Updates from sanfordsquires to fix RS decoding for Datamatrix

git-svn-id: https://zxing.googlecode.com/svn/trunk@688 59b500cc-1b3d-0410-9834-0bbf25fbcc57

AUTHORS

@@ -12,5 +12,6 @@ Joseph Wain (Google)
 Matthew Schulkind (Google)
 Matt York (LifeMarks)
 Paul Hackenberger
+sanfordsquires (?)
 Sean Owen (Google)
 Vince Francis (LifeMarks)

core/src/com/google/zxing/common/reedsolomon/ReedSolomonDecoder.java

@@ -36,6 +36,7 @@ package com.google.zxing.common.reedsolomon;
  *
  * @author srowen@google.com (Sean Owen)
  * @author William Rucklidge
+ * @author sanfordsquires
  */
 public final class ReedSolomonDecoder {
 
@@ -52,16 +53,15 @@ public final class ReedSolomonDecoder {
    *
    * @param received data and error-correction codewords
    * @param twoS number of error-correction codewords available
-   * @param dataMatrix if true, then uses a calculation that matches the Data Matrix
-   *  standard rather than the one used in QR Code
    * @throws ReedSolomonException if decoding fails for any reason
    */
-  public void decode(int[] received, int twoS, boolean dataMatrix) throws ReedSolomonException {
+  public void decode(int[] received, int twoS) throws ReedSolomonException {
     GF256Poly poly = new GF256Poly(field, received);
     int[] syndromeCoefficients = new int[twoS];
+    boolean dataMatrix = field.equals(GF256.DATA_MATRIX_FIELD);
     boolean noError = true;
     for (int i = 0; i < twoS; i++) {
-      // This difference in syndrome calculation appears to be correct, but then causes issues below
+      // Thanks to sanfordsquires for this fix:
       int eval = poly.evaluateAt(field.exp(dataMatrix ? i + 1 : i));
       syndromeCoefficients[syndromeCoefficients.length - 1 - i] = eval;
       if (eval != 0) {
@@ -72,19 +72,17 @@ public final class ReedSolomonDecoder {
       return;
     }
     GF256Poly syndrome = new GF256Poly(field, syndromeCoefficients);
-    if (dataMatrix) {
-      // TODO Not clear this is correct for DataMatrix, but it gives almost-correct behavior;
-      // works except when number of errors is the maximum allowable.
-      syndrome = syndrome.multiply(field.buildMonomial(1, 1));
-    }
     GF256Poly[] sigmaOmega =
         runEuclideanAlgorithm(field.buildMonomial(twoS, 1), syndrome, twoS);
     GF256Poly sigma = sigmaOmega[0];
     GF256Poly omega = sigmaOmega[1];
     int[] errorLocations = findErrorLocations(sigma);
-    int[] errorMagnitudes = findErrorMagnitudes(omega, errorLocations);
+    int[] errorMagnitudes = findErrorMagnitudes(omega, errorLocations, dataMatrix);
     for (int i = 0; i < errorLocations.length; i++) {
       int position = received.length - 1 - field.log(errorLocations[i]);
+      if (position < 0) {
+        throw new ReedSolomonException("Bad error location");
+      }
       received[position] = GF256.addOrSubtract(received[position], errorMagnitudes[i]);
     }
   }
@@ -165,7 +163,7 @@ public final class ReedSolomonDecoder {
     return result;
   }
 
-  private int[] findErrorMagnitudes(GF256Poly errorEvaluator, int[] errorLocations) {
+  private int[] findErrorMagnitudes(GF256Poly errorEvaluator, int[] errorLocations, boolean dataMatrix) {
     // This is directly applying Forney's Formula
     int s = errorLocations.length;
     int[] result = new int[s];
@@ -180,6 +178,10 @@ public final class ReedSolomonDecoder {
       }
       result[i] = field.multiply(errorEvaluator.evaluateAt(xiInverse),
           field.inverse(denominator));
+      // Thanks to sanfordsquires for this fix:
+      if (dataMatrix) {
+        result[i] = field.multiply(result[i], xiInverse);
+      }
     }
     return result;
   }

core/src/com/google/zxing/datamatrix/decoder/Decoder.java

@@ -118,7 +118,7 @@ public final class Decoder {
     }
     int numECCodewords = codewordBytes.length - numDataCodewords;
     try {
-      rsDecoder.decode(codewordsInts, numECCodewords, true);
+      rsDecoder.decode(codewordsInts, numECCodewords);
     } catch (ReedSolomonException rse) {
       throw new ReaderException(rse.toString());
     }

core/src/com/google/zxing/qrcode/decoder/Decoder.java

@@ -118,7 +118,7 @@ public final class Decoder {
     }
     int numECCodewords = codewordBytes.length - numDataCodewords;
     try {
-      rsDecoder.decode(codewordsInts, numECCodewords, false);
+      rsDecoder.decode(codewordsInts, numECCodewords);
     } catch (ReedSolomonException rse) {
       throw new ReaderException(rse.toString());
     }

core/test/src/com/google/zxing/common/reedsolomon/AbstractReedSolomonTestCase.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2008 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.common.reedsolomon;
+
+import junit.framework.TestCase;
+
+import java.util.BitSet;
+import java.util.Random;
+
+/**
+ * @author srowen@google.com (Sean Owen)
+ */
+abstract class AbstractReedSolomonTestCase extends TestCase {
+
+  static void corrupt(int[] received, int howMany, Random random) {
+    BitSet corrupted = new BitSet(received.length);
+    for (int j = 0; j < howMany; j++) {
+      int location = random.nextInt(received.length);
+      if (corrupted.get(location)) {
+        j--;
+      } else {
+        corrupted.set(location);
+        received[location] = (received[location] + 1 + random.nextInt(255)) & 0xFF;
+      }
+    }
+  }
+
+}

core/test/src/com/google/zxing/common/reedsolomon/ReedSolomonDecoderDataMatrixTestCase.java

@@ -16,15 +16,13 @@
 
 package com.google.zxing.common.reedsolomon;
 
-import junit.framework.TestCase;
-
-import java.util.BitSet;
 import java.util.Random;
 
 /**
  * @author srowen@google.com (Sean Owen)
+ * @author sanfordsquires
  */
-public final class ReedSolomonDecoderDataMatrixTestCase extends TestCase {
+public final class ReedSolomonDecoderDataMatrixTestCase extends AbstractReedSolomonTestCase {
 
   private static final int[] DM_CODE_TEST = { 142, 164, 186 };
   private static final int[] DM_CODE_TEST_WITH_EC = { 142, 164, 186, 114, 25, 5, 88, 102 };
@@ -54,8 +52,7 @@ public final class ReedSolomonDecoderDataMatrixTestCase extends TestCase {
     Random random = new Random(0xDEADBEEFL);
     for (int i = 0; i < DM_CODE_TEST.length; i++) { // # iterations is kind of arbitrary
       System.arraycopy(DM_CODE_TEST_WITH_EC, 0, received, 0, received.length);
-      // TODO we aren't testing max errors really since there is a known bug here
+      corrupt(received, DM_CODE_CORRECTABLE, random);
-      corrupt(received, DM_CODE_CORRECTABLE - 1, random);
       checkQRRSDecode(received);
     }
   }
@@ -74,23 +71,10 @@ public final class ReedSolomonDecoderDataMatrixTestCase extends TestCase {
   }
 
   private void checkQRRSDecode(int[] received) throws ReedSolomonException {
-    dmRSDecoder.decode(received, 2 * DM_CODE_CORRECTABLE, true);
+    dmRSDecoder.decode(received, 2 * DM_CODE_CORRECTABLE);
     for (int i = 0; i < DM_CODE_TEST.length; i++) {
       assertEquals(received[i], DM_CODE_TEST[i]);
     }
   }
 
-  private static void corrupt(int[] received, int howMany, Random random) {
-    BitSet corrupted = new BitSet(received.length);
-    for (int j = 0; j < howMany; j++) {
-      int location = random.nextInt(received.length);
-      if (corrupted.get(location)) {
-        j--;
-      } else {
-        corrupted.set(location);
-        int newByte = random.nextInt(256);
-        received[location] = newByte;
-      }
-    }
-  }
 }

core/test/src/com/google/zxing/common/reedsolomon/ReedSolomonDecoderQRCodeTestCase.java

@@ -16,15 +16,12 @@
 
 package com.google.zxing.common.reedsolomon;
 
-import junit.framework.TestCase;
-
-import java.util.BitSet;
 import java.util.Random;
 
 /**
  * @author srowen@google.com (Sean Owen)
  */
-public final class ReedSolomonDecoderQRCodeTestCase extends TestCase {
+public final class ReedSolomonDecoderQRCodeTestCase extends AbstractReedSolomonTestCase {
 
   /** See ISO 18004, Appendix I, from which this example is taken. */
   private static final int[] QR_CODE_TEST =
@@ -79,23 +76,10 @@ public final class ReedSolomonDecoderQRCodeTestCase extends TestCase {
   }
 
   private void checkQRRSDecode(int[] received) throws ReedSolomonException {
-    qrRSDecoder.decode(received, 2*QR_CODE_CORRECTABLE, false);
+    qrRSDecoder.decode(received, 2*QR_CODE_CORRECTABLE);
     for (int i = 0; i < QR_CODE_TEST.length; i++) {
       assertEquals(received[i], QR_CODE_TEST[i]);
     }
   }
 
-  private static void corrupt(int[] received, int howMany, Random random) {
-    BitSet corrupted = new BitSet(received.length);
-    for (int j = 0; j < howMany; j++) {
-      int location = random.nextInt(received.length);
-      if (corrupted.get(location)) {
-        j--;
-      } else {
-        corrupted.set(location);
-        int newByte = random.nextInt(256);
-        received[location] = newByte;
-      }
-    }
-  }
 }