/*
 * Copyright 2011 Google Inc.
 * 
 * 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 com.google.gwt.safecss.shared;

import com.google.gwt.core.shared.GWT;
import com.google.gwt.thirdparty.guava.common.base.Preconditions;

import java.util.HashMap;
import java.util.Stack;

/**
 * SafeStyles utilities whose implementation differs between Development and
 * Production Mode.
 * 
 * <p>
 * This class has a super-source peer that provides the Production Mode
 * implementation.
 * 
 * <p>
 * Do not use this class - it is used for implementation only, and its methods
 * may change in the future.
 */
public class SafeStylesHostedModeUtils {

  /**
   * Name of system property that if set, enables checks in server-side code
   * (even if assertions are disabled).
   */
  public static final String FORCE_CHECK_VALID_STYLES =
      "com.google.gwt.safecss.ForceCheckValidStyles";

  private static boolean forceCheck;

  static {
    setForceCheckValidStyleFromProperty();
  }

  /**
   * Check if the specified style property name is valid.
   * 
   * <p>
   * NOTE: This method does <em>NOT</em> guarantee the safety of a style name.
   * It looks for common errors, but does not check for every possible error. It
   * is intended to help validate a string that the user has already asserted is
   * safe.
   * </p>
   * 
   * @param name the name to check
   * @return null if valid, an error string if not
   * @see <a
   *      href="http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">CSS
   *      2.1 identifiers</a>
   */
  // VisibleForTesting.
  public static String isValidStyleName(String name) {
    // Empty name.
    if (name == null || name.isEmpty()) {
      return "Style property names cannot be null or empty";
    }

    // Starts with a digit, or a hyphen followed by a digit.
    char firstReal = name.charAt(0);
    if (firstReal == '-' && name.length() > 0) {
      firstReal = name.charAt(1);
    }
    if (firstReal >= '0' && firstReal <= '9') {
      return "Style property names cannot start with a digit or a hyphen followed by a digit: "
          + name;
    }

    // Starts with double hyphen.
    if (name.startsWith("--")) {
      return "Style property names cannot start with a double hyphen: " + name;
    }

    /*
     * Technically an escaped colon/semi-colon (eg. "property\;name") is valid,
     * but in practice nobody uses it and its safer to exclude it.
     */
    if (name.indexOf(';') >= 0) {
      return "Style property names cannot contain a semi-colon: " + name;
    } else if (name.indexOf(':') >= 0) {
      return "Style property names cannot contain a colon: " + name;
    }

    return null;
  }

  /**
   * Check if the specified style property value is valid.
   * 
   * <p>
   * NOTE: This method does <em>NOT</em> guarantee the safety of a style value.
   * It looks for common errors, but does not check for every possible error. It
   * is intended to help validate a string that the user has already asserted is
   * safe.
   * </p>
   * 
   * @param value the value to check
   * @return null if valid, an error string if not
   * @see <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">CSS 2.1
   *      declarations and properties</a>
   */
  // VisibleForTesting.
  public static String isValidStyleValue(String value) {
    // Empty value.
    if (value == null || value.length() == 0) {
      return "Style property values cannot be null or empty";
    }

    /*
     * A value can contain any token, but parenthesis, brackets, braces, and
     * single/double quotes must be paired. Semi-colons are only allowed in
     * strings, such as in a url.
     */

    // Create a map of pairable 'open' characters to 'close' characters.
    final HashMap<Character, Character> pairs = new HashMap<Character, Character>();
    pairs.put('(', ')');
    pairs.put('[', ']');
    pairs.put('{', '}');

    // Iterate over the characters in the value.
    final Stack<Character> pairsStack = new Stack<Character>();
    final Stack<Integer> pairsPos = new Stack<Integer>();
    Character inQuote = null; // The current quote character.
    int inQuotePos = -1;
    boolean inUrl = false;
    boolean ignoreNext = false;
    for (int i = 0; i < value.length(); i++) {
      // This character was escaped.
      if (ignoreNext) {
        ignoreNext = false;
        continue;
      }

      char ch = value.charAt(i);
      if (ch == '\\') {
        // Ignore the character immediately after an escape character.
        ignoreNext = true;
        continue;
      }

      if (inUrl) {
        // Check for closing parenthesis.
        if (ch == ')') {
          // Close the URL.
          inUrl = false;
        } else if (ch == '(') {
          // Parenthesis must be escaped within a url.
          return "Unescaped parentheses within a url at index " + i + ": " + value;
        }

        /*
         * Almost all tokens are valid within a URL.
         * 
         * 
         * TODO(jlabanca): Check for unescaped quotes and whitespace in URLs.
         * Quotes and whitespace (other than the optional ones that wrap the
         * URL) should be escaped.
         */
        continue;
      }

      if (inQuote != null) {
        // Check for a matching end quote.
        if (ch == inQuote) {
          inQuote = null;
        }

        // Else - still in quote, all tokens valid.
        continue;
      }

      if (ch == '"' || ch == '\'') {
        // Found an open quote.
        inQuote = ch;
        inQuotePos = i;
      } else if ((ch == 'u' || ch == 'U') && value.length() >= i + 4
          && value.substring(i, i + 4).equalsIgnoreCase("url(")) {
        // Starting a URL.
        inUrl = true;
        i = i + 3; // Advance to the URL.
      } else if (pairs.containsKey(ch)) {
        // Opened a pairable.
        pairsStack.push(ch);
        pairsPos.push(i);
      } else if (pairs.values().contains(ch)) {
        // Closed a pairable.
        if (pairsStack.isEmpty() || pairs.get(pairsStack.pop()) != ch) {
          // Unmatched close token.
          return "Style property value contains unpaired '" + ch + "' at index " + i + ": " + value;
        }
        pairsPos.pop();
      } else if (ch == ';') {
        // Contains an unescaped semi-colon.
        return "Style property values cannot contain a semi-colon (except within quotes): " + value;
      } else if (ch == ':') {
        // Contains an unescaped colon.
        return "Style property values cannot contain a colon (except within quotes): " + value;
      }
    }

    // Unmatched open quote.
    if (inQuote != null) {
      return "Style property value contains unpaired open quote at index " + inQuotePos + ": "
          + value;
    }

    // Unterminated url.
    if (inUrl) {
      return "Style property value contains an unterminated url: " + value;
    }

    // Unmatched open pairable.
    if (!pairsStack.isEmpty()) {
      char openToken = pairsStack.pop();
      int index = pairsPos.pop();
      return "Style property value contains unpaired '" + openToken + "' at index " + index + ": "
          + value;
    }

    // Ends in an escape character,
    if (ignoreNext) {
      return "Style property values cannot end in an escape character: " + value;
    }

    return null;
  }

  /**
   * Checks if the provided string is a valid style property name.
   * 
   * @param name the style name
   * @see <a
   *      href="http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">CSS
   *      2.1 identifiers</a>
   */
  public static void maybeCheckValidStyleName(String name) {
    if (GWT.isClient() || forceCheck) {
      String errorText = isValidStyleName(name);
      Preconditions.checkArgument(errorText == null, errorText);
    } else {
      assert isValidStyleName(name) == null : isValidStyleName(name);
    }
  }

  /**
   * Checks if the provided string is a valid style property value.
   * 
   * @param value the style value
   * @see <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">CSS 2.1
   *      declarations and properties</a>
   */
  public static void maybeCheckValidStyleValue(String value) {
    if (GWT.isClient() || forceCheck) {
      String errorText = isValidStyleValue(value);
      Preconditions.checkArgument(errorText == null, errorText);
    } else {
      assert isValidStyleValue(value) == null : isValidStyleValue(value);
    }
  }

  /**
   * Sets a global flag that controls whether or not
   * {@link #maybeCheckValidStyleName(String)} and
   * {@link #maybeCheckValidStyleValue(String)} should perform their checks in a
   * server-side environment.
   * 
   * @param check if true, perform server-side checks.
   */
  public static void setForceCheckValidStyle(boolean check) {
    forceCheck = check;
  }

  /**
   * Sets a global flag that controls whether or not
   * {@link #maybeCheckValidStyleName(String)} and
   * {@link #maybeCheckValidStyleValue(String)} should perform their checks in a
   * server-side environment from the value of the
   * {@value #FORCE_CHECK_VALID_STYLES} property.
   */
  static void setForceCheckValidStyleFromProperty() {
    forceCheck = System.getProperty(FORCE_CHECK_VALID_STYLES) != null;
  }
}