DataMask.java

/*
 * Copyright 2007 ZXing 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.zxing.qrcode.decoder;

import com.google.zxing.common.BitMatrix;

/**
 * <p>Encapsulates data masks for the data bits in a QR code, per ISO 18004:2006 6.8. Implementations
 * of this class can un-mask a raw BitMatrix. For simplicity, they will unmask the entire BitMatrix,
 * including areas used for finder patterns, timing patterns, etc. These areas should be unused
 * after the point they are unmasked anyway.</p>
 *
 * <p>Note that the diagram in section 6.8.1 is misleading since it indicates that i is column position
 * and j is row position. In fact, as the text says, i is row position and j is column position.</p>
 *
 * @author Sean Owen
 */
abstract class DataMask {

  /**
   * See ISO 18004:2006 6.8.1
   */
  private static final DataMask[] DATA_MASKS = {
      new DataMask000(),
      new DataMask001(),
      new DataMask010(),
      new DataMask011(),
      new DataMask100(),
      new DataMask101(),
      new DataMask110(),
      new DataMask111(),
  };

  private DataMask() {
  }

  /**
   * <p>Implementations of this method reverse the data masking process applied to a QR Code and
   * make its bits ready to read.</p>
   *
   * @param bits representation of QR Code bits
   * @param dimension dimension of QR Code, represented by bits, being unmasked
   */
  final void unmaskBitMatrix(BitMatrix bits, int dimension) {
    for (int i = 0; i < dimension; i++) {
      for (int j = 0; j < dimension; j++) {
        if (isMasked(i, j)) {
          bits.flip(j, i);
        }
      }
    }
  }

  abstract boolean isMasked(int i, int j);

  /**
   * @param reference a value between 0 and 7 indicating one of the eight possible
   * data mask patterns a QR Code may use
   * @return DataMask encapsulating the data mask pattern
   */
  static DataMask forReference(int reference) {
    if (reference < 0 || reference > 7) {
      throw new IllegalArgumentException();
    }
    return DATA_MASKS[reference];
  }

  /**
   * 000: mask bits for which (x + y) mod 2 == 0
   */
  private static final class DataMask000 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return ((i + j) & 0x01) == 0;
    }
  }

  /**
   * 001: mask bits for which x mod 2 == 0
   */
  private static final class DataMask001 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return (i & 0x01) == 0;
    }
  }

  /**
   * 010: mask bits for which y mod 3 == 0
   */
  private static final class DataMask010 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return j % 3 == 0;
    }
  }

  /**
   * 011: mask bits for which (x + y) mod 3 == 0
   */
  private static final class DataMask011 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return (i + j) % 3 == 0;
    }
  }

  /**
   * 100: mask bits for which (x/2 + y/3) mod 2 == 0
   */
  private static final class DataMask100 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return (((i >>> 1) + (j /3)) & 0x01) == 0;
    }
  }

  /**
   * 101: mask bits for which xy mod 2 + xy mod 3 == 0
   */
  private static final class DataMask101 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      int temp = i * j;
      return (temp & 0x01) + (temp % 3) == 0;
    }
  }

  /**
   * 110: mask bits for which (xy mod 2 + xy mod 3) mod 2 == 0
   */
  private static final class DataMask110 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      int temp = i * j;
      return (((temp & 0x01) + (temp % 3)) & 0x01) == 0;
    }
  }

  /**
   * 111: mask bits for which ((x+y)mod 2 + xy mod 3) mod 2 == 0
   */
  private static final class DataMask111 extends DataMask {
    @Override
    boolean isMasked(int i, int j) {
      return ((((i + j) & 0x01) + ((i * j) % 3)) & 0x01) == 0;
    }
  }
}