help in convolutional encoding and decoding

Hello friends, here is the code of convolutional encoding and decoding, convolutional encoding is simple enough, but the logic in decoding is not understandable. can u tell me what logic is he putting here.

convolutional encoder:
UINT16 SR = 0;
UINT16 uPoly1 = 11001;
UINT16 uPoly2 = 11011;
UINT8 ucDataMask = 0x01;
INT i;
for (i = 0 ; i < iBitCount ; i++)
{
SR <<= 1;
SR |= (*puc8PackedData & ucDataMask) ? 1 : 0;
pucUnpackedOutput[*piImTable++] = Xor8((UINT8)(SR & uPoly1));
pucUnpackedOutput[*piImTable++] = Xor8((UINT8)(SR & uPoly2));
ucDataMask<<=1;
if (!ucDataMask)
ucDataMask = 0x01, puc8PackedData++;
}


Convolutional decoder:

uPoly1 >>= 1;
uPoly2 >>= 1; //why these two polynomials have been shifted which is not shifted in encoder

for (i = 0 ; i < iOutputBitCount ; i++)
{

bit1 = pucUnpackedInput;
bit1 ^= Xor8((UINT8)(SR & uPoly1)) ? 1 : 0;

bit2 = pucUnpackedInput;
bit2 ^= Xor8((UINT8)(SR & uPoly2)) ? 1 : 0;

if (bit1 != bit2) iErrors++;

SR <<= 1;

if (bit1) // Write bit to packed output and SR
{
*puc8PackedOutput |= ucDataMask;
SR |= 0x01;
}

// move output byte/bit pointer
ucDataMask <<= 1;
if (!ucDataMask) ucDataMask = 0x01, puc8PackedOutput++;
}

what is the logic behind this decoder, can anybody draw the sequential diagram with xor gates and tell me.