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A packaging of the IntelliJ Community Edition java-analysis-impl library. This is release number 1 of trunk branch 142.
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/*
 * Copyright 2000-2014 JetBrains s.r.o.
 *
 * 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.intellij.psi.util;

import com.intellij.psi.PsiElement;

public class PsiMatcherImpl implements PsiMatcher {
  private PsiElement myElement;

  public PsiMatcherImpl(PsiElement element) {
    myElement = element;
  }

  @Override
  public PsiMatcher parent(PsiMatcherExpression e) {
    myElement = myElement.getParent();
    if (myElement == null || e != null && e.match(myElement) != Boolean.TRUE) return NullPsiMatcherImpl.INSTANCE;
    return this;
  }

  @Override
  public PsiMatcher firstChild(PsiMatcherExpression e) {
    final PsiElement[] children = myElement.getChildren();
    for (PsiElement child : children) {
      myElement = child;
      if (e == null || e.match(myElement) == Boolean.TRUE) {
        return this;
      }
    }
    return NullPsiMatcherImpl.INSTANCE;
  }

  @Override
  public PsiMatcher ancestor(PsiMatcherExpression e) {
    while (myElement != null) {
      Boolean res = e == null ? Boolean.TRUE : e.match(myElement);
      if (res == Boolean.TRUE) break;
      if (res == null) return NullPsiMatcherImpl.INSTANCE;
      myElement = myElement.getParent();
    }
    if (myElement == null) return NullPsiMatcherImpl.INSTANCE;
    return this;
  }

  @Override
  public PsiMatcher descendant(PsiMatcherExpression e) {
    final PsiElement[] children = myElement.getChildren();
    for (PsiElement child : children) {
      myElement = child;
      final Boolean res = e == null ? Boolean.TRUE : e.match(myElement);
      if (res == Boolean.TRUE) {
        return this;
      }
      else if (res == Boolean.FALSE) {
        final PsiMatcher grandChild = descendant(e);
        if (grandChild != NullPsiMatcherImpl.INSTANCE) return grandChild;
      }
    }
    return NullPsiMatcherImpl.INSTANCE;
  }

  @Override
  public PsiMatcher dot(PsiMatcherExpression e) {
    return e == null || e.match(myElement) == Boolean.TRUE ? this : NullPsiMatcherImpl.INSTANCE;
  }


  @Override
  public PsiElement getElement() {
    return myElement;
  }

  private static class NullPsiMatcherImpl implements PsiMatcher {
    @Override
    public PsiMatcher parent(PsiMatcherExpression e) {
      return this;
    }

    @Override
    public PsiMatcher firstChild(PsiMatcherExpression e) {
      return this;
    }

    @Override
    public PsiMatcher ancestor(PsiMatcherExpression e) {
      return this;
    }

    @Override
    public PsiMatcher descendant(PsiMatcherExpression e) {
      return this;
    }

    @Override
    public PsiMatcher dot(PsiMatcherExpression e) {
      return this;
    }

    @Override
    public PsiElement getElement() {
      return null;
    }

    private static final NullPsiMatcherImpl INSTANCE = new NullPsiMatcherImpl();
  }
}