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#include "rar.hpp"
#define Clean(D,S) {for (int I=0;I<(S);I++) (D)[I]=0;}
void RSCoder::Init(int ParSize)
{
RSCoder::ParSize=ParSize; // Store the number of recovery volumes.
FirstBlockDone=false;
gfInit();
pnInit();
}
// Initialize logarithms and exponents Galois field tables.
void RSCoder::gfInit()
{
for (int I=0,J=1;I<MAXPAR;I++)
{
gfLog[J]=I;
gfExp[I]=J;
J<<=1;
if (J > MAXPAR)
J^=0x11D; // 0x11D field-generator polynomial (x^8+x^4+x^3+x^2+1).
}
for (int I=MAXPAR;I<MAXPOL;I++) // Avoid gfExp overflow check.
gfExp[I]=gfExp[I-MAXPAR];
}
// Multiplication over Galois field.
inline int RSCoder::gfMult(int a,int b)
{
return(a==0 || b == 0 ? 0:gfExp[gfLog[a]+gfLog[b]]);
}
// 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];
}
}
// Multiply polynomial 'p1' to 'p2' and store the result in 'r'.
void RSCoder::pnMult(int *p1,int *p2,int *r)
{
Clean(r,ParSize);
for (int I=0;I<ParSize;I++)
if (p1[I]!=0)
for(int J=0;J<ParSize-I;J++)
r[I+J]^=gfMult(p1[I],p2[J]);
}
void RSCoder::Encode(byte *Data,int DataSize,byte *DestData)
{
int ShiftReg[MAXPAR+1]; // Linear Feedback Shift Register.
Clean(ShiftReg,ParSize+1);
for (int I=0;I<DataSize;I++)
{
int D=Data[I]^ShiftReg[ParSize-1];
// Use g(x) to define feedback taps.
for (int J=ParSize-1;J>0;J--)
ShiftReg[J]=ShiftReg[J-1]^gfMult(GXPol[J],D);
ShiftReg[0]=gfMult(GXPol[0],D);
}
for (int I=0;I<ParSize;I++)
DestData[I]=ShiftReg[ParSize-I-1];
}
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.
}
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