/*
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * 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 com.google.errorprone.BugPattern;
import com.google.errorprone.BugPattern.Category;
import com.google.errorprone.BugPattern.SeverityLevel;
import com.google.errorprone.VisitorState;
import com.google.errorprone.bugpatterns.BugChecker.BinaryTreeMatcher;
import com.google.errorprone.fixes.SuggestedFix;
import com.google.errorprone.matchers.Description;
import com.google.errorprone.matchers.Matcher;
import com.google.errorprone.matchers.Matchers;
import com.google.errorprone.util.ASTHelpers;
import com.sun.source.tree.BinaryTree;
import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.MethodInvocationTree;
import com.sun.source.tree.Tree.Kind;

/** @author Louis Wasserman */
@BugPattern(
  name = "RandomModInteger",
  summary = "Use Random.nextInt(int).  Random.nextInt() % n can have negative results",
  explanation =
      "`Random.nextInt() % n` has \n\n"
          + "* a 1/n chance of being 0\n"
          + "* a 1/2n chance of being each number from `1` to `n-1` inclusive\n"
          + "* a 1/2n chance of being each number from `-1` to `-(n-1)` inclusive\n\n"
          + "Many users expect a uniformly distributed random integer between `0` and `n-1` "
          + "inclusive, but you must use random.nextInt(n) to get that behavior.  If the original "
          + "behavior is truly desired, use `(random.nextBoolean() ? 1 : -1) * random.nextInt(n)`.",
  severity = SeverityLevel.ERROR,
  category = Category.JDK
)
public class RandomModInteger extends BugChecker implements BinaryTreeMatcher {

  private static final Matcher<ExpressionTree> RANDOM_NEXT_INT =
      Matchers.instanceMethod().onDescendantOf("java.util.Random").withSignature("nextInt()");

  @Override
  public Description matchBinary(BinaryTree tree, VisitorState state) {
    if (tree.getKind() == Kind.REMAINDER
        && tree.getLeftOperand() instanceof MethodInvocationTree
        && RANDOM_NEXT_INT.matches(tree.getLeftOperand(), state)) {
      ExpressionTree randomExpr = ASTHelpers.getReceiver(tree.getLeftOperand());
      ExpressionTree modulus = tree.getRightOperand();
      return describeMatch(
          tree,
          SuggestedFix.replace(
              tree,
              String.format(
                  "%s.nextInt(%s)",
                  state.getSourceForNode(randomExpr), state.getSourceForNode(modulus))));
    }
    return Description.NO_MATCH;
  }
}