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Less compiler in Java, based on less.js
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/**
 * Copyright (c) 2014 SQUARESPACE, 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.squarespace.less.parse;

import static com.squarespace.less.parse.Parselets.DETACHED_RULESET;
import static com.squarespace.less.parse.Parselets.EXPRESSION_LIST;
import static com.squarespace.less.parse.Parselets.RULE_PROPERTY;

import com.squarespace.less.LessException;
import com.squarespace.less.core.CharClass;
import com.squarespace.less.core.Chars;
import com.squarespace.less.model.Anonymous;
import com.squarespace.less.model.Definition;
import com.squarespace.less.model.Node;
import com.squarespace.less.model.Property;
import com.squarespace.less.model.Rule;
import com.squarespace.less.model.Variable;


public class RuleParselet implements Parselet {

  @Override
  public Node parse(LessStream stm) throws LessException {
    if (!CharClass.ruleStart(stm.peek())) {
      return null;
    }

    Mark ruleMark = stm.mark();
    Node key = stm.parse(RULE_PROPERTY);
    if (key == null) {
      stm.restore(ruleMark);
      return null;
    }

    Node value = null;
    stm.skipWs();

    Mark valueMark = stm.mark();

    // The 'font' rule has a special shorthand syntax which resembles
    // dimension division, but must be left unevaluated. We switch into
    // strict math mode for the entire rule.
    if (key instanceof Property && ((Property)key).name().equals("font")) {
      stm.setRequireStrictMath(true);
    }

    // If we're creating a variable definition, try
    // to parse a detached ruleset.
    if (key instanceof Variable) {
      Variable var = (Variable)key;
      if (!var.curly()) {
        value = stm.parse(DETACHED_RULESET);
      }
    }

    // Fall back to parsing a normal expression list.
    if (value == null) {
      value = stm.parse(EXPRESSION_LIST);
    }

    stm.setRequireStrictMath(false);

    stm.skipWs();
    boolean important = important(stm);

    // If we didn't read a full rule, back up and try to parse an opaque value.
    if (!endPeek(stm)) {
      important = false;
      stm.restore(valueMark);
      if (stm.matchAnonRuleValue()) {
        value = new Anonymous(stm.token().trim());
      }
    } else if (value == null) {
      value = new Anonymous();
    }

    // Nothing was parsed or we failed to find the ending sequence.
    if (value == null || !end(stm)) {
      stm.restore(ruleMark);
      return null;
    }

    // If we parsed an AT-prefixed name, it may be a definition.
    if (key instanceof Variable) {
      Variable var = (Variable)key;
      if (!var.curly()) {
        Definition def = stm.context().nodeBuilder().buildDefinition(var.name(), value, important);
        def.fileName(stm.fileName());
        return def;
      }
    }

    // Rule whose property is either a Property or curly Variable.
    Rule rule = stm.context().nodeBuilder().buildRule(key, value, important);
    rule.fileName(stm.fileName());
    return rule;
  }

  private boolean important(LessStream stm) {
    return stm.peek() == Chars.EXCLAMATION_MARK && stm.matchImportant();
  }

  static boolean endPeek(LessStream stm) {
    stm.skipWs();
    char ch = stm.peek();
    return ch == Chars.SEMICOLON || ch == Chars.RIGHT_CURLY_BRACKET || ch == Chars.EOF;
  }

  static boolean end(LessStream stm) {
    stm.skipWs();
    switch (stm.peek()) {

      case Chars.SEMICOLON:
        stm.seek1();
        return true;

      case Chars.EOF:
      case Chars.RIGHT_CURLY_BRACKET:
        return true;

      default:
        break;
    }
    return false;
  }

}