/*
 * 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.commons.el;

import java.io.Reader;
import java.io.StringReader;
import java.text.MessageFormat;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.jsp.el.ELException;
import javax.servlet.jsp.el.ExpressionEvaluator;
import javax.servlet.jsp.el.FunctionMapper;
import javax.servlet.jsp.el.VariableResolver;

import org.apache.commons.el.parser.ELParser;
import org.apache.commons.el.parser.ParseException;
import org.apache.commons.el.parser.Token;
import org.apache.commons.el.parser.TokenMgrError;

/**
 *
 * <p>This is the main class for evaluating expression Strings.  An
 * expression String is a String that may contain expressions of the
 * form ${...}.  Multiple expressions may appear in the same
 * expression String.  In such a case, the expression String's value
 * is computed by concatenating the String values of those evaluated
 * expressions and any intervening non-expression text, then
 * converting the resulting String to the expected type using the
 * PropertyEditor mechanism.
 *
 * <p>In the special case where the expression String is a single
 * expression, the value of the expression String is determined by
 * evaluating the expression, without any intervening conversion to a
 * String.
 *
 * <p>The evaluator maintains a cache mapping expression Strings to
 * their parsed results.  For expression Strings containing no
 * expression elements, it maintains a cache mapping
 * ExpectedType/ExpressionString to parsed value, so that static
 * expression Strings won't have to go through a conversion step every
 * time they are used.  All instances of the evaluator share the same
 * cache.  The cache may be bypassed by setting a flag on the
 * evaluator's constructor.
 *
 * <p>The evaluator must be passed a VariableResolver in its
 * constructor.  The VariableResolver is used to resolve variable
 * names encountered in expressions, and can also be used to implement
 * "implicit objects" that are always present in the namespace.
 * Different applications will have different policies for variable
 * lookups and implicit objects - these differences can be
 * encapsulated in the VariableResolver passed to the evaluator's
 * constructor.
 *
 * <p>Most VariableResolvers will need to perform their resolution
 * against some context.  For example, a JSP environment needs a
 * PageContext to resolve variables.  The evaluate() method takes a
 * generic Object context which is eventually passed to the
 * VariableResolver - the VariableResolver is responsible for casting
 * the context to the proper type.
 *
 * <p>Once an evaluator instance has been constructed, it may be used
 * multiple times, and may be used by multiple simultaneous Threads.
 * In other words, an evaluator instance is well-suited for use as a
 * singleton.
 * 
 * @author Nathan Abramson - Art Technology Group
 * @author Shawn Bayern
 * @version $Change: 181177 $$DateTime: 2001/06/26 08:45:09 $$Author: bayard $
 **/
public class ExpressionEvaluatorImpl
  extends ExpressionEvaluator
{
  //-------------------------------------
  // Statics
  //-------------------------------------
  /** The mapping from expression String to its parsed form (String,
      Expression, or ExpressionString) **/
  static Map sCachedExpressionStrings =
    Collections.synchronizedMap (new HashMap ());

  /** The mapping from ExpectedType to Maps mapping literal String to
      parsed value **/
  static Map sCachedExpectedTypes = new HashMap ();

  //-------------------------------------
  // Member variables
  //-------------------------------------

  /** Flag if the cache should be bypassed **/
  boolean mBypassCache;

  //-------------------------------------
  /**
   *
   * Constructor
   **/
  public ExpressionEvaluatorImpl () { }

  /**
   *
   * Constructor
   *
   * @param pBypassCache flag indicating if the cache should be
   * bypassed
   **/
  public ExpressionEvaluatorImpl (boolean pBypassCache)
  {
    mBypassCache = pBypassCache;
  }

  //-------------------------------------

  /**
   *
   * Prepare an expression for later evaluation.  This method should perform
   * syntactic validation of the expression; if in doing so it detects 
   * errors, it should raise an ELParseException.
   *
   * @param expression The expression to be evaluated.
   * @param expectedType The expected type of the result of the evaluation
   * @param fMapper A FunctionMapper to resolve functions found in
   *     the expression.  It can be null, in which case no functions
   *     are supported for this invocation.  The ExpressionEvaluator
   *     must not hold on to the FunctionMapper reference after
   *     returning from <code>parseExpression()</code>.  The
   *     <code>Expression</code> object returned must invoke the same
   *     functions regardless of whether the mappings in the
   *     provided <code>FunctionMapper</code> instance change between
   *     calling <code>ExpressionEvaluator.parseExpression()</code>
   *     and <code>Expression.evaluate()</code>.
   * @return The Expression object encapsulating the arguments.
   *
   * @exception ELException Thrown if parsing errors were found.
   **/
  public javax.servlet.jsp.el.Expression parseExpression(String expression,
                                                        Class expectedType,
                                                        FunctionMapper fMapper)
    throws ELException
  {
       // Create an Expression object that knows how to evaluate this.
       final Object parsedExpression = parseExpressionString(expression);
       if (parsedExpression instanceof Expression) {
           return new JSTLExpression(this, (Expression)parsedExpression, expectedType, fMapper);
       } else {
           // this had better be a string
           return new JSTLExpression(this, (String)parsedExpression, expectedType, fMapper);
       }
  }

  //-------------------------------------
  /**
   *
   * Evaluates the given expression String
   *
   * @param pExpressionString The expression to be evaluated.
   * @param pExpectedType The expected type of the result of the evaluation
   * @param pResolver A VariableResolver instance that can be used at 
   *     runtime to resolve the name of implicit objects into Objects.
   * @param functions A FunctionMapper to resolve functions found in 
   *     the expression.  It can be null, in which case no functions 
   *     are supported for this invocation.
   * @return the expression String evaluated to the given expected type
   **/
  public Object evaluate (String pExpressionString,
                Class pExpectedType,
                VariableResolver pResolver,
                FunctionMapper functions)
    throws ELException
  {
    // Check for null expression strings
    if (pExpressionString == null) {
      throw new ELException
       (Constants.NULL_EXPRESSION_STRING);
    }

    // Get the parsed version of the expression string
    Object parsedValue = parseExpressionString (pExpressionString);
    return evaluate (parsedValue, pExpectedType, pResolver, functions);
  }

  //-------------------------------------
  /**
   *
   * Evaluates the given parsed expression.
   *
   * @param parsedExpression The expression to be evaluated.
   * @param pExpectedType The expected type of the result of the evaluation
   * @param pResolver A VariableResolver instance that can be used at 
   *     runtime to resolve the name of implicit objects into Objects.
   * @param functions A FunctionMapper to resolve functions found in 
   *     the expression.  It can be null, in which case no functions 
   *     are supported for this invocation.
   * @return the expression evaluated to the given expected type
   **/
  public Object evaluate (Object parsedExpression, Class pExpectedType,
      VariableResolver pResolver, FunctionMapper functions) throws ELException
  {
      return evaluateParsedValue(parsedExpression, pExpectedType, pResolver, functions);
  }

  private Object evaluateParsedValue(Object parsedValue, Class pExpectedType, VariableResolver pResolver, FunctionMapper functions) throws ELException {
        // Evaluate differently based on the parsed type
        if (parsedValue instanceof String) {
          // Convert the String, and cache the conversion
          String strValue = (String) parsedValue;
          return convertStaticValueToExpectedType (strValue, pExpectedType);
        }

        else if (parsedValue instanceof Expression) {
          // Evaluate the expression and convert
          Object value =
        ((Expression) parsedValue).evaluate (pResolver,
                            functions);
          return convertToExpectedType (value, pExpectedType);
        }

        else {
          // This should never be reached
          return null;
        }
    }

  //-------------------------------------
  /**
   *
   * Gets the parsed form of the given expression string.  If the
   * parsed form is cached (and caching is not bypassed), return the
   * cached form, otherwise parse and cache the value.  Returns either
   * a String, Expression, or ExpressionString.
   **/
  public Object parseExpressionString (String pExpressionString)
    throws ELException
  {
    // See if it's an empty String
    if (pExpressionString.length () == 0) {
      return "";
    }

    // See if it's in the cache
    Object ret =
      mBypassCache ?
      null :
      sCachedExpressionStrings.get (pExpressionString);

    if (ret == null) {
      // Parse the expression
      Reader r = new StringReader (pExpressionString);
      ELParser parser = new ELParser (r);
      try {
        ret = parser.ExpressionString ();
        sCachedExpressionStrings.put (pExpressionString, ret);
      }
      catch (ParseException exc)
      {
        throw new ELException
          (formatParseException (pExpressionString,
              exc));
      }
      catch (TokenMgrError exc)
      {
        // Note - this should never be reached, since the parser is
        // constructed to tokenize any input (illegal inputs get
        // parsed to <BADLY_ESCAPED_STRING_LITERAL> or
        // <ILLEGAL_CHARACTER>
        throw new ELException (exc.getMessage ());
      }
    }
    return ret;
  }

  //-------------------------------------
  /**
   *
   * Converts the given value to the specified expected type.
   **/
  Object convertToExpectedType (Object pValue,
                               Class pExpectedType)
    throws ELException
  {
    return Coercions.coerce (pValue, pExpectedType);
  }

  //-------------------------------------
  /**
   *
   * Converts the given String, specified as a static expression
   * string, to the given expected type.  The conversion is cached.
   **/
  Object convertStaticValueToExpectedType (String pValue, Class pExpectedType)
    throws ELException
  {
    // See if the value is already of the expected type
    if (pExpectedType == String.class ||
      pExpectedType == Object.class) {
      return pValue;
    }

    // Find the cached value
    Map valueByString = getOrCreateExpectedTypeMap (pExpectedType);
    if (!mBypassCache &&
      valueByString.containsKey (pValue)) {
      return valueByString.get (pValue);
    }
    else {
      // Convert from a String
      Object ret = Coercions.coerce (pValue, pExpectedType);
      valueByString.put (pValue, ret);
      return ret;
    }
  }

  //-------------------------------------
  /**
   *
   * Creates or returns the Map that maps string literals to parsed
   * values for the specified expected type.
   **/
  static Map getOrCreateExpectedTypeMap (Class pExpectedType)
  {
    synchronized (sCachedExpectedTypes) {
      Map ret = (Map) sCachedExpectedTypes.get (pExpectedType);
      if (ret == null) {
        ret = Collections.synchronizedMap (new HashMap ());
        sCachedExpectedTypes.put (pExpectedType, ret);
      }
      return ret;
    }
  }

  //-------------------------------------
  // Formatting ParseException
  //-------------------------------------
  /**
   *
   * Formats a ParseException into an error message suitable for
   * displaying on a web page
   **/
  static String formatParseException (String pExpressionString,
                                     ParseException pExc)
  {
    // Generate the String of expected tokens
    StringBuffer expectedBuf = new StringBuffer ();
    int maxSize = 0;
    boolean printedOne = false;

    if (pExc.expectedTokenSequences == null)
      return pExc.toString();

    for (int i = 0; i < pExc.expectedTokenSequences.length; i++) {
      if (maxSize < pExc.expectedTokenSequences [i].length) {
        maxSize = pExc.expectedTokenSequences [i].length;
      }
      for (int j = 0; j < pExc.expectedTokenSequences[i].length; j++) {
        if (printedOne) {
          expectedBuf.append (", ");
        }
        expectedBuf.append
          (pExc.tokenImage [pExc.expectedTokenSequences [i] [j]]);
        printedOne = true;
      }
    }
    String expected = expectedBuf.toString ();

    // Generate the String of encountered tokens
    StringBuffer encounteredBuf = new StringBuffer ();
    Token tok = pExc.currentToken.next;
    for (int i = 0; i < maxSize; i++) {
      if (i != 0) encounteredBuf.append (" ");

      if (tok.kind == 0) {
        encounteredBuf.append (pExc.tokenImage[0]);
        break;
      }
      encounteredBuf.append (addEscapes (tok.image));
      tok = tok.next;
    }
    String encountered = encounteredBuf.toString ();

    // Format the error message
    return MessageFormat.format
      (Constants.PARSE_EXCEPTION,
       new Object[] {
        expected,
        encountered,
       });
  }

  //-------------------------------------
  /**
   *
   * Used to convert raw characters to their escaped version when
   * these raw version cannot be used as part of an ASCII string
   * literal.
   **/
  static String addEscapes (String str)
  {
    StringBuffer retval = new StringBuffer ();
    char ch;
    for (int i = 0, length = str.length (); i < length; i++) {
      switch (str.charAt (i)) {
      case 0:
        continue;
      case '\b':
        retval.append ("\\b");
        continue;
      case '\t':
        retval.append ("\\t");
        continue;
      case '\n':
        retval.append ("\\n");
        continue;
      case '\f':
        retval.append ("\\f");
        continue;
      case '\r':
        retval.append ("\\r");
        continue;
      default:
        if ((ch = str.charAt (i)) < 0x20 || ch > 0x7e) {
          String s = "0000" + Integer.toString (ch, 16);
          retval.append ("\\u" + s.substring (s.length () - 4, s.length ()));
        }
        else {
          retval.append (ch);
        }
        continue;
      }
    }
    return retval.toString ();
  }

  //-------------------------------------
  // Testing methods
  //-------------------------------------
  /**
   *
   * Parses the given expression string, then converts it back to a
   * String in its canonical form.  This is used to test parsing.
   **/
  public String parseAndRender (String pExpressionString)
    throws ELException
  {
    Object val = parseExpressionString (pExpressionString);
    if (val instanceof String) {
      return (String) val;
    }
    else if (val instanceof Expression) {
      return "${" + ((Expression) val).getExpressionString () + "}";
    }
    else if (val instanceof ExpressionString) {
      return ((ExpressionString) val).getExpressionString ();
    }
    else {
      return "";
    }
  }

  /**
   * An object that encapsulates an expression to be evaluated by 
   * the JSTL evaluator.
   */
  private class JSTLExpression
    extends javax.servlet.jsp.el.Expression
  {
    private ExpressionEvaluatorImpl evaluator;
    private Object parsedExpression;
    private Class expectedType;

    public JSTLExpression(
            final ExpressionEvaluatorImpl evaluator,
            final Expression expression,
            final Class expectedType,
            final FunctionMapper fMapper)
    throws ELException {
      this.evaluator = evaluator;
      this.parsedExpression = expression.bindFunctions(fMapper);
      this.expectedType = expectedType;
    }
    public JSTLExpression(
            final ExpressionEvaluatorImpl evaluator,
            final String expressionString,
            final Class expectedType,
            final FunctionMapper fMapper)
    throws ELException {
       this.evaluator = evaluator;
       this.parsedExpression = expressionString;
       this.expectedType = expectedType;
     }
    
     public Object evaluate( VariableResolver vResolver )
       throws ELException
     {
      return evaluator.evaluateParsedValue(this.parsedExpression,
               this.expectedType,
               vResolver,
               null);
     }
   }

  //-------------------------------------

}