sdrangel/plugins/channel/lora/lorabits.h

/*
 Interleaving is "easiest" if the same number of bits is used per symbol as for FEC
 Chosen mode "spreading 8, low rate" has 6 bits per symbol, so use 4:6 FEC

 More spreading needs higher frequency resolution and longer time on air, increasing drift errors.
 Want higher bandwidth when using more spreading, which needs more CPU.

 Six bit Hamming can only correct drift errors. Want 7 or 8 bit FEC for QRM
*/

// Needs adjusting for different sizes
void LoRaDemod::interleave6(char* inout, int size)
{
	int i, j;
	char in[6 * 2];
	short s;

	for (j = 0; j < size; j+=6) {
		for (i = 0; i < 6; i++)
			in[i] = in[i + 6] = inout[i + j];
		// top bits are swapped
		for (i = 0; i < 6; i++) {
			s = (32 & in[2 + i]) | (16 & in[1 + i]) | (8 & in[3 + i])
				| (4 & in[4 + i]) | (2 & in[5 + i]) | (1 & in[6 + i]);
			// bits are also  rotated
			s = (s << 3) | (s >> 3);
			s &= 63;
			s = (s >> i) | (s << (6 - i));
			inout[i + j] = s & 63;
		}
	}
}

short LoRaDemod::toGray(short num)
{
        return (num >> 1) ^ num;
}

// ignore FEC, try to extract raw bits
void LoRaDemod::hamming6(char* c, int size)
{
	int i;

	for (i = 0; i < size; i++) {
		c[i] = ((c[i] & 1)<<3) | ((c[i] & 2)<<0) | ((c[i] & 4)>>0) | ((c[i] & 8)>>3);
		i++;
		c[i] = ((c[i] & 1)<<2) | ((c[i] & 2)<<2) | ((c[i] & 4)>>1) | ((c[i] & 8)>>3);
		i++;
		c[i] = ((c[i] & 32)>>2) | ((c[i] & 2)<<1) | ((c[i] & 4)>>1) | ((c[i] & 8)>>3);
		i++;
		c[i] = ((c[i] & 1)<<3) | ((c[i] & 2)<<1) | ((c[i] & 4)>>1) | ((c[i] & 8)>>3);
		i++;
		c[i] = ((c[i] & 1)<<3) | ((c[i] & 2)<<1) | ((c[i] & 4)>>1) | ((c[i] & 16)>>4);
		i++;
		c[i] = ((c[i] & 1)<<3) | ((c[i] & 2)<<1) | ((c[i] & 4)>>2) | ((c[i] & 8)>>2);
	}
	c[i] = 0;
}

// data whitening (6 bit)
void LoRaDemod::prng6(char* inout, int size)
{
	const char otp[] = {
	"5^ZSm0=cOGMgUB=bNcb<@a^T;_f=6DEB]2ImPIKg:j]RlYT4YZ<`9hZ\\PPb;@8X8i]Zmc_6B52\\8oUPHIcBOc>dY?d9[n5Lg]b]R8hR<0`T008h9c9QJm[c?a:lQEGa;nU=b_UbTW3=W5Aa<9i;F;ondS[LBA;[4S9]kkh]Vc2j>kX"
	};
	int i, maxchars;

	maxchars = sizeof( otp );
	if (size < maxchars)
		maxchars = size;
	for (i = 0; i < maxchars; i++)
		inout[i] ^= (otp[i] - 48);
}