words.c

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	    else
		wps->w.error_limit [1] = 0;
	}
	else {
	    wps->w.error_limit [0] = exp2s (bitrate_0);
	    wps->w.error_limit [1] = exp2s (bitrate_1);
	}
    }
}

static ulong read_code (Bitstream *bs, ulong maxcode);

// Read the next word from the bitstream "wvbits" and return the value. This
// function can be used for hybrid or lossless streams, but since an
// optimized version is available for lossless this function would normally
// be used for hybrid only. If a hybrid lossless stream is being read then
// the "correction" offset is written at the specified pointer. A return value
// of WORD_EOF indicates that the end of the bitstream was reached (all 1s) or
// some other error occurred.

long get_word (WavpackStream *wps, int chan)
{
    ulong ones_count, low, mid, high;
    int sign;

    if (wps->w.zeros_acc) {
	if (--wps->w.zeros_acc) {
	    wps->w.slow_level [chan] -= (wps->w.slow_level [chan] + SLO) >> SLS;
	    return 0;
	}
    }
    else if (!wps->w.holding_zero && !wps->w.holding_one && !(wps->w.median [0] [0] & ~1) && !(wps->w.median [0] [1] & ~1)) {
	ulong mask;
	int cbits;

	for (cbits = 0; cbits < 33 && getbit (&wps->wvbits); ++cbits);

	if (cbits == 33)
	    return WORD_EOF;

	if (cbits < 2)
	    wps->w.zeros_acc = cbits;
	else {
	    for (mask = 1, wps->w.zeros_acc = 0; --cbits; mask <<= 1)
		if (getbit (&wps->wvbits))
		    wps->w.zeros_acc |= mask;

	    wps->w.zeros_acc |= mask;
	}

	if (wps->w.zeros_acc) {
	    wps->w.slow_level [chan] -= (wps->w.slow_level [chan] + SLO) >> SLS;
	    CLEAR (wps->w.median);
	    return 0;
	}
    }

    if (wps->w.holding_zero)
	ones_count = wps->w.holding_zero = 0;
    else {
#ifdef LIMIT_ONES
	for (ones_count = 0; ones_count < (LIMIT_ONES + 1) && getbit (&wps->wvbits); ++ones_count);

	if (ones_count == (LIMIT_ONES + 1))
	    return WORD_EOF;

	if (ones_count == LIMIT_ONES) {
	    ulong mask;
	    int cbits;

	    for (cbits = 0; cbits < 33 && getbit (&wps->wvbits); ++cbits);

	    if (cbits == 33)
		return WORD_EOF;

	    if (cbits < 2)
		ones_count = cbits;
	    else {
		for (mask = 1, ones_count = 0; --cbits; mask <<= 1)
		    if (getbit (&wps->wvbits))
			ones_count |= mask;

		ones_count |= mask;
	    }

	    ones_count += LIMIT_ONES;
	}
#else
	for (ones_count = 0; getbit (&wps->wvbits); ++ones_count);
#endif

	if (wps->w.holding_one) {
	    wps->w.holding_one = ones_count & 1;
	    ones_count = (ones_count >> 1) + 1;
	}
	else {
	    wps->w.holding_one = ones_count & 1;
	    ones_count >>= 1;
	}

	wps->w.holding_zero = ~wps->w.holding_one & 1;
    }

    if ((wps->wphdr.flags & HYBRID_FLAG) && !chan)
	update_error_limit (wps);

    if (ones_count == 0) {
	low = 0;
	high = GET_MED (0) - 1;
	DEC_MED0 ();
    }
    else {
	low = GET_MED (0);
	INC_MED0 ();

	if (ones_count == 1) {
	    high = low + GET_MED (1) - 1;
	    DEC_MED1 ();
	}
	else {
	    low += GET_MED (1);
	    INC_MED1 ();

	    if (ones_count == 2) {
		high = low + GET_MED (2) - 1;
		DEC_MED2 ();
	    }
	    else {
		low += (ones_count - 2) * GET_MED (2);
		high = low + GET_MED (2) - 1;
		INC_MED2 ();
	    }
	}
    }

    mid = (high + low + 1) >> 1;

    if (!wps->w.error_limit [chan])
	mid = read_code (&wps->wvbits, high - low) + low;
    else while (high - low > wps->w.error_limit [chan]) {
	if (getbit (&wps->wvbits))
	    mid = (high + (low = mid) + 1) >> 1;
	else
	    mid = ((high = mid - 1) + low + 1) >> 1;
    }

    sign = getbit (&wps->wvbits);

    if (wps->wphdr.flags & HYBRID_BITRATE) {
	wps->w.slow_level [chan] -= (wps->w.slow_level [chan] + SLO) >> SLS;
	wps->w.slow_level [chan] += log2 (mid);
    }

    return sign ? ~mid : mid;
}

// Read a single unsigned value from the specified bitstream with a value
// from 0 to maxcode. If there are exactly a power of two number of possible
// codes then this will read a fixed number of bits; otherwise it reads the
// minimum number of bits and then determines whether another bit is needed
// to define the code.

static ulong read_code (Bitstream *bs, ulong maxcode)
{
    int bitcount = count_bits (maxcode);
    ulong extras = (1L << bitcount) - maxcode - 1, code;

    if (!bitcount)
	return 0;

    getbits (&code, bitcount - 1, bs);
    code &= (1L << (bitcount - 1)) - 1;

    if (code >= extras) {
	code = (code << 1) - extras;

	if (getbit (bs))
	    ++code;
    }

    return code;
}

// The concept of a base 2 logarithm is used in many parts of WavPack. It is
// a way of sufficiently accurately representing 32-bit signed and unsigned
// values storing only 16 bits (actually fewer). It is also used in the hybrid
// mode for quickly comparing the relative magnitude of large values (i.e.
// division) and providing smooth exponentials using only addition.

// These are not strict logarithms in that they become linear around zero and
// can therefore represent both zero and negative values. They have 8 bits
// of precision and in "roundtrip" conversions the total error never exceeds 1
// part in 225 except for the cases of +/-115 and +/-195 (which error by 1).


// This function returns the log2 for the specified 32-bit unsigned value.
// The maximum value allowed is about 0xff800000 and returns 8447.

static int log2 (unsigned long avalue)
{
    int dbits;

    if ((avalue += avalue >> 9) < (1 << 8)) {
	dbits = nbits_table [avalue];
	return (dbits << 8) + log2_table [(avalue << (9 - dbits)) & 0xff];
    }
    else {
	if (avalue < (1L << 16))
	    dbits = nbits_table [avalue >> 8] + 8;
	else if (avalue < (1L << 24))
	    dbits = nbits_table [avalue >> 16] + 16;
	else
	    dbits = nbits_table [avalue >> 24] + 24;

	return (dbits << 8) + log2_table [(avalue >> (dbits - 9)) & 0xff];
    }
}

// This function returns the original integer represented by the supplied
// logarithm (at least within the provided accuracy). The log is signed,
// but since a full 32-bit value is returned this can be used for unsigned
// conversions as well (i.e. the input range is -8192 to +8447).

long exp2s (int log)
{
    ulong value;

    if (log < 0)
	return -exp2s (-log);

    value = exp2_table [log & 0xff] | 0x100;

    if ((log >>= 8) <= 9)
	return value >> (9 - log);
    else
	return value << (log - 9);
}

// These two functions convert internal weights (which are normally +/-1024)
// to and from an 8-bit signed character version for storage in metadata. The
// weights are clipped here in the case that they are outside that range.

int restore_weight (char weight)
{
    int result;

    if ((result = (int) weight << 3) > 0)
	result += (result + 64) >> 7;

    return result;
}