package org.wikimedia.search.extra.superdetectnoop;
   
   import static java.lang.Math.abs;
   import static org.wikimedia.search.extra.superdetectnoop.ChangeHandler.TypeSafe.nullAndTypeSafe;
   
   import javax.annotation.Nonnull;
   
   /**
    * Checks if a number is different by some percentage.
   */
  public class WithinPercentageHandler implements ChangeHandler.NonnullChangeHandler<Number> {
      public static class Recognizer implements ChangeHandler.Recognizer {
          private static final String PREFIX = "within ";
          private static final String SUFFIX = "%";
  
          @Override
          public ChangeHandler<Object> build(String description) {
              if (!description.startsWith(PREFIX)) {
                  return null;
              }
              if (!description.endsWith(SUFFIX)) {
                  return null;
              }
              try {
                  double percentage = Double.parseDouble(description.substring(PREFIX.length(), description.length() - SUFFIX.length()));
                  return nullAndTypeSafe(Number.class, new WithinPercentageHandler(percentage / 100));
              } catch (NumberFormatException e) {
                  // Not a valid number even with the % sign....
                  return null;
              }
          }
      }
  
      private final double absoluteDifference;
  
      public WithinPercentageHandler(double absoluteDifference) {
          this.absoluteDifference = absoluteDifference;
      }
  
      @Override
      public ChangeHandler.Result handle(@Nonnull Number oldValue, @Nonnull Number newValue) {
          if (oldValue.doubleValue() == 0) {
              return ChangeHandler.Changed.forBoolean(newValue.doubleValue() == 0, newValue);
          }
          return ChangeHandler.Changed.forBoolean(
                  abs((newValue.doubleValue() - oldValue.doubleValue()) / oldValue.doubleValue()) < absoluteDifference, newValue);
      }
  }