void RSCoder::pnInit()
{
int p1[MAXPAR+1],p2[MAXPAR+1];
Clean(p2,ParSize);
p2[0]=1;
for (int I=1;I<=ParSize;I++)
{
Clean(p1,ParSize);
p1[0]=gfExp[I];
p1[1]=1;
pnMult(p1,p2,GXPol);
for (int J=0;J<ParSize;J++)
p2[J]=GXPol[J];
}
}
// Create the generator polynomial g(x).
// g(x)=(x-a)(x-a^2)(x-a^3)..(x-a^N)
void RSCoder::pnInit()
{
int p2[MAXPAR + 1]; // Currently calculated part of g(x).
Clean(p2, ParSize);
p2[0] = 1; // Set p2 polynomial to 1.
for (int I = 1; I <= ParSize; I++)
{
int p1[MAXPAR + 1]; // We use p1 as current (x+a^i) expression.
Clean(p1, ParSize);
p1[0] = gfExp[I];
p1[1] = 1; // Set p1 polynomial to x+a^i.
// Multiply the already calucated part of g(x) to next (x+a^i).
pnMult(p1, p2, GXPol);
// p2=g(x).
for (int J = 0; J < ParSize; J++)
p2[J] = GXPol[J];
}
}
bool RSCoder::Decode(byte *Data,int DataSize,int *EraLoc,int EraSize)
{
int SynData[MAXPOL]; // Syndrome data.
bool AllZeroes=true;
for (int I=0;I<ParSize;I++)
{
int Sum=0;
for (int J=0;J<DataSize;J++)
Sum=Data[J]^gfMult(gfExp[I+1],Sum);
if ((SynData[I]=Sum)!=0)
AllZeroes=false;
}
// If all syndrome numbers are zero, message does not have errors.
if (AllZeroes)
return(true);
if (!FirstBlockDone) // Do things which we need to do once for all data.
{
FirstBlockDone=true;
// Calculate the error locator polynomial.
Clean(ELPol,ParSize+1);
ELPol[0]=1;
for (int EraPos=0;EraPos<EraSize;EraPos++)
for (int I=ParSize,M=gfExp[DataSize-EraLoc[EraPos]-1];I>0;I--)
ELPol[I]^=gfMult(M,ELPol[I-1]);
ErrCount=0;
// Find roots of error locator polynomial.
for (int Root=MAXPAR-DataSize;Root<MAXPAR+1;Root++)
{
int Sum=0;
for (int B=0;B<ParSize+1;B++)
Sum^=gfMult(gfExp[(B*Root)%MAXPAR],ELPol[B]);
if (Sum==0) // Root found.
{
ErrorLocs[ErrCount]=MAXPAR-Root; // Location of error.
// Calculate the denominator for every error location.
Dnm[ErrCount]=0;
for (int I=1;I<ParSize+1;I+=2)
Dnm[ErrCount]^= gfMult(ELPol[I],gfExp[Root*(I-1)%MAXPAR]);
ErrCount++;
}
}
}
int EEPol[MAXPOL]; // Error Evaluator Polynomial.
pnMult(ELPol,SynData,EEPol);
// If errors are present and their number is correctable.
if ((ErrCount<=ParSize) && ErrCount>0)
for (int I=0;I<ErrCount;I++)
{
int Loc=ErrorLocs[I],DLoc=MAXPAR-Loc,N=0;
for (int J=0;J<ParSize;J++)
N^=gfMult(EEPol[J],gfExp[DLoc*J%MAXPAR]);
int DataPos=DataSize-Loc-1;
// Perform bounds check and correct the data error.
if (DataPos>=0 && DataPos<DataSize)
Data[DataPos]^=gfMult(N,gfExp[MAXPAR-gfLog[Dnm[I]]]);
}
return(ErrCount<=ParSize); // Return true if success.
}
bool RSCoder::Decode(unsigned char *Data,int DataSize,int *EraLoc,int EraSize)
{
int SynData[MAXPOL];
bool AllZeroes=true;
for (int I=0;I<ParSize;I++)
{
int Sum=Data[0],J=1,Exp=gfExp[I+1];
for (;J+8<=DataSize;J+=8)
{
Sum=Data[J]^gfMult(Exp,Sum);
Sum=Data[J+1]^gfMult(Exp,Sum);
Sum=Data[J+2]^gfMult(Exp,Sum);
Sum=Data[J+3]^gfMult(Exp,Sum);
Sum=Data[J+4]^gfMult(Exp,Sum);
Sum=Data[J+5]^gfMult(Exp,Sum);
Sum=Data[J+6]^gfMult(Exp,Sum);
Sum=Data[J+7]^gfMult(Exp,Sum);
}
for (;J<DataSize;J++)
Sum=Data[J]^gfMult(Exp,Sum);
if ((SynData[I]=Sum)!=0)
AllZeroes=false;
}
if (AllZeroes)
return(true);
if (!FirstBlockDone)
{
FirstBlockDone=true;
Clean(PolB,ParSize+1);
PolB[0]=1;
for (int EraPos=0;EraPos<EraSize;EraPos++)
for (int I=ParSize,M=gfExp[DataSize-EraLoc[EraPos]-1];I>0;I--)
PolB[I]^=gfMult(M,PolB[I-1]);
ErrCount=0;
for (int Root=MAXPAR-DataSize;Root<MAXPAR+1;Root++)
{
int Sum=0;
for (int B=0;B<ParSize+1;B++)
Sum^=gfMult(gfExp[(B*Root)%MAXPAR],PolB[B]);
if (Sum==0)
{
Dn[ErrCount]=0;
for (int I=1;I<ParSize+1;I+=2)
Dn[ErrCount]^= gfMult(PolB[I],gfExp[Root*(I-1)%MAXPAR]);
ErrorLocs[ErrCount++]=MAXPAR-Root;
}
}
}
int PolD[MAXPOL];
pnMult(PolB,SynData,PolD);
if ((ErrCount<=ParSize) && ErrCount>0)
for (int I=0;I<ErrCount;I++)
{
int Loc=ErrorLocs[I],DLoc=MAXPAR-Loc,N=0;
for (int J=0;J<ParSize;J++)
N^=gfMult(PolD[J],gfExp[DLoc*J%MAXPAR]);
int DataPos=DataSize-Loc-1;
if (DataPos>=0 && DataPos<DataSize)
Data[DataPos]^=gfMult(N,gfExp[MAXPAR-gfLog[Dn[I]]]);
}
return(ErrCount<=ParSize);
}
