/*
 * Copyright 2014 The Error Prone 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 com.google.errorprone.bugpatterns;

import static com.google.common.collect.Iterables.getOnlyElement;
import static com.google.errorprone.BugPattern.SeverityLevel.ERROR;
import static com.google.errorprone.matchers.Matchers.allOf;
import static com.google.errorprone.matchers.Matchers.anyOf;
import static com.google.errorprone.matchers.Matchers.isSubtypeOf;
import static com.google.errorprone.matchers.Matchers.kindIs;
import static com.google.errorprone.suppliers.Suppliers.ANNOTATION_TYPE;
import static com.sun.source.tree.Tree.Kind.CLASS;
import static com.sun.source.tree.Tree.Kind.ENUM;

import com.google.common.base.Verify;
import com.google.errorprone.BugPattern;
import com.google.errorprone.VisitorState;
import com.google.errorprone.bugpatterns.BugChecker.ClassTreeMatcher;
import com.google.errorprone.matchers.Description;
import com.google.errorprone.matchers.Matcher;
import com.google.errorprone.suppliers.Supplier;
import com.google.errorprone.util.ASTHelpers;
import com.sun.source.tree.ClassTree;
import com.sun.source.tree.Tree.Kind;
import com.sun.tools.javac.code.Flags;
import com.sun.tools.javac.code.Scope;
import com.sun.tools.javac.code.Symbol;
import com.sun.tools.javac.code.Symbol.MethodSymbol;
import com.sun.tools.javac.code.Type;
import com.sun.tools.javac.code.Types;
import com.sun.tools.javac.util.Name;
import java.lang.annotation.Annotation;
import java.util.function.Predicate;
import org.jspecify.annotations.Nullable;

/**
 * Checker that ensures implementations of {@link Annotation} override equals and hashCode.
 * Otherwise, the implementation inherits equals and hashCode from {@link Object}, and those do not
 * meet the contract specified by the {@link Annotation} interface.
 */
@BugPattern(
    summary =
        "Classes that implement Annotation must override equals and hashCode. Consider "
            + "using AutoAnnotation instead of implementing Annotation by hand.",
    severity = ERROR)
public class BadAnnotationImplementation extends BugChecker implements ClassTreeMatcher {

  private static final Matcher<ClassTree> CLASS_TREE_MATCHER =
      allOf(anyOf(kindIs(CLASS), kindIs(ENUM)), isSubtypeOf(ANNOTATION_TYPE));

  @Override
  public Description matchClass(ClassTree classTree, VisitorState state) {
    if (!CLASS_TREE_MATCHER.matches(classTree, state)) {
      return Description.NO_MATCH;
    }

    // If this is an enum that is trying to implement Annotation, give a special error message.
    if (classTree.getKind() == Kind.ENUM) {
      return buildDescription(classTree)
          .setMessage(
              "Enums cannot correctly implement Annotation because their equals and hashCode "
                  + "methods are final. Consider using AutoAnnotation instead of implementing "
                  + "Annotation by hand.")
          .build();
    }

    // Otherwise walk up type hierarchy looking for equals and hashcode methods
    MethodSymbol equals = null;
    MethodSymbol hashCode = null;
    Types types = state.getTypes();
    Name equalsName = EQUALS.get(state);
    Predicate<MethodSymbol> equalsPredicate =
        methodSymbol ->
            !methodSymbol.isStatic()
                && ((methodSymbol.flags() & Flags.SYNTHETIC) == 0)
                && ((methodSymbol.flags() & Flags.ABSTRACT) == 0)
                && methodSymbol.getParameters().size() == 1
                && types.isSameType(
                    getOnlyElement(methodSymbol.getParameters()).type,
                    state.getSymtab().objectType);
    Name hashCodeName = HASHCODE.get(state);
    Predicate<MethodSymbol> hashCodePredicate =
        methodSymbol ->
            !methodSymbol.isStatic()
                && ((methodSymbol.flags() & Flags.SYNTHETIC) == 0)
                && ((methodSymbol.flags() & Flags.ABSTRACT) == 0)
                && methodSymbol.getParameters().isEmpty();

    for (Type sup : types.closure(ASTHelpers.getSymbol(classTree).type)) {
      if (equals == null) {
        equals = getMatchingMethod(sup, equalsName, equalsPredicate);
      }
      if (hashCode == null) {
        hashCode = getMatchingMethod(sup, hashCodeName, hashCodePredicate);
      }
    }
    Verify.verifyNotNull(equals);
    Verify.verifyNotNull(hashCode);

    Symbol objectSymbol = state.getSymtab().objectType.tsym;
    if (equals.owner.equals(objectSymbol) || hashCode.owner.equals(objectSymbol)) {
      return describeMatch(classTree);
    }

    return Description.NO_MATCH;
  }

  private static @Nullable MethodSymbol getMatchingMethod(
      Type type, Name name, Predicate<MethodSymbol> predicate) {
    Scope scope = type.tsym.members();
    for (Symbol sym : scope.getSymbolsByName(name)) {
      if (!(sym instanceof MethodSymbol methodSymbol)) {
        continue;
      }
      if (predicate.test(methodSymbol)) {
        return methodSymbol;
      }
    }
    return null;
  }

  private static final Supplier<Name> EQUALS =
      VisitorState.memoize(state -> state.getName("equals"));

  private static final Supplier<Name> HASHCODE =
      VisitorState.memoize(state -> state.getName("hashCode"));
}
