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Optimized 0- and 1-error case of Reed Solomon decoding a bit

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git-svn-id: https://zxing.googlecode.com/svn/trunk@309 59b500cc-1b3d-0410-9834-0bbf25fbcc57
This commit is contained in:
srowen 2008-03-24 20:02:06 +00:00
parent b88df41dca
commit 3a1e3dc156
2 changed files with 42 additions and 25 deletions
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57
core/src/com/google/zxing/common/reedsolomon/ReedSolomonDecoder.java
View file

@ -16,8 +16,6 @@
package com.google.zxing.common.reedsolomon;
import java.util.Vector;
/**
* <p>Implements Reed-Solomon decoding, as the name implies.</p>
*
@ -59,19 +57,25 @@ public final class ReedSolomonDecoder {
public void decode(int[] received, int twoS) throws ReedSolomonException {
GF256Poly poly = new GF256Poly(field, received);
int[] syndromeCoefficients = new int[twoS];
boolean noError = true;
for (int i = 0; i < twoS; i++) {
syndromeCoefficients[syndromeCoefficients.length - 1 - i] = poly.evaluateAt(field.exp(i));
int eval = poly.evaluateAt(field.exp(i));
syndromeCoefficients[syndromeCoefficients.length - 1 - i] = eval;
if (eval != 0) {
noError = false;
}
}
if (noError) {
return;
}
GF256Poly syndrome = new GF256Poly(field, syndromeCoefficients);
if (!syndrome.isZero()) { // Error
GF256Poly[] sigmaOmega =
runEuclideanAlgorithm(field.buildMonomial(twoS, 1), syndrome, twoS);
int[] errorLocations = findErrorLocations(sigmaOmega[0]);
int[] errorMagnitudes = findErrorMagnitudes(sigmaOmega[1], errorLocations);
for (int i = 0; i < errorLocations.length; i++) {
int position = received.length - 1 - field.log(errorLocations[i]);
received[position] = GF256.addOrSubtract(received[position], errorMagnitudes[i]);
}
GF256Poly[] sigmaOmega =
runEuclideanAlgorithm(field.buildMonomial(twoS, 1), syndrome, twoS);
int[] errorLocations = findErrorLocations(sigmaOmega[0]);
int[] errorMagnitudes = findErrorMagnitudes(sigmaOmega[1], errorLocations);
for (int i = 0; i < errorLocations.length; i++) {
int position = received.length - 1 - field.log(errorLocations[i]);
received[position] = GF256.addOrSubtract(received[position], errorMagnitudes[i]);
}
}
@ -131,31 +135,34 @@ public final class ReedSolomonDecoder {
return new GF256Poly[]{sigma, omega};
}
private int[] findErrorLocations(GF256Poly errorLocator)
throws ReedSolomonException {
private int[] findErrorLocations(GF256Poly errorLocator) throws ReedSolomonException {
// This is a direct application of Chien's search
Vector errorLocations = new Vector(3);
for (int i = 1; i < 256; i++) {
int numErrors = errorLocator.getDegree();
if (numErrors == 1) { // shortcut
return new int[] { errorLocator.getCoefficient(1) };
}
int[] result = new int[numErrors];
int e = 0;
for (int i = 1; i < 256 && e < numErrors; i++) {
if (errorLocator.evaluateAt(i) == 0) {
errorLocations.addElement(new Integer(field.inverse(i)));
result[e] = field.inverse(i);
e++;
}
}
if (errorLocations.size() != errorLocator.getDegree()) {
if (e != numErrors) {
throw new ReedSolomonException("Error locator degree does not match number of roots");
}
int[] result = new int[errorLocations.size()]; // Can't use toArray() here
for (int i = 0; i < result.length; i++) {
result[i] = ((Integer) errorLocations.elementAt(i)).intValue();
}
return result;
}
private int[] findErrorMagnitudes(GF256Poly errorEvaluator,
int[] errorLocations) {
private int[] findErrorMagnitudes(GF256Poly errorEvaluator, int[] errorLocations) {
// This is directly applying Forney's Formula
int s = errorLocations.length;
if (s == 1) { // shortcut
return new int[] { errorEvaluator.getCoefficient(0) };
}
int[] result = new int[s];
for (int i = 0; i < errorLocations.length; i++) {
for (int i = 0; i < s; i++) {
int xiInverse = field.inverse(errorLocations[i]);
int denominator = 1;
for (int j = 0; j < s; j++) {

10
core/test/src/com/google/zxing/common/reedsolomon/ReedSolomonDecoderTestCase.java
View file

@ -45,6 +45,16 @@ public final class ReedSolomonDecoderTestCase extends TestCase {
checkQRRSDecode(received);
}
public void testOneError() throws ReedSolomonException {
int[] received = new int[QR_CODE_TEST_WITH_EC.length];
Random random = new Random(0xDEADBEEFL);
for (int i = 0; i < received.length; i++) {
System.arraycopy(QR_CODE_TEST_WITH_EC, 0, received, 0, received.length);
received[i] = random.nextInt(256);
checkQRRSDecode(received);
}
}
public void testMaxErrors() throws ReedSolomonException {
int[] received = new int[QR_CODE_TEST_WITH_EC.length];
Random random = new Random(0xDEADBEEFL);