/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed 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.springframework.expression.spel;

import java.util.ArrayDeque;
import java.util.Collections;
import java.util.Deque;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;

import org.springframework.core.convert.TypeDescriptor;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.EvaluationException;
import org.springframework.expression.Operation;
import org.springframework.expression.OperatorOverloader;
import org.springframework.expression.PropertyAccessor;
import org.springframework.expression.TypeComparator;
import org.springframework.expression.TypeConverter;
import org.springframework.expression.TypedValue;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;

/**
 * An ExpressionState is for maintaining per-expression-evaluation state, any changes to
 * it are not seen by other expressions but it gives a place to hold local variables and
 * for component expressions in a compound expression to communicate state. This is in
 * contrast to the EvaluationContext, which is shared amongst expression evaluations, and
 * any changes to it will be seen by other expressions or any code that chooses to ask
 * questions of the context.
 *
 * <p>It also acts as a place for to define common utility routines that the various AST
 * nodes might need.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 3.0
 */
public class ExpressionState {

        private final EvaluationContext relatedContext;

        private final TypedValue rootObject;

        private final SpelParserConfiguration configuration;

        @Nullable
        private Deque<TypedValue> contextObjects;

        @Nullable
        private Deque<VariableScope> variableScopes;

        // When entering a new scope there is a new base object which should be used
        // for '#this' references (or to act as a target for unqualified references).
        // This ArrayDeque captures those objects at each nested scope level.
        // For example:
        // #list1.?[#list2.contains(#this)]
        // On entering the selection we enter a new scope, and #this is now the
        // element from list1
        @Nullable
        private ArrayDeque<TypedValue> scopeRootObjects;


        public ExpressionState(EvaluationContext context) {
                this(context, context.getRootObject(), new SpelParserConfiguration(false, false));
        }

        public ExpressionState(EvaluationContext context, SpelParserConfiguration configuration) {
                this(context, context.getRootObject(), configuration);
        }

        public ExpressionState(EvaluationContext context, TypedValue rootObject) {
                this(context, rootObject, new SpelParserConfiguration(false, false));
        }

        public ExpressionState(EvaluationContext context, TypedValue rootObject, SpelParserConfiguration configuration) {
                Assert.notNull(context, "EvaluationContext must not be null");
                Assert.notNull(configuration, "SpelParserConfiguration must not be null");
                this.relatedContext = context;
                this.rootObject = rootObject;
                this.configuration = configuration;
        }


        /**
         * The active context object is what unqualified references to properties/etc are resolved against.
         */
        public TypedValue getActiveContextObject() {
                if (CollectionUtils.isEmpty(this.contextObjects)) {
                        return this.rootObject;
                }
                return this.contextObjects.element();
        }

        public void pushActiveContextObject(TypedValue obj) {
                if (this.contextObjects == null) {
                        this.contextObjects = new ArrayDeque<>();
                }
                this.contextObjects.push(obj);
        }

        public void popActiveContextObject() {
                if (this.contextObjects == null) {
                        this.contextObjects = new ArrayDeque<>();
                }
                try {
                        this.contextObjects.pop();
                }
                catch (NoSuchElementException ex) {
                        throw new IllegalStateException("Cannot pop active context object: stack is empty");
                }
        }

        public TypedValue getRootContextObject() {
                return this.rootObject;
        }

        public TypedValue getScopeRootContextObject() {
                if (CollectionUtils.isEmpty(this.scopeRootObjects)) {
                        return this.rootObject;
                }
                return this.scopeRootObjects.element();
        }

        public void setVariable(String name, @Nullable Object value) {
                this.relatedContext.setVariable(name, value);
        }

        public TypedValue lookupVariable(String name) {
                Object value = this.relatedContext.lookupVariable(name);
                return (value != null ? new TypedValue(value) : TypedValue.NULL);
        }

        public TypeComparator getTypeComparator() {
                return this.relatedContext.getTypeComparator();
        }

        public Class<?> findType(String type) throws EvaluationException {
                return this.relatedContext.getTypeLocator().findType(type);
        }

        public Object convertValue(Object value, TypeDescriptor targetTypeDescriptor) throws EvaluationException {
                Object result = this.relatedContext.getTypeConverter().convertValue(
                                value, TypeDescriptor.forObject(value), targetTypeDescriptor);
                if (result == null) {
                        throw new IllegalStateException("Null conversion result for value [" + value + "]");
                }
                return result;
        }

        public TypeConverter getTypeConverter() {
                return this.relatedContext.getTypeConverter();
        }

        @Nullable
        public Object convertValue(TypedValue value, TypeDescriptor targetTypeDescriptor) throws EvaluationException {
                Object val = value.getValue();
                return this.relatedContext.getTypeConverter().convertValue(
                                val, TypeDescriptor.forObject(val), targetTypeDescriptor);
        }

        /*
         * A new scope is entered when a function is invoked.
         */
        public void enterScope(Map<String, Object> argMap) {
                initVariableScopes().push(new VariableScope(argMap));
                initScopeRootObjects().push(getActiveContextObject());
        }

        public void enterScope() {
                initVariableScopes().push(new VariableScope(Collections.emptyMap()));
                initScopeRootObjects().push(getActiveContextObject());
        }

        public void enterScope(String name, Object value) {
                initVariableScopes().push(new VariableScope(name, value));
                initScopeRootObjects().push(getActiveContextObject());
        }

        public void exitScope() {
                initVariableScopes().pop();
                initScopeRootObjects().pop();
        }

        public void setLocalVariable(String name, Object value) {
                initVariableScopes().element().setVariable(name, value);
        }

        @Nullable
        public Object lookupLocalVariable(String name) {
                for (VariableScope scope : initVariableScopes()) {
                        if (scope.definesVariable(name)) {
                                return scope.lookupVariable(name);
                        }
                }
                return null;
        }

        private Deque<VariableScope> initVariableScopes() {
                if (this.variableScopes == null) {
                        this.variableScopes = new ArrayDeque<>();
                        // top-level empty variable scope
                        this.variableScopes.add(new VariableScope());
                }
                return this.variableScopes;
        }

        private Deque<TypedValue> initScopeRootObjects() {
                if (this.scopeRootObjects == null) {
                        this.scopeRootObjects = new ArrayDeque<>();
                }
                return this.scopeRootObjects;
        }

        public TypedValue operate(Operation op, @Nullable Object left, @Nullable Object right) throws EvaluationException {
                OperatorOverloader overloader = this.relatedContext.getOperatorOverloader();
                if (overloader.overridesOperation(op, left, right)) {
                        Object returnValue = overloader.operate(op, left, right);
                        return new TypedValue(returnValue);
                }
                else {
                        String leftType = (left == null ? "null" : left.getClass().getName());
                        String rightType = (right == null? "null" : right.getClass().getName());
                        throw new SpelEvaluationException(SpelMessage.OPERATOR_NOT_SUPPORTED_BETWEEN_TYPES, op, leftType, rightType);
                }
        }

        public List<PropertyAccessor> getPropertyAccessors() {
                return this.relatedContext.getPropertyAccessors();
        }

        public EvaluationContext getEvaluationContext() {
                return this.relatedContext;
        }

        public SpelParserConfiguration getConfiguration() {
                return this.configuration;
        }


        /**
         * A new scope is entered when a function is called and it is used to hold the
         * parameters to the function call. If the names of the parameters clash with
         * those in a higher level scope, those in the higher level scope will not be
         * accessible whilst the function is executing. When the function returns,
         * the scope is exited.
         */
        private static class VariableScope {

                private final Map<String, Object> vars = new HashMap<>();

                public VariableScope() {
                }

                public VariableScope(@Nullable Map<String, Object> arguments) {
                        if (arguments != null) {
                                this.vars.putAll(arguments);
                        }
                }

                public VariableScope(String name, Object value) {
                        this.vars.put(name,value);
                }

                public Object lookupVariable(String name) {
                        return this.vars.get(name);
                }

                public void setVariable(String name, Object value) {
                        this.vars.put(name,value);
                }

                public boolean definesVariable(String name) {
                        return this.vars.containsKey(name);
                }
        }

}