161 lines
3.9 KiB
C++
161 lines
3.9 KiB
C++
#include "rar.hpp"
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#define Clean(D,S) {for (int I=0;I<(S);I++) (D)[I]=0;}
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void RSCoder::Init(int ParSize)
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{
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RSCoder::ParSize=ParSize; // Store the number of recovery volumes.
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FirstBlockDone=false;
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gfInit();
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pnInit();
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}
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// Initialize logarithms and exponents Galois field tables.
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void RSCoder::gfInit()
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{
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for (int I=0,J=1;I<MAXPAR;I++)
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{
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gfLog[J]=I;
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gfExp[I]=J;
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J<<=1;
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if (J > MAXPAR)
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J^=0x11D; // 0x11D field-generator polynomial (x^8+x^4+x^3+x^2+1).
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}
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for (int I=MAXPAR;I<MAXPOL;I++) // Avoid gfExp overflow check.
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gfExp[I]=gfExp[I-MAXPAR];
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}
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// Multiplication over Galois field.
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inline int RSCoder::gfMult(int a,int b)
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{
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return(a==0 || b == 0 ? 0:gfExp[gfLog[a]+gfLog[b]]);
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}
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// Create the generator polynomial g(x).
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// g(x)=(x-a)(x-a^2)(x-a^3)..(x-a^N)
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void RSCoder::pnInit()
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{
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int p2[MAXPAR+1]; // Currently calculated part of g(x).
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Clean(p2,ParSize);
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p2[0]=1; // Set p2 polynomial to 1.
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for (int I=1;I<=ParSize;I++)
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{
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int p1[MAXPAR+1]; // We use p1 as current (x+a^i) expression.
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Clean(p1,ParSize);
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p1[0]=gfExp[I];
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p1[1]=1; // Set p1 polynomial to x+a^i.
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// Multiply the already calucated part of g(x) to next (x+a^i).
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pnMult(p1,p2,GXPol);
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// p2=g(x).
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for (int J=0;J<ParSize;J++)
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p2[J]=GXPol[J];
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}
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}
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// Multiply polynomial 'p1' to 'p2' and store the result in 'r'.
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void RSCoder::pnMult(int *p1,int *p2,int *r)
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{
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Clean(r,ParSize);
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for (int I=0;I<ParSize;I++)
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if (p1[I]!=0)
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for(int J=0;J<ParSize-I;J++)
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r[I+J]^=gfMult(p1[I],p2[J]);
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}
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void RSCoder::Encode(byte *Data,int DataSize,byte *DestData)
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{
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int ShiftReg[MAXPAR+1]; // Linear Feedback Shift Register.
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Clean(ShiftReg,ParSize+1);
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for (int I=0;I<DataSize;I++)
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{
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int D=Data[I]^ShiftReg[ParSize-1];
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// Use g(x) to define feedback taps.
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for (int J=ParSize-1;J>0;J--)
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ShiftReg[J]=ShiftReg[J-1]^gfMult(GXPol[J],D);
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ShiftReg[0]=gfMult(GXPol[0],D);
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}
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for (int I=0;I<ParSize;I++)
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DestData[I]=ShiftReg[ParSize-I-1];
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}
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bool RSCoder::Decode(byte *Data,int DataSize,int *EraLoc,int EraSize)
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{
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int SynData[MAXPOL]; // Syndrome data.
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bool AllZeroes=true;
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for (int I=0;I<ParSize;I++)
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{
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int Sum=0;
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for (int J=0;J<DataSize;J++)
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Sum=Data[J]^gfMult(gfExp[I+1],Sum);
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if ((SynData[I]=Sum)!=0)
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AllZeroes=false;
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}
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// If all syndrome numbers are zero, message does not have errors.
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if (AllZeroes)
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return(true);
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if (!FirstBlockDone) // Do things which we need to do once for all data.
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{
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FirstBlockDone=true;
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// Calculate the error locator polynomial.
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Clean(ELPol,ParSize+1);
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ELPol[0]=1;
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for (int EraPos=0;EraPos<EraSize;EraPos++)
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for (int I=ParSize,M=gfExp[DataSize-EraLoc[EraPos]-1];I>0;I--)
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ELPol[I]^=gfMult(M,ELPol[I-1]);
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ErrCount=0;
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// Find roots of error locator polynomial.
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for (int Root=MAXPAR-DataSize;Root<MAXPAR+1;Root++)
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{
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int Sum=0;
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for (int B=0;B<ParSize+1;B++)
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Sum^=gfMult(gfExp[(B*Root)%MAXPAR],ELPol[B]);
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if (Sum==0) // Root found.
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{
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ErrorLocs[ErrCount]=MAXPAR-Root; // Location of error.
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// Calculate the denominator for every error location.
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Dnm[ErrCount]=0;
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for (int I=1;I<ParSize+1;I+=2)
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Dnm[ErrCount]^= gfMult(ELPol[I],gfExp[Root*(I-1)%MAXPAR]);
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ErrCount++;
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}
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}
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}
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int EEPol[MAXPOL]; // Error Evaluator Polynomial.
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pnMult(ELPol,SynData,EEPol);
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// If errors are present and their number is correctable.
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if ((ErrCount<=ParSize) && ErrCount>0)
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for (int I=0;I<ErrCount;I++)
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{
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int Loc=ErrorLocs[I],DLoc=MAXPAR-Loc,N=0;
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for (int J=0;J<ParSize;J++)
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N^=gfMult(EEPol[J],gfExp[DLoc*J%MAXPAR]);
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int DataPos=DataSize-Loc-1;
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// Perform bounds check and correct the data error.
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if (DataPos>=0 && DataPos<DataSize)
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Data[DataPos]^=gfMult(N,gfExp[MAXPAR-gfLog[Dnm[I]]]);
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}
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return(ErrCount<=ParSize); // Return true if success.
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}
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