/*
 * Copyright 2002-2009 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
 *
 *      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.springframework.expression.common;

import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.ParseException;
import org.springframework.expression.ParserContext;

/**
 * An expression parser that understands templates. It can be subclassed
 * by expression parsers that do not offer first class support for templating.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @author Andy Clement
 * @since 3.0
 */
public abstract class TemplateAwareExpressionParser implements ExpressionParser {

	/**
	 * Default ParserContext instance for non-template expressions.
	 */
	private static final ParserContext NON_TEMPLATE_PARSER_CONTEXT = new ParserContext() {
		public String getExpressionPrefix() {
			return null;
		}
		public String getExpressionSuffix() {
			return null;
		}
		public boolean isTemplate() {
			return false;
		}
	};


	public Expression parseExpression(String expressionString) throws ParseException {
		return parseExpression(expressionString, NON_TEMPLATE_PARSER_CONTEXT);
	}

	public Expression parseExpression(String expressionString, ParserContext context) throws ParseException {
		if (context == null) {
			context = NON_TEMPLATE_PARSER_CONTEXT;
		}
		if (context.isTemplate()) {
			return parseTemplate(expressionString, context);
		} else {
			return doParseExpression(expressionString, context);
		}
	}

	private Expression parseTemplate(String expressionString, ParserContext context) throws ParseException {
		if (expressionString.length() == 0) {
			return new LiteralExpression("");
		}
		Expression[] expressions = parseExpressions(expressionString, context);
		if (expressions.length == 1) {
			return expressions[0];
		} else {
			return new CompositeStringExpression(expressionString, expressions);
		}
	}


	/**
	 * Helper that parses given expression string using the configured parser. The expression string can contain any
	 * number of expressions all contained in "${...}" markers. For instance: "foo${expr0}bar${expr1}". The static
	 * pieces of text will also be returned as Expressions that just return that static piece of text. As a result,
	 * evaluating all returned expressions and concatenating the results produces the complete evaluated string.
	 * Unwrapping is only done of the outermost delimiters found, so the string 'hello ${foo${abc}}' would break into
	 * the pieces 'hello ' and 'foo${abc}'. This means that expression languages that used ${..} as part of their
	 * functionality are supported without any problem.
	 * The parsing is aware of the structure of an embedded expression.  It assumes that parentheses '(', 
	 * square brackets '[' and curly brackets '}' must be in pairs within the expression unless they are within a 
	 * string literal and a string literal starts and terminates with a single quote '.
	 * 
	 * @param expressionString the expression string
	 * @return the parsed expressions
	 * @throws ParseException when the expressions cannot be parsed
	 */
	private Expression[] parseExpressions(String expressionString, ParserContext context) throws ParseException {
		List<Expression> expressions = new LinkedList<Expression>();
		String prefix = context.getExpressionPrefix();
		String suffix = context.getExpressionSuffix();
		int startIdx = 0;
		while (startIdx < expressionString.length()) {
			int prefixIndex = expressionString.indexOf(prefix,startIdx);
			if (prefixIndex >= startIdx) {
				// an inner expression was found - this is a composite
				if (prefixIndex > startIdx) {
					expressions.add(createLiteralExpression(context,expressionString.substring(startIdx, prefixIndex)));
				}
				int afterPrefixIndex = prefixIndex + prefix.length();
				int suffixIndex = skipToCorrectEndSuffix(prefix,suffix,expressionString,afterPrefixIndex);
				if (suffixIndex == -1) {
					throw new ParseException(expressionString, prefixIndex, "No ending suffix '" + suffix +
							"' for expression starting at character " + prefixIndex + ": " +
							expressionString.substring(prefixIndex));
				}
				if (suffixIndex == afterPrefixIndex) {
					throw new ParseException(expressionString, prefixIndex, "No expression defined within delimiter '" +
							prefix + suffix + "' at character " + prefixIndex);
				} else {
					String expr = expressionString.substring(prefixIndex + prefix.length(), suffixIndex);
					expr = expr.trim();
					if (expr.length()==0) {
						throw new ParseException(expressionString, prefixIndex, "No expression defined within delimiter '" +
								prefix + suffix + "' at character " + prefixIndex);
					}
					expressions.add(doParseExpression(expr, context));
					startIdx = suffixIndex + suffix.length();
				}
			} else {
				// no more ${expressions} found in string, add rest as static text
				expressions.add(createLiteralExpression(context,expressionString.substring(startIdx)));
				startIdx = expressionString.length();
			}
		}
		return expressions.toArray(new Expression[expressions.size()]);
	}
	
	private Expression createLiteralExpression(ParserContext context, String text) {
		return new LiteralExpression(text);
	}
	
	/**
	 * Return true if the specified suffix can be found at the supplied position in the supplied expression string.
	 * @param expressionString the expression string which may contain the suffix
	 * @param pos the start position at which to check for the suffix
	 * @param suffix the suffix string
	 * @return
	 */
	private boolean isSuffixHere(String expressionString,int pos,String suffix) {
		int suffixPosition = 0;
		for (int i=0;i<suffix.length() && pos<expressionString.length();i++) {
			if (expressionString.charAt(pos++)!=suffix.charAt(suffixPosition++)) {
				return false;
			}
		}
		if (suffixPosition!=suffix.length()) {
			// the expressionString ran out before the suffix could entirely be found
			return false;
		}
		return true;
	}
	
	/**
	 * Copes with nesting, for example '${...${...}}' where the correct end for the first ${ is the final }.
	 * 
	 * @param prefix the prefix 
	 * @param suffix the suffix
	 * @param expressionString the expression string
	 * @param afterPrefixIndex the most recently found prefix location for which the matching end suffix is being sought
	 * @return the position of the correct matching nextSuffix or -1 if none can be found
	 */
	private int skipToCorrectEndSuffix(String prefix, String suffix, String expressionString, int afterPrefixIndex) throws ParseException {
		// Chew on the expression text - relying on the rules:
		// brackets must be in pairs: () [] {}
		// string literals are "..." or '...' and these may contain unmatched brackets
		int pos = afterPrefixIndex;
		int maxlen = expressionString.length();
		int nextSuffix = expressionString.indexOf(suffix,afterPrefixIndex);
		if (nextSuffix ==-1 ) {
			return -1; // the suffix is missing
		}
		Stack<Bracket> stack = new Stack<Bracket>();
		while (pos<maxlen) {
			if (isSuffixHere(expressionString,pos,suffix) && stack.isEmpty()) {
				break;
			}
			char ch = expressionString.charAt(pos);
			switch (ch) {
			case '{': case '[': case '(':
				stack.push(new Bracket(ch,pos));
				break;
			case '}':case ']':case ')':
				if (stack.isEmpty()) {
					throw new ParseException(expressionString, pos, "Found closing '"+ch+"' at position "+pos+" without an opening '"+Bracket.theOpenBracketFor(ch)+"'");
				}
				Bracket p = stack.pop();
				if (!p.compatibleWithCloseBracket(ch)) {
					throw new ParseException(expressionString, pos, "Found closing '"+ch+"' at position "+pos+" but most recent opening is '"+p.bracket+"' at position "+p.pos);
				}
				break;
			case '\'':
			case '"':
				// jump to the end of the literal
				int endLiteral = expressionString.indexOf(ch,pos+1);
				if (endLiteral==-1) {
					throw new ParseException(expressionString, pos, "Found non terminating string literal starting at position "+pos);
				}
				pos=endLiteral;
				break;				
			}
			pos++;
		}
		if (!stack.isEmpty()) {
			Bracket p = stack.pop();
			throw new ParseException(expressionString, p.pos, "Missing closing '"+Bracket.theCloseBracketFor(p.bracket)+"' for '"+p.bracket+"' at position "+p.pos);
		}
		if (!isSuffixHere(expressionString, pos, suffix)) {
			return -1;
		}
		return pos;		
	}

	/**
	 * This captures a type of bracket and the position in which it occurs in the expression.  The positional
	 * information is used if an error has to be reported because the related end bracket cannot be found.
	 * Bracket is used to describe: square brackets [] round brackets () and curly brackets {}
	 */
	private static class Bracket {
		
		char bracket;
		int pos;
		
		Bracket(char bracket,int pos) {
			this.bracket = bracket;
			this.pos = pos;
		}

		boolean compatibleWithCloseBracket(char closeBracket) {
			if (bracket=='{') { 
				return closeBracket=='}';
			} else if (bracket=='[') {
				return closeBracket==']';
			}
			return closeBracket==')';
		}
		
		static char theOpenBracketFor(char closeBracket) {
			if (closeBracket=='}') { 
				return '{';
			} else if (closeBracket==']') {
				return '[';
			}
			return '(';
		}

		static char theCloseBracketFor(char openBracket) {
			if (openBracket=='{') { 
				return '}';
			} else if (openBracket=='[') {
				return ']';
			}
			return ')';
		}
	}

	/**
	 * Actually parse the expression string and return an Expression object.
	 * @param expressionString the raw expression string to parse
	 * @param context a context for influencing this expression parsing routine (optional)
	 * @return an evaluator for the parsed expression
	 * @throws ParseException an exception occurred during parsing
	 */
	protected abstract Expression doParseExpression(String expressionString, ParserContext context)
			throws ParseException;

}
