satdump/plugins/dvb_support/codings/dvb-s2/s2_scrambling.cpp

#include "s2_scrambling.h"

/*
Adapted from LeanDVB
*/

namespace dvbs2
{
    S2Scrambling::S2Scrambling(int codenum)
    {
        uint32_t stx = 0x00001, sty = 0x3ffff;
        // x starts at codenum, wraps at index 2^18-1 by design
        for (int i = 0; i < codenum; ++i)
            stx = lfsr_x(stx);
        // First half of sequence is LSB of scrambling angle
        for (int i = 0; i < 131072; ++i)
        {
            int zn = (stx ^ sty) & 1;
            Rn[i] = zn;
            stx = lfsr_x(stx);
            sty = lfsr_y(sty);
        }
        // Second half is MSB
        for (int i = 0; i < 131072; ++i)
        {
            int zn = (stx ^ sty) & 1;
            Rn[i] |= zn << 1;
            stx = lfsr_x(stx);
            sty = lfsr_y(sty);
        }
    }

    S2Scrambling::~S2Scrambling()
    {
    }

    void S2Scrambling::reset()
    {
        pos = 0;
    }

    complex_t S2Scrambling::descramble(complex_t &p)
    {
        r = Rn[pos++];

        switch (r)
        {
        case 3:
            res.real = -p.imag;
            res.imag = p.real;
            break;
        case 2:
            res.real = -p.real;
            res.imag = -p.imag;
            break;
        case 1:
            res.real = p.imag;
            res.imag = -p.real;
            break;
        default:
            res = p;
        }

        return res;
    }

    complex_t S2Scrambling::scramble(complex_t &p)
    {
        r = Rn[pos++];

        switch (r)
        {
        case 3:
            res.imag = -p.real;
            res.real = p.imag;
            break;
        case 2:
            res.real = -p.real;
            res.imag = -p.imag;
            break;
        case 1:
            res.imag = p.real;
            res.real = -p.imag;
            break;
        default:
            res = p;
        }

        return res;
    }
}