/*
 * 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
 *
 *      http://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.jasper.interpreter;

import java.io.StringReader;
import java.util.ArrayList;
import java.util.List;

import org.apache.el.parser.AstDeferredExpression;
import org.apache.el.parser.AstDynamicExpression;
import org.apache.el.parser.AstLiteralExpression;
import org.apache.el.parser.ELParser;
import org.apache.el.parser.Node;
import org.apache.el.parser.ParseException;
import org.apache.jasper.JspCompilationContext;
import org.apache.jasper.compiler.ELInterpreter;
import org.apache.jasper.compiler.JspUtil;

/**
 * Simple ELInterpreter. It can transfer simple ELs to java code directly.
 * EL is a big bottleneck of JSP. EL resolving takes much CPU expense.
 * 
 * The performance is better when simple ELs were transfered to java code.
 *  
 * However the code will by pass all ELResolvers. This interpreter is not a standard of JSP specification. 
 * When activate this ELInterpreter. User must know how it works.
 * 
 * Here are the cases of Simple ELs,
 * 
 * 1.Simple EL, only contains one part of expression, for example ${elemId}. This kind of EL will be generated as
 * 
 * this.getJspContext().findAttribute("elemId")
 * 
 * 2.EL with two parts of expression and type of first part is specified by attribute in Tag File. 
 *   For example, ${model.location} .  "model" is specified on the top of tag file, 
 *   
 *   <%@ attribute name="model" required="true" type="org.apache.jasper.model.results.ItemModel" %>.
 *   
 *   It is generated as (getModel() != null ? getModel().getLocation() : null)
 *   
 * 3.EL with logic or arithmetic and the value part can be generated.
 *   For example: ${(intlExpansion eq 'true' && not empty model.location) || sortType==7}
 *   
 *   It is generated as 
 *   (org.apache.jasper.runtime.ELRuntimeUtil.equals(getIntlExpansion(), "true")&&(!org.apache.jasper.runtime.ELRuntimeUtil.isEmpty((getModel() != null ? getModel().getLocation() : null))))
 * 
 * How to apply this ELInterpreter?
 * 
 * It is based on BUG 54239.
 * 
 * @author Sheldon Shao <xshao@ebay.com>
 */
public class JasperELInterpreter implements ELInterpreter {

    public static enum ELFormat {
        SINGLE,

        COUPLE,
        
        MULTIPLE,

        COMPLEX
    };

    public String interpreterCall(JspCompilationContext context, boolean isTagFile, String expression,
            Class<?> expectedType, String fnmapvar, boolean xmlEscape) {

        if (fnmapvar == null) {
            List<String> nodes = new ArrayList<String>(2);
            ELFormat format = split(expression, nodes);
            if (format == ELFormat.SINGLE) {
                return ELCodegen.genSingleNode(context, isTagFile, nodes.get(0), expectedType, xmlEscape, true);
            } else if (format == ELFormat.COUPLE && isTagFile) {
                String firstNode = nodes.get(0);
                String secondNode = nodes.get(1);
                return ELCodegen.genCoupleNodes(context, isTagFile, firstNode, secondNode, expression, expectedType, xmlEscape, true);
            }
            
            String extension = forExtend(format, context, isTagFile, expression, expectedType, fnmapvar, xmlEscape, nodes);
            if (extension != null) {
                return extension;
            }
            else {
                Node node = parseAsNode(expression);
                if (node != null) {
                    try {
                        return generate(context, isTagFile, expectedType, node, xmlEscape);
                    }
                    catch(IllegalStateException ise) {
                    }
                }
            }
        }

        return JspUtil.interpreterCall(isTagFile, expression, expectedType,
                fnmapvar, xmlEscape);
    }
    
    protected String forExtend(ELFormat format, JspCompilationContext context, boolean isTagFile, String expression,
            Class<?> expectedType, String fnmapvar, boolean xmlEscape, List<String> nodes) {
        return null;
    }
    
    private String generate(JspCompilationContext context, boolean isTagFile, Class<?> expectedType, Node node, boolean xmlEscape) {
        String code = ELCodegen.generate(context, isTagFile, expectedType, node, true);
        if (xmlEscape && String.class == expectedType) {
            return "org.apache.jasper.runtime.ELRuntimeUtil.xmlEscape(" + code + ")";
        }
        return code;
    }
    
    private Node parseAsNode(String expression) {
        ELParser parser = new ELParser(new StringReader(expression));
        Node n;
        try {
            n = parser.CompositeExpression();
        } catch (ParseException e) {
            return null;
        }
        
        // validate composite expression
        int numChildren = n.jjtGetNumChildren();
        if (numChildren == 1) {
            n = n.jjtGetChild(0);
        } else {
            Class<?> type = null;
            Node child = null;
            for (int i = 0; i < numChildren; i++) {
                child = n.jjtGetChild(i);
                if (child instanceof AstLiteralExpression)
                    continue;
                if (type == null)
                    type = child.getClass();
                else {
                    if (!type.equals(child.getClass())) {
                        return null;
                    }
                }
            }
        }

        if (n instanceof AstDeferredExpression
                || n instanceof AstDynamicExpression) {
            n = n.jjtGetChild(0);
        }
        
        return n;
    }
    
    
    private static int[] trim(String text, int start, int end) {
        char ch;
        int i = start;
        for (; i < end; i++) {
            ch = text.charAt(i);
            if (ch == ' ' || ch == '\t') {
                continue;
            }
            else {
                break;
            }
        }
        start = i;
        for (i = end -1; i > start; i--) {
            ch = text.charAt(i);
            if (ch == ' ' || ch == '\t') {
                continue;
            }
            else {
                break;
            }
        }
        end = i + 1;
        return new int[] { start, end };
    }

    private ELFormat split(String text, List<String> nodes) {
        char ch = text.charAt(0);
        int len = text.length();
        if (ch == '$' || ch == '#') {
            int end = len - 1;
            if (text.charAt(1) == '{' && text.charAt(end) == '}') {
                int[] trimed = trim(text, 2, end);
                ch = text.charAt(trimed[0]);
                if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) {
                    int nodeCount = 1;
                    int pos = trimed[0];
                    end = trimed[1];
                    for (int i = pos+1; i < end; i++) {
                        ch = text.charAt(i);
                        if ((ch >= 'A' && ch <= 'Z')
                                || (ch >= 'a' && ch <= 'z')
                                || (ch >= '0' && ch <= '9') || ch == '_') {
                            continue;
                        } else if (ch == '.') {
                            String el = text.substring(pos, i);
                            nodes.add(el);
                            pos = i + 1;
                            nodeCount++;
                        } else {
                            return ELFormat.COMPLEX;
                        }
                    }
                    if (pos < end) {
                        String el = text.substring(pos, end);
                        nodes.add(el);
                    }
                    return nodeCount == 1 ? ELFormat.SINGLE :(nodeCount == 2 ? ELFormat.COUPLE : ELFormat.MULTIPLE);
                }
            }
        }
        return ELFormat.COMPLEX;
    }
}