/*
* Copyright 2012 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.
*/
namespace ZXing.PDF417.Internal.EC
{
/// <summary>
/// <p>PDF417 error correction implementation.</p>
/// <p>This <a href="http://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction#Example">example
/// is quite useful in understanding the algorithm.</p>
/// <author>Sean Owen</author>
/// <see cref="ZXing.Common.ReedSolomon.ReedSolomonDecoder" />
/// </summary>
public sealed class ErrorCorrection
{
private readonly ModulusGF field;
/// <summary>
/// Initializes a new instance of the <see cref="ErrorCorrection"/> class.
/// </summary>
public ErrorCorrection()
{
this.field = ModulusGF.PDF417_GF;
}
/// <summary>
/// Decodes the specified received.
/// </summary>
/// <param name="received">received codewords</param>
/// <param name="numECCodewords">number of those codewords used for EC</param>
/// <param name="erasures">location of erasures</param>
/// <param name="errorLocationsCount">The error locations count.</param>
/// <returns></returns>
public bool decode(int[] received, int numECCodewords, int[] erasures, out int errorLocationsCount)
{
ModulusPoly poly = new ModulusPoly(field, received);
int[] S = new int[numECCodewords];
bool error = false;
errorLocationsCount = 0;
for (int i = numECCodewords; i > 0; i--)
{
int eval = poly.evaluateAt(field.exp(i));
S[numECCodewords - i] = eval;
if (eval != 0)
{
error = true;
}
}
if (!error)
{
return true;
}
ModulusPoly knownErrors = field.One;
if (erasures != null)
{
foreach (int erasure in erasures)
{
int b = field.exp(received.Length - 1 - erasure);
// Add (1 - bx) term:
ModulusPoly term = new ModulusPoly(field, new int[] {field.subtract(0, b), 1});
knownErrors = knownErrors.multiply(term);
}
}
ModulusPoly syndrome = new ModulusPoly(field, S);
//syndrome = syndrome.multiply(knownErrors);
ModulusPoly[] sigmaOmega = runEuclideanAlgorithm(field.buildMonomial(numECCodewords, 1), syndrome, numECCodewords);
if (sigmaOmega == null)
{
return false;
}
ModulusPoly sigma = sigmaOmega[0];
ModulusPoly omega = sigmaOmega[1];
if (sigma == null || omega == null)
{
return false;
}
//sigma = sigma.multiply(knownErrors);
int[] errorLocations = findErrorLocations(sigma);
if (errorLocations == null)
{
return false;
}
int[] errorMagnitudes = findErrorMagnitudes(omega, sigma, errorLocations);
for (int i = 0; i < errorLocations.Length; i++)
{
int position = received.Length - 1 - field.log(errorLocations[i]);
if (position < 0)
{
return false;
}
received[position] = field.subtract(received[position], errorMagnitudes[i]);
}
errorLocationsCount = errorLocations.Length;
return true;
}
/// <summary>
/// Runs the euclidean algorithm (Greatest Common Divisor) until r's degree is less than R/2
/// </summary>
/// <returns>The euclidean algorithm.</returns>
private ModulusPoly[] runEuclideanAlgorithm(ModulusPoly a, ModulusPoly b, int R)
{
// Assume a's degree is >= b's
if (a.Degree < b.Degree)
{
ModulusPoly temp = a;
a = b;
b = temp;
}
ModulusPoly rLast = a;
ModulusPoly r = b;
ModulusPoly tLast = field.Zero;
ModulusPoly t = field.One;
// Run Euclidean algorithm until r's degree is less than R/2
while (r.Degree >= R / 2)
{
ModulusPoly rLastLast = rLast;
ModulusPoly tLastLast = tLast;
rLast = r;
tLast = t;
// Divide rLastLast by rLast, with quotient in q and remainder in r
if (rLast.isZero)
{
// Oops, Euclidean algorithm already terminated?
return null;
}
r = rLastLast;
ModulusPoly q = field.Zero;
int denominatorLeadingTerm = rLast.getCoefficient(rLast.Degree);
int dltInverse = field.inverse(denominatorLeadingTerm);
while (r.Degree >= rLast.Degree && !r.isZero)
{
int degreeDiff = r.Degree - rLast.Degree;
int scale = field.multiply(r.getCoefficient(r.Degree), dltInverse);
q = q.add(field.buildMonomial(degreeDiff, scale));
r = r.subtract(rLast.multiplyByMonomial(degreeDiff, scale));
}
t = q.multiply(tLast).subtract(tLastLast).getNegative();
}
int sigmaTildeAtZero = t.getCoefficient(0);
if (sigmaTildeAtZero == 0)
{
return null;
}
int inverse = field.inverse(sigmaTildeAtZero);
ModulusPoly sigma = t.multiply(inverse);
ModulusPoly omega = r.multiply(inverse);
return new ModulusPoly[] { sigma, omega };
}
/// <summary>
/// Finds the error locations as a direct application of Chien's search
/// </summary>
/// <returns>The error locations.</returns>
/// <param name="errorLocator">Error locator.</param>
private int[] findErrorLocations(ModulusPoly errorLocator)
{
// This is a direct application of Chien's search
int numErrors = errorLocator.Degree;
int[] result = new int[numErrors];
int e = 0;
for (int i = 1; i < field.Size && e < numErrors; i++)
{
if (errorLocator.evaluateAt(i) == 0)
{
result[e] = field.inverse(i);
e++;
}
}
if (e != numErrors)
{
return null;
}
return result;
}
/// <summary>
/// Finds the error magnitudes by directly applying Forney's Formula
/// </summary>
/// <returns>The error magnitudes.</returns>
/// <param name="errorEvaluator">Error evaluator.</param>
/// <param name="errorLocator">Error locator.</param>
/// <param name="errorLocations">Error locations.</param>
private int[] findErrorMagnitudes(ModulusPoly errorEvaluator,
ModulusPoly errorLocator,
int[] errorLocations)
{
int errorLocatorDegree = errorLocator.Degree;
int[] formalDerivativeCoefficients = new int[errorLocatorDegree];
for (int i = 1; i <= errorLocatorDegree; i++)
{
formalDerivativeCoefficients[errorLocatorDegree - i] =
field.multiply(i, errorLocator.getCoefficient(i));
}
ModulusPoly formalDerivative = new ModulusPoly(field, formalDerivativeCoefficients);
// This is directly applying Forney's Formula
int s = errorLocations.Length;
int[] result = new int[s];
for (int i = 0; i < s; i++)
{
int xiInverse = field.inverse(errorLocations[i]);
int numerator = field.subtract(0, errorEvaluator.evaluateAt(xiInverse));
int denominator = field.inverse(formalDerivative.evaluateAt(xiInverse));
result[i] = field.multiply(numerator, denominator);
}
return result;
}
}
}
