fluxengine/lib/decoders/fmmfm.cc

#include "globals.h"
#include "decoders/decoders.h"

Bytes decodeFmMfm(
        std::vector<bool>::const_iterator ii, std::vector<bool>::const_iterator end)
{
    /* 
     * FM is dumb as rocks, consisting on regular clock pulses with data pulses in
     * the gaps. 0x00 is:
     * 
     *     X-X-X-X-X-X-X-X-
     * 
     * 0xff is:
     * 
     *     XXXXXXXXXXXXXXXX
     *
     * So we just need to extract all the odd bits.
     * 
     * MFM and M2FM are slightly more complicated, where the first bit of each
     * pair can be either 0 or 1... but the second bit is always the data bit,
     * and at this point we simply don't care what the first bit is, so
     * decoding MFM uses just the same code!
     */

    Bytes bytes;
    ByteWriter bw(bytes);

    int bitcount = 0;
    uint8_t fifo = 0;

    while (ii != end)
    {
        ii++; /* skip clock bit */
        if (ii == end)
            break;
        fifo = (fifo<<1) | *ii++;

        bitcount++;
        if (bitcount == 8)
        {
            bw.write_8(fifo);
            bitcount = 0;
        }
    }

    if (bitcount != 0)
    {
        fifo <<= 8-bitcount;
        bw.write_8(fifo);
    }

    return bytes;
}

void encodeMfm(std::vector<bool>& bits, unsigned& cursor, const Bytes& input)
{
    bool lastBit = false;
    unsigned len = bits.size()-1;

    for (uint8_t b : input)
    {
        for (int i=0; i<8; i++)
        {
            bool bit = b & 0x80;
            b <<= 1;

            if (cursor >= len)
                return;
            
            bits[cursor++] = !lastBit && !bit;
            bits[cursor++] = bit;
            lastBit = bit;
        }
    }
}