// borrowed from Wine

static inline USHORT get_fixed_windescent(USHORT windescent)
{
    return abs((SHORT)windescent);
}

static LONG calc_ppem_for_height(FT_Face ft_face, LONG height)
{
    TT_OS2 *pOS2;
    TT_HoriHeader *pHori;

    LONG ppem;
    const LONG MAX_PPEM = (1 << 16) - 1;

    pOS2 = FT_Get_Sfnt_Table(ft_face, FT_SFNT_OS2);
    pHori = FT_Get_Sfnt_Table(ft_face, FT_SFNT_HHEA);
    ASSERT(pOS2);
    ASSERT(pHori);

    if (height == 0)
        height = 16;

    /* For lfHeight > 0:
     * ppem = units_per_em * lfheight / (winAscent + winDescent).
     *
     * For lfHeight < 0:
     * ppem = |lfHeight| = (winAscent + winDescent - IL) * ppem / units_per_em
     *                   ; with IL = winAscent + winDescent - units_per_em.
     */
    if (height > 0)
    {
        USHORT windescent = get_fixed_windescent(pOS2->usWinDescent);
        if (pOS2->usWinAscent + windescent == 0)
        {
            ppem = MulDiv(ft_face->units_per_EM, height, pHori->Ascender - pHori->Descender);
        }
        else
        {
            ppem = MulDiv(ft_face->units_per_EM, height, pOS2->usWinAscent + windescent);
        }

        if (ppem > MAX_PPEM)
        {
            ppem = 1;
        }
    }
    else if (height >= -MAX_PPEM)
    {
        ppem = -height;
    }
    else
    {
        DPRINT("Ignoring too large height %d\n", height);
        ppem = 1;
    }

    return ppem;
}