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
*/

// Needs adjusting for different sizes
void LoRaDemod::interleave(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::hamming(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] & 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] & 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)>>2) | ((c[i] & 8)>>2);
	}
	c[i] = 0;
}

// example data whitening (4 bit only - needs 6 bit for FEC)
void LoRaDemod::prng(char* inout, int size)
{
	const char otp[] = {
	"LBMGEJJENKKKJBN@KB@KAEDDMMDONIN@N@KFBGBCMCMCMIBJHBDHNJDJALDHE@A@DFGOBOMIM@M@BBBCBAMBMKDENDLEDKNKNBNHKJ@JADD@EAAADBOCGAGBLCDANFLGDCNGLOMOBIHHMLBHB@BHMJGJBLMLED@B"
	};
	int i, maxchars;

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