/*
 * 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
 *
 *      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.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;

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

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, Juergen Hoeller, Andy Clement
Since:
3.0/**
 * 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 {

	@Override
	public Expression parseExpression(String expressionString) throws ParseException {
		return parseExpression(expressionString, null);
	}

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


	private Expression parseTemplate(String expressionString, ParserContext context) throws ParseException {
		if (expressionString.isEmpty()) {
			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 ${..} as part of their
functionality are supported without any problem. The parsing is aware of the
structure of an embedded expression. It assumes that parentheses '(', square
brackets '[' and curly brackets '}' must be in pairs within the expression unless
they are within a string literal and a string literal starts and terminates with a
single quote '.
Params:
expressionString – the expression string
Throws:
ParseException – when the expressions cannot be parsed
Returns:
the parsed expressions/**
	 * Helper that parses given expression string using the configured parser. The
	 * expression string can contain any number of expressions all contained in "${...}"
	 * 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 ArrayList<>();
		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(new LiteralExpression(expressionString.substring(startIdx, prefixIndex)));
				}
				int afterPrefixIndex = prefixIndex + prefix.length();
				int suffixIndex = skipToCorrectEndSuffix(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);
				}
				String expr = expressionString.substring(prefixIndex + prefix.length(), suffixIndex);
				expr = expr.trim();
				if (expr.isEmpty()) {
					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(new LiteralExpression(expressionString.substring(startIdx)));
				startIdx = expressionString.length();
			}
		}

		return expressions.toArray(new Expression[0]);
	}

	
Return true if the specified suffix can be found at the supplied position in the
supplied expression string.
Params:
expressionString – the expression string which may contain the suffix
pos – the start position at which to check for the suffix
suffix – the suffix string/**
	 * 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
	 */
	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
${ is the final }.
Params:
suffix – the suffix
expressionString – the expression string
afterPrefixIndex – the most recently found prefix location for which the
matching end suffix is being sought
Returns:
the position of the correct matching nextSuffix or -1 if none can be found/**
	 * Copes with nesting, for example '${...${...}}' where the correct end for the first
	 * ${ is the final }.
	 * @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 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
		}
		Deque<Bracket> stack = new ArrayDeque<>();
		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;
	}


	
Actually parse the expression string and return an Expression object.
Params:
expressionString – the raw expression string to parse
context – a context for influencing this expression parsing routine (optional)
Throws:
ParseException – an exception occurred during parsing
Returns:
an evaluator for the parsed expression/**
	 * 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, @Nullable ParserContext context)
			throws ParseException;


	
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 {}
/**
	 * 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 (this.bracket == '{') {
				return closeBracket == '}';
			}
			else if (this.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 ')';
		}
	}

}