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/*
Copyright (C) 2012, 2013 Fredrik Johansson
This file is part of Arb.
Arb is free software: you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License (LGPL) as published
by the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version. See <http://www.gnu.org/licenses/>.
*/
#include "arb_poly.h"
#define MULLOW(z, x, xn, y, yn, nn, prec) \
if ((xn) >= (yn)) \
_arb_poly_mullow(z, x, xn, y, yn, nn, prec); \
else \
_arb_poly_mullow(z, y, yn, x, xn, nn, prec); \
void
_arb_poly_inv_series(arb_ptr Qinv,
arb_srcptr Q, slong Qlen, slong len, slong prec)
{
Qlen = FLINT_MIN(Qlen, len);
arb_inv(Qinv, Q, prec);
if (Qlen == 1)
{
_arb_vec_zero(Qinv + 1, len - 1);
}
else if (len == 2)
{
arb_mul(Qinv + 1, Qinv, Qinv, prec);
arb_mul(Qinv + 1, Qinv + 1, Q + 1, prec);
arb_neg(Qinv + 1, Qinv + 1);
}
else
{
slong i, blen;
/* The basecase algorithm is faster for much larger Qlen or len than
this, but unfortunately also much less numerically stable. */
if (Qlen == 2 || len <= 8)
blen = len;
else
blen = FLINT_MIN(len, 4);
for (i = 1; i < blen; i++)
{
arb_dot(Qinv + i, NULL, 1,
Q + 1, 1, Qinv + i - 1, -1, FLINT_MIN(i, Qlen - 1), prec);
if (!arb_is_one(Qinv))
arb_mul(Qinv + i, Qinv + i, Qinv, prec);
}
if (len > blen)
{
slong Qnlen, Wlen, W2len;
arb_ptr W;
W = _arb_vec_init(len);
NEWTON_INIT(blen, len)
NEWTON_LOOP(m, n)
Qnlen = FLINT_MIN(Qlen, n);
Wlen = FLINT_MIN(Qnlen + m - 1, n);
W2len = Wlen - m;
MULLOW(W, Q, Qnlen, Qinv, m, Wlen, prec);
MULLOW(Qinv + m, Qinv, m, W + m, W2len, n - m, prec);
_arb_vec_neg(Qinv + m, Qinv + m, n - m);
NEWTON_END_LOOP
NEWTON_END
_arb_vec_clear(W, len);
}
}
}
void
arb_poly_inv_series(arb_poly_t Qinv, const arb_poly_t Q, slong n, slong prec)
{
if (n == 0)
{
arb_poly_zero(Qinv);
return;
}
if (Q->length == 0)
{
arb_poly_fit_length(Qinv, n);
_arb_vec_indeterminate(Qinv->coeffs, n);
_arb_poly_set_length(Qinv, n);
return;
}
if (Qinv == Q)
{
arb_poly_t t;
arb_poly_init(t);
arb_poly_inv_series(t, Q, n, prec);
arb_poly_swap(Qinv, t);
arb_poly_clear(t);
return;
}
arb_poly_fit_length(Qinv, n);
_arb_poly_inv_series(Qinv->coeffs, Q->coeffs, Q->length, n, prec);
_arb_poly_set_length(Qinv, n);
_arb_poly_normalise(Qinv);
}
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