNode.value
                 vs.invoke(object, value);
                 if (node != null && cache && vs.isCacheable()) {
                     node.jjtSetValue(vs);
                 }
                 return;
             }
         } catch (final Exception xany) {
             xcause = xany;
         }
         // lets fail
         if (node == null) {
             // direct call
             final String error = "unable to set object property"
                     + ", class: " + object.getClass().getName()
                     + ", property: " + attribute
                     + ", argument: " + value.getClass().getSimpleName();
             throw new UnsupportedOperationException(error, xcause);
         }
         final String attrStr = Objects.toString(attribute, null);
         unsolvableProperty(node, attrStr, true, xcause);
     }
 
     /**
      * Sets a variable in the global context.
      * <p>If interpretation applies lexical shade, the variable must exist (ie
      * the context has(...) method returns true) otherwise an error occurs.
      * @param node the node
      * @param name the variable name
      * @param value the variable value
      */
     protected void setContextVariable(final JexlNode node, final String name, final Object value) {
         boolean lexical = options.isLexicalShade();
         if (!lexical && node instanceof ASTIdentifier) {
             lexical = ((ASTIdentifier) node).isLexical();
         }
         if (lexical && !context.has(name)) {
             throw new JexlException.Variable(node, name, true);
         }
         try {
             context.set(name, value);
         } catch (final UnsupportedOperationException xsupport) {
             throw new JexlException(node, "context is readonly", xsupport);
         }
     }
 
     /**
      * Pretty-prints a failing property (de)reference.
      * <p>Used by calls to unsolvableProperty(...).</p>
      * @param node the property node
      * @return the (pretty) string
      */
     protected String stringifyProperty(final JexlNode node) {
         if (node instanceof ASTArrayAccess) {
             return "[" + stringifyPropertyValue(node.jjtGetChild(0)) + "]";
         }
         if (node instanceof ASTMethodNode) {
             return stringifyPropertyValue(node.jjtGetChild(0));
         }
         if (node instanceof ASTFunctionNode) {
             return stringifyPropertyValue(node.jjtGetChild(0));
         }
         if (node instanceof ASTIdentifier) {
             return ((ASTIdentifier) node).getName();
         }
         if (node instanceof ASTReference) {
             return stringifyProperty(node.jjtGetChild(0));
         }
         return stringifyPropertyValue(node);
     }
 
     /**
      * Triggered when a variable is lexically known as undefined.
      * @param node  the node where the error originated from
      * @param variable   the variable name
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object undefinedVariable(final JexlNode node, final String variable) {
         return variableError(node, variable, VariableIssue.UNDEFINED);
     }
 
     /**
      * Triggered when a method cannot be resolved.
      * @param node   the node where the error originated from
      * @param method the method name
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object unsolvableMethod(final JexlNode node, final String method) {
         return unsolvableMethod(node, method, null);
     }
 
     /**
      * Triggered when a method cannot be resolved.
      * @param node   the node where the error originated from
      * @param method the method name
      * @param args the method arguments
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object unsolvableMethod(final JexlNode node, final String method, final Object[] args) {
         if (isStrictEngine()) {
             throw new JexlException.Method(node, method, args);
         }
         if (logger.isDebugEnabled()) {
             logger.debug(JexlException.methodError(node, method, args));
         }
         return null;
     }
 
     /**
      * Triggered when a property cannot be resolved.
      * @param node  the node where the error originated from
      * @param property   the property node
      * @param cause the cause if any
      * @param undef whether the property is undefined or null
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object unsolvableProperty(final JexlNode node, final String property, final boolean undef, final Throwable cause) {
         if (isStrictEngine() && !isTernaryProtected(node)) {
             throw new JexlException.Property(node, property, undef, cause);
         }
         if (logger.isDebugEnabled()) {
             logger.debug(JexlException.propertyError(node, property, undef));
         }
         return null;
     }
 
     /**
      * Triggered when a variable cannot be resolved.
      * @param node  the node where the error originated from
      * @param variable   the variable name
      * @param undef whether the variable is undefined or null
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object unsolvableVariable(final JexlNode node, final String variable, final boolean undef) {
         return variableError(node, variable, undef? VariableIssue.UNDEFINED : VariableIssue.NULLVALUE);
     }
 
     /**
      * Triggered when a variable generates an issue.
      * @param node  the node where the error originated from
      * @param variable   the variable name
      * @param issue the issue type
      * @return throws JexlException if strict and not silent, null otherwise
      */
     protected Object variableError(final JexlNode node, final String variable, final VariableIssue issue) {
         if (isStrictEngine() && !isTernaryProtected(node)) {
             throw new JexlException.Variable(node, variable, issue);
         }
         if (logger.isDebugEnabled()) {
             logger.debug(JexlException.variableError(node, variable, issue));
         }
         return null;
     }
 }