content: package act.controller.captcha.generator;
  
  /*-
   * #%L
   * ACT Framework
   * %%
   * Copyright (C) 2014 - 2018 ActFramework
   * %%
   * 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.
   * #L%
   */
  
  import act.act_messages;
  import act.controller.captcha.CaptchaSession;
  import act.controller.captcha.CaptchaSessionGenerator;
  import act.i18n.I18n;
  import org.osgl.$;
  import org.osgl.util.N;
  import org.osgl.util.S;
  
  /**
   * Generate {@link act.controller.captcha.CaptchaSession} with
   * a random arithmetic expression.
   */
  public class ArithmeticGenerator implements CaptchaSessionGenerator {
  
      private enum Operator {
          ADD() {
              @Override
              int apply(int left, int right) {
                  return left + right;
              }
  
              @Override
              public String toString() {
                  return "+";
              }
          },
          SUBSTRACT() {
              @Override
              int apply(int left, int right) {
                  return left - right;
              }
  
              @Override
              public String toString() {
                  return "-";
              }
          },
          TIMES() {
              @Override
              int apply(int left, int right) {
                  return left * right;
              }
  
              @Override
              public String toString() {
                  return "x";
              }
          };
  
          abstract int apply(int left, int right);
  
          public abstract String toString();
      }
  
      // a simple ternary arithmetic expression
      private static class ArithmeticExpression {
          int first = N.randInt(10);
          Operator o1 = $.random(Operator.class);
          int second = N.randInt(10);
          Operator o2 = $.random(Operator.class);
          int third = N.randInt(10);
  
          int evaluate() {
              if (o2 == Operator.TIMES) {
                  return o1.apply(first, o2.apply(second, third));
              } else {
                  return o2.apply(o1.apply(first, second), third);
              }
          }
  
          @Override
          public String toString() {
              return S.concat(first, o1, second, o2, third, " = ?");
          }
      }
  
      @Override
      public CaptchaSession generate() {
          ArithmeticExpression expression = new ArithmeticExpression();
          return new CaptchaSession(
                  expression.toString(), S.string(expression.evaluate()), null,
                  I18n.i18n(act_messages.class, "act.captcha.generator.arithmetic.instruction"),
                  null
          );
      }
  
  }
  