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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 2003-2010 Dave Griffith, Bas Leijdekkers
 *
 * 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.siyeh.ig.psiutils;

import com.intellij.psi.*;
import com.intellij.psi.search.searches.MethodReferencesSearch;
import com.intellij.util.Processor;
import com.intellij.util.Query;
import org.jetbrains.annotations.NotNull;

public class SingletonUtil {

  private SingletonUtil() {
  }

  public static boolean isSingleton(@NotNull PsiClass aClass) {
    if (aClass.isInterface() || aClass.isEnum() ||
        aClass.isAnnotationType()) {
      return false;
    }
    if (aClass instanceof PsiTypeParameter ||
        aClass instanceof PsiAnonymousClass) {
      return false;
    }
    final PsiMethod[] constructors = getIfOnlyInvisibleConstructors(aClass);
    if (constructors.length == 0) {
      return false;
    }
    final PsiField selfInstance = getIfOneStaticSelfInstance(aClass);
    if (selfInstance == null) {
      return false;
    }
    return newOnlyAssignsToStaticSelfInstance(constructors[0], selfInstance);
  }

  private static PsiField getIfOneStaticSelfInstance(PsiClass aClass) {
    final PsiField[] fields = aClass.getFields();
    PsiField result = null;
    for (final PsiField field : fields) {
      final String className = aClass.getQualifiedName();
      if (!field.hasModifierProperty(PsiModifier.STATIC)) {
        continue;
      }
      final PsiType type = field.getType();
      final String fieldTypeName = type.getCanonicalText();
      if (!fieldTypeName.equals(className)) {
        continue;
      }
      if (result != null) {
        return null;
      }
      result = field;
    }
    return result;
  }

  private static PsiMethod[] getIfOnlyInvisibleConstructors(PsiClass aClass) {
    final PsiMethod[] constructors = aClass.getConstructors();
    if (constructors.length == 0) {
      return PsiMethod.EMPTY_ARRAY;
    }
    for (final PsiMethod constructor : constructors) {
      if (constructor.hasModifierProperty(PsiModifier.PUBLIC)) {
        return PsiMethod.EMPTY_ARRAY;
      }
      if (!constructor.hasModifierProperty(PsiModifier.PRIVATE) &&
          !constructor.hasModifierProperty(PsiModifier.PROTECTED)) {
        return PsiMethod.EMPTY_ARRAY;
      }
    }
    return constructors;
  }

  private static boolean newOnlyAssignsToStaticSelfInstance(
    PsiMethod method, final PsiField field) {
    final Query search =
      MethodReferencesSearch.search(method, field.getUseScope(),
                                    false);
    final NewOnlyAssignedToFieldProcessor processor =
      new NewOnlyAssignedToFieldProcessor(field);
    search.forEach(processor);
    return processor.isNewOnlyAssignedToField();
  }

  private static class NewOnlyAssignedToFieldProcessor
    implements Processor {

    private boolean newOnlyAssignedToField = true;
    private final PsiField field;

    public NewOnlyAssignedToFieldProcessor(PsiField field) {
      this.field = field;
    }

    @Override
    public boolean process(PsiReference reference) {
      final PsiElement element = reference.getElement();
      final PsiElement parent = element.getParent();
      if (!(parent instanceof PsiNewExpression)) {
        newOnlyAssignedToField = false;
        return false;
      }
      final PsiElement grandParent = parent.getParent();
      if (field.equals(grandParent)) {
        return true;
      }
      if (!(grandParent instanceof PsiAssignmentExpression)) {
        newOnlyAssignedToField = false;
        return false;
      }
      final PsiAssignmentExpression assignmentExpression =
        (PsiAssignmentExpression)grandParent;
      final PsiExpression lhs = assignmentExpression.getLExpression();
      if (!(lhs instanceof PsiReferenceExpression)) {
        newOnlyAssignedToField = false;
        return false;
      }
      final PsiReferenceExpression referenceExpression =
        (PsiReferenceExpression)lhs;
      final PsiElement target = referenceExpression.resolve();
      if (!field.equals(target)) {
        newOnlyAssignedToField = false;
        return false;
      }
      return true;
    }

    public boolean isNewOnlyAssignedToField() {
      return newOnlyAssignedToField;
    }
  }
}