.. _ulong-extras:
**ulong_extras.h** -- arithmetic and number-theoretic functions for single-word integers
========================================================================================
Random functions
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.. function:: ulong n_randlimb(flint_rand_t state)
Returns a uniformly pseudo random limb.
The algorithm generates two random half limbs `s_j`, `j = 0, 1`,
by iterating respectively `v_{i+1} = (v_i a + b) \bmod{p_j}` for
some initial seed `v_0`, randomly chosen values `a` and `b` and
``p_0 = 4294967311 = nextprime(2^32)`` on a 64-bit machine
and ``p_0 = nextprime(2^16)`` on a 32-bit machine and
``p_1 = nextprime(p_0)``.
.. function:: ulong n_randbits(flint_rand_t state, unsigned int bits)
Returns a uniformly pseudo random number with the given number of
bits. The most significant bit is always set, unless zero is passed,
in which case zero is returned.
.. function:: ulong n_randtest_bits(flint_rand_t state, int bits)
Returns a uniformly pseudo random number with the given number of
bits. The most significant bit is always set, unless zero is passed,
in which case zero is returned. The probability of a value with a
sparse binary representation being returned is increased. This
function is intended for use in test code.
.. function:: ulong n_randint(flint_rand_t state, ulong limit)
Returns a uniformly pseudo random number up to but not including
the given limit. If zero is passed as a parameter, an entire random
limb is returned.
.. function:: ulong n_urandint(flint_rand_t state, ulong limit)
Returns a uniformly pseudo random number up to but not including
the given limit. If zero is passed as a parameter, an entire
random limb is returned. This function provides somewhat better
randomness as compared to :func:`n_randint`, especially for larger
values of limit.
.. function:: ulong n_randtest(flint_rand_t state)
Returns a pseudo random number with a random number of bits,
from `0` to ``FLINT_BITS``. The probability of the special
values `0`, `1`, ``COEFF_MAX`` and ``WORD_MAX`` is increased
as is the probability of a value with sparse binary representation.
This random function is mainly used for testing purposes.
This function is intended for use in test code.
.. function:: ulong n_randtest_not_zero(flint_rand_t state)
As for :func:`n_randtest`, but does not return `0`.
This function is intended for use in test code.
.. function:: ulong n_randprime(flint_rand_t state, ulong bits, int proved)
Returns a random prime number ``(proved = 1)`` or probable prime
``(proved = 0)``
with ``bits`` bits, where ``bits`` must be at least 2 and
at most ``FLINT_BITS``.
.. function:: ulong n_randtest_prime(flint_rand_t state, int proved)
Returns a random prime number ``(proved = 1)`` or probable
prime ``(proved = 0)``
with size randomly chosen between 2 and ``FLINT_BITS`` bits.
This function is intended for use in test code.
Basic arithmetic
--------------------------------------------------------------------------------
.. function:: ulong n_pow(ulong n, ulong exp)
Returns ``n^exp``. No checking is done for overflow. The exponent
may be zero. We define `0^0 = 1`.
The algorithm simply uses a for loop. Repeated squaring is
unlikely to speed up this algorithm.
.. function:: ulong n_flog(ulong n, ulong b)
Returns `\lfloor\log_b n\rfloor`.
Assumes that `n \geq 1` and `b \geq 2`.
.. function:: ulong n_clog(ulong n, ulong b)
Returns `\lceil\log_b n\rceil`.
Assumes that `n \geq 1` and `b \geq 2`.
.. function:: ulong n_clog_2exp(ulong n, ulong b)
Returns `\lceil\log_b 2^n\rceil`.
Assumes that `b \geq 2`.
Miscellaneous
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.. function:: ulong n_revbin(ulong n, ulong b)
Returns the binary reverse of `n`, assuming it is `b` bits in length,
e.g. ``n_revbin(10110, 6)`` will return ``110100``.
.. function:: int n_sizeinbase(ulong n, int base)
Returns the exact number of digits needed to represent `n` as a
string in base ``base`` assumed to be between 2 and 36.
Returns 1 when `n = 0`.
Basic arithmetic with precomputed inverses
--------------------------------------------------------------------------------
.. function:: ulong n_preinvert_limb(ulong n)
Returns a precomputed inverse of `n`, as defined in [GraMol2010]_.
This precomputed inverse can be used with all of the functions that
take a precomputed inverse whose names are suffixed by ``_preinv``.
We require `n > 0`.
.. function:: double n_precompute_inverse(ulong n)
Returns a precomputed inverse of `n` with double precision value `1/n`.
This precomputed inverse can be used with all of the functions that
take a precomputed inverse whose names are suffixed by ``_precomp``.
We require `n > 0`.
.. function:: ulong n_mod_precomp(ulong a, ulong n, double ninv)
Returns `a \bmod{n}` given a precomputed inverse of `n` computed
by :func:`n_precompute_inverse`. We require ``n < 2^FLINT_D_BITS``
and ``a < 2^(FLINT_BITS-1)`` and `0 \leq a < n^2`.
We assume the processor is in the standard round to nearest
mode. Thus ``ninv`` is correct to `53` binary bits, the least
significant bit of which we shall call a place, and can be at most
half a place out. When `a` is multiplied by `ninv`, the binary
representation of `a` is exact and the mantissa is less than `2`, thus we
see that ``a * ninv`` can be at most one out in the mantissa. We now
truncate ``a * ninv`` to the nearest integer, which is always a round
down. Either we already have an integer, or we need to make a change down
of at least `1` in the last place. In the latter case we either get
precisely the exact quotient or below it as when we rounded the
product to the nearest place we changed by at most half a place.
In the case that truncating to an integer takes us below the
exact quotient, we have rounded down by less than `1` plus half a
place. But as the product is less than `n` and `n` is less than `2^{53}`,
half a place is less than `1`, thus we are out by less than `2` from
the exact quotient, i.e.\ the quotient we have computed is the
quotient we are after or one too small. That leaves only the case
where we had to round up to the nearest place which happened to
be an integer, so that truncating to an integer didn't change
anything. But this implies that the exact quotient `a/n` is less
than `2^{-54}` from an integer. We deal with this rare case by
subtracting 1 from the quotient. Then the quotient we have computed is
either exactly what we are after, or one too small.
.. function:: ulong n_mod2_precomp(ulong a, ulong n, double ninv)
Returns `a \bmod{n}` given a precomputed inverse of `n` computed by
:func:`n_precompute_inverse`. There are no restrictions on `a` or
on `n`.
As for :func:`n_mod_precomp` for `n < 2^{53}` and `a < n^2` the
computed quotient is either what we are after or one too large or small.
We deal with these cases. Otherwise we can be sure that the
top `52` bits of the quotient are computed correctly. We take
the remainder and adjust the quotient by multiplying the
remainder by ``ninv`` to compute another approximate quotient as
per :func:`mod_precomp`. Now the remainder may be either
negative or positive, so the quotient we compute may be one
out in either direction.
.. function:: ulong n_divrem2_preinv(ulong * q, ulong a, ulong n, ulong ninv)
Returns `a \bmod{n}` and sets `q` to the quotient of `a` by `n`, given a
precomputed inverse of `n` computed by :func:`n_preinvert_limb()`. There are
no restrictions on `a` and the only restriction on `n` is that it be
nonzero.
This uses the algorithm of Granlund and M\"oller [GraMol2010]_. First
`n` is normalised and `a` is shifted into two limbs to compensate. Then
their algorithm is applied verbatim and the remainder shifted back.
.. function:: ulong n_div2_preinv(ulong a, ulong n, ulong ninv)
Returns the Euclidean quotient of `a` by `n` given a precomputed inverse of
`n` computed by :func:`n_preinvert_limb`. There are no restrictions on `a`
and the only restriction on `n` is that it be nonzero.
This uses the algorithm of Granlund and M\"oller [GraMol2010]_. First
`n` is normalised and `a` is shifted into two limbs to compensate. Then
their algorithm is applied verbatim.
.. function:: ulong n_mod2_preinv(ulong a, ulong n, ulong ninv)
Returns `a \bmod{n}` given a precomputed inverse of `n` computed by
:func:`n_preinvert_limb()`. There are no restrictions on `a` and the only
restriction on `n` is that it be nonzero.
This uses the algorithm of Granlund and M\"oller [GraMol2010]_. First
`n` is normalised and `a` is shifted into two limbs to compensate. Then
their algorithm is applied verbatim and the result shifted back.
.. function:: ulong n_divrem2_precomp(ulong * q, ulong a, ulong n, double npre)
Returns `a \bmod{n}` given a precomputed inverse of `n` computed by
:func:`n_precompute_inverse` and sets `q` to the quotient. There
are no restrictions on `a` or on `n`.
This is as for :func:`n_mod2_precomp` with some additional care taken
to retain the quotient information. There are also special
cases to deal with the case where `a` is already reduced modulo
`n` and where `n` is `64` bits and `a` is not reduced modulo `n`.
.. function:: ulong n_ll_mod_preinv(ulong a_hi, ulong a_lo, ulong n, ulong ninv)
Returns `a \bmod{n}` given a precomputed inverse of `n` computed by
:func:`n_preinvert_limb`. There are no restrictions on `a`, which
will be two limbs ``(a_hi, a_lo)``, or on `n`.
The old version of this function merely reduced the top limb
``a_hi`` modulo `n` so that :func:`udiv_qrnnd_preinv()` could
be used.
The new version reduces the top limb modulo `n` as per
:func:`n_mod2_preinv` and then the algorithm of Granlund and
M\"oller [GraMol2010]_ is used again to reduce modulo `n`.
.. function:: ulong n_lll_mod_preinv(ulong a_hi, ulong a_mi, ulong a_lo, ulong n, ulong ninv)
Returns `a \bmod{n}`, where `a` has three limbs ``(a_hi, a_mi, a_lo)``,
given a precomputed inverse of `n` computed by :func:`n_preinvert_limb`.
It is assumed that ``a_hi`` is reduced modulo `n`. There are no
restrictions on `n`.
This function uses the algorithm of Granlund and
M\"oller [GraMol2010]_ to first reduce the top two limbs
modulo `n`, then does the same on the bottom two limbs.
.. function:: ulong n_mulmod_precomp(ulong a, ulong b, ulong n, double ninv)
Returns `a b \bmod{n}` given a precomputed inverse of `n`
computed by :func:`n_precompute_inverse`. We require
``n < 2^FLINT_D_BITS`` and `0 \leq a, b < n`.
We assume the processor is in the standard round to nearest
mode. Thus ``ninv`` is correct to `53` binary bits, the least
significant bit of which we shall call a place, and can be at most half
a place out. The product of `a` and `b` is computed with error at most
half a place. When ``a * b`` is multiplied by `ninv` we find that the
exact quotient and computed quotient differ by less than two places. As
the quotient is less than `n` this means that the exact quotient is at
most `1` away from the computed quotient. We truncate this quotient to
an integer which reduces the value by less than `1`. We end up with a
value which can be no more than two above the quotient we are after and
no less than two below. However an argument similar to that for
:func:`n_mod_precomp` shows that the truncated computed quotient cannot
be two smaller than the truncated exact quotient. In other words the
computed integer quotient is at most two above and one below the quotient
we are after.
.. function:: ulong n_mulmod2_preinv(ulong a, ulong b, ulong n, ulong ninv)
Returns `a b \bmod{n}` given a precomputed inverse of `n` computed by
:func:`n_preinvert_limb`. There are no restrictions on `a`, `b` or
on `n`. This is implemented by multiplying using :func:`umul_ppmm` and
then reducing using :func:`n_ll_mod_preinv`.
.. function:: ulong n_mulmod2(ulong a, ulong b, ulong n)
Returns `a b \bmod{n}`. There are no restrictions on `a`, `b` or
on `n`. This is implemented by multiplying using :func:`umul_ppmm` and
then reducing using :func:`n_ll_mod_preinv` after computing a precomputed
inverse.
.. function:: ulong n_mulmod_preinv(ulong a, ulong b, ulong n, ulong ninv, ulong norm)
Returns `a b \pmod{n}` given a precomputed inverse of `n` computed by
:func:`n_preinvert_limb`, assuming `a` and `b` are reduced modulo `n`
and `n` is normalised, i.e. with most significant bit set. There are
no other restrictions on `a`, `b` or `n`.
The value ``norm`` is provided for convenience. As `n` is required
to be normalised, it may be that `a` and `b` have been shifted to the
left by ``norm`` bits before calling the function. Their product
then has an extra factor of `2^\text{norm}`. Specifying a nonzero
``norm`` will shift the product right by this many bits before
reducing it.
The algorithm use is that of Granlund and M\"oller [GraMol2010]_.
Greatest common divisor
--------------------------------------------------------------------------------
.. function:: ulong n_gcd(ulong x, ulong y)
Returns the greatest common divisor `g` of `x` and `y`. No assumptions
are made about the values `x` and `y`.
The algorithm is a slight embellishment of the Euclidean algorithm
which uses some branches to avoid most divisions.
One wishes to compute the quotient and remainder of `u_3 / v_3` without
division where possible. This is accomplished when `u_3 < 4 v_3`, i.e.
the quotient is either `1`, `2` or `3`.
We first compute `s = u_3 - v_3`. If `s < v_3`, i.e.\ `u_3 < 2 v_3`, we
know the quotient is `1`, else if `s < 2 v_3`, i.e.\ `u_3 < 3 v_3` we
know the quotient is `2`. In the remaining cases, the quotient must
be `3`. When the quotient is `4` or above, we use division. However this
happens rarely for generic inputs.
.. function:: ulong n_gcd_full(ulong x, ulong y)
Returns the greatest common divisor `g` of `x` and `y`.
No assumptions are made about `x` and `y`.
This function is deprecated.
.. function:: ulong n_gcdinv(ulong * a, ulong x, ulong y)
Returns the greatest common divisor `g` of `x` and `y` and computes
`a` such that `0 \leq a < y` and `a x = \gcd(x, y) \bmod{y}`, when
this is defined. We require `x < y`.
When `y = 1` the greatest common divisor is set to `1` and `a` is
set to `0`.
This is merely an adaption of the extended Euclidean algorithm
computing just one cofactor and reducing it modulo `y`.
.. function:: ulong n_xgcd(ulong * a, ulong * b, ulong x, ulong y)
Returns the greatest common divisor `g` of `x` and `y` and unsigned
values `a` and `b` such that `a x - b y = g`. We require `x \geq y`.
We claim that computing the extended greatest common divisor via the
Euclidean algorithm always results in cofactor `\lvert a \rvert < x/2`,
`\lvert b\rvert < x/2`, with perhaps some small degenerate exceptions.
We proceed by induction.
Suppose we are at some step of the algorithm, with `x_n = q y_n + r`
with `r \geq 1`, and suppose `1 = s y_n - t r` with
`s < y_n / 2`, `t < y_n / 2` by hypothesis.
Write `1 = s y_n - t (x_n - q y_n) = (s + t q) y_n - t x_n`.
It suffices to show that `(s + t q) < x_n / 2` as `t < y_n / 2 < x_n / 2`,
which will complete the induction step.
But at the previous step in the backsubstitution we would have had
`1 = s r - c d` with `s < r/2` and `c < r/2`.
Then `s + t q < r/2 + y_n / 2 q = (r + q y_n)/2 = x_n / 2`.
See the documentation of :func:`n_gcd` for a description of the
branching in the algorithm, which is faster than using division.
Jacobi and Kronecker symbols
--------------------------------------------------------------------------------
.. function:: int n_jacobi(mp_limb_signed_t x, ulong y)
Computes the Jacobi symbol `\left(\frac{x}{y}\right)` for any x and odd `y`.
.. function:: int n_jacobi_unsigned(ulong x, ulong y)
Computes the Jacobi symbol, allowing `x` to go up to a full limb.
Modular Arithmetic
--------------------------------------------------------------------------------
.. function:: ulong n_addmod(ulong a, ulong b, ulong n)
Returns `(a + b) \bmod{n}`.
.. function:: ulong n_submod(ulong a, ulong b, ulong n)
Returns `(a - b) \bmod{n}`.
.. function:: ulong n_invmod(ulong x, ulong y)
Returns the inverse of `x` modulo `y`, if it exists. Otherwise an exception
is thrown.
This is merely an adaption of the extended Euclidean algorithm
with appropriate normalisation.
.. function:: ulong n_powmod_precomp(ulong a, mp_limb_signed_t exp, ulong n, double npre)
Returns ``a^exp`` modulo `n` given a precomputed inverse of `n`
computed by :func:`n_precompute_inverse`. We require `n < 2^{53}`
and `0 \leq a < n`. There are no restrictions on ``exp``, i.e.
it can be negative.
This is implemented as a standard binary powering algorithm using
repeated squaring and reducing modulo `n` at each step.
.. function:: ulong n_powmod_ui_precomp(ulong a, ulong exp, ulong n, double npre)
Returns ``a^exp`` modulo `n` given a precomputed inverse of `n`
computed by :func:`n_precompute_inverse`. We require `n < 2^{53}`
and `0 \leq a < n`. The exponent ``exp`` is unsigned and so
can be larger than allowed by :func:`n_powmod_precomp`.
This is implemented as a standard binary powering algorithm using
repeated squaring and reducing modulo `n` at each step.
.. function:: ulong n_powmod(ulong a, mp_limb_signed_t exp, ulong n)
Returns ``a^exp`` modulo `n`. We require ``n < 2^FLINT_D_BITS``
and `0 \leq a < n`. There are no restrictions on ``exp``, i.e.\
it can be negative.
This is implemented by precomputing an inverse and calling the
``precomp`` version of this function.
.. function:: ulong n_powmod2_preinv(ulong a, mp_limb_signed_t exp, ulong n, ulong ninv)
Returns ``(a^exp) % n`` given a precomputed inverse of `n` computed
by :func:`n_preinvert_limb`. We require `0 \leq a < n`, but there are no
restrictions on `n` or on ``exp``, i.e.\ it can be negative.
This is implemented as a standard binary powering algorithm using
repeated squaring and reducing modulo `n` at each step.
If ``exp`` is negative but `a` is not invertible modulo `n`, an
exception is raised.
.. function:: ulong n_powmod2(ulong a, mp_limb_signed_t exp, ulong n)
Returns ``(a^exp) % n``. We require `0 \leq a < n`, but there are
no restrictions on `n` or on ``exp``, i.e.\ it can be negative.
This is implemented by precomputing an inverse limb and calling the
``preinv`` version of this function.
If ``exp`` is negative but `a` is not invertible modulo `n`, an
exception is raised.
.. function:: ulong n_powmod2_ui_preinv(ulong a, ulong exp, ulong n, ulong ninv)
Returns ``(a^exp) % n`` given a precomputed inverse of `n` computed
by :func:`n_preinvert_limb`. We require `0 \leq a < n`, but there are no
restrictions on `n`. The exponent ``exp`` is unsigned and so can be
larger than allowed by :func:`n_powmod2_preinv`.
This is implemented as a standard binary powering algorithm using
repeated squaring and reducing modulo `n` at each step.
.. function:: ulong n_powmod2_fmpz_preinv(ulong a, const fmpz_t exp, ulong n, ulong ninv)
Returns ``(a^exp) % n`` given a precomputed inverse of `n` computed
by :func:`n_preinvert_limb`. We require `0 \leq a < n`, but there are no
restrictions on `n`. The exponent ``exp`` must not be negative.
This is implemented as a standard binary powering algorithm using
repeated squaring and reducing modulo `n` at each step.
.. function:: ulong n_sqrtmod(ulong a, ulong p)
If `p` is prime, compute a square root of `a` modulo `p` if `a` is a
quadratic residue modulo `p`, otherwise return `0`.
If `p` is not prime the result is with high probability `0`, indicating
that `p` is not prime, or `a` is not a square modulo `p`. Otherwise the
result is meaningless.
Assumes that `a` is reduced modulo `p`.
.. function:: slong n_sqrtmod_2pow(ulong ** sqrt, ulong a, slong exp)
Computes all the square roots of ``a`` modulo ``2^exp``. The roots
are stored in an array which is created and whose address is stored in
the location pointed to by ``sqrt``. The array of roots is allocated
by the function but must be cleaned up by the user by calling
``flint_free``. The number of roots is returned by the function. If
``a`` is not a quadratic residue modulo ``2^exp`` then 0 is
returned by the function and the location ``sqrt`` points to is set to
NULL.
.. function:: slong n_sqrtmod_primepow(ulong ** sqrt, ulong a, ulong p, slong exp)
Computes all the square roots of ``a`` modulo ``p^exp``. The roots
are stored in an array which is created and whose address is stored in
the location pointed to by ``sqrt``. The array of roots is allocated
by the function but must be cleaned up by the user by calling
``flint_free``. The number of roots is returned by the function. If
``a`` is not a quadratic residue modulo ``p^exp`` then 0 is
returned by the function and the location ``sqrt`` points to is set to
NULL.
.. function:: slong n_sqrtmodn(ulong ** sqrt, ulong a, n_factor_t * fac)
Computes all the square roots of ``a`` modulo ``m`` given the
factorisation of ``m`` in ``fac``. The roots are stored in an array
which is created and whose address is stored in the location pointed to by
``sqrt``. The array of roots is allocated by the function but must be
cleaned up by the user by calling :func:`flint_free`. The number of roots
is returned by the function. If ``a`` is not a quadratic residue modulo
``m`` then 0 is returned by the function and the location ``sqrt``
points to is set to NULL.
.. function:: mp_limb_t n_mulmod_shoup(mp_limb_t w, mp_limb_t t, mp_limb_t w_precomp, mp_limb_t p)
Returns `w t \bmod{p}` given a precomputed scaled approximation of `w / p`
computed by :func:`n_mulmod_precomp_shoup`. The value of `p` should be
less than `2^{\mathtt{FLINT\_BITS} - 1}`. `w` and `t` should be less than `p`.
Works faster than :func:`n_mulmod2_preinv` if `w` fixed and `t` from array
(for example, scalar multiplication of vector).
.. function:: mp_limb_t n_mulmod_precomp_shoup(mp_limb_t w, mp_limb_t p)
Returns `w'`, scaled approximation of `w / p`. `w'` is equal to the integer
part of `w * 2^{\mathtt{FLINT\_BITS}} / p`.
Divisibility testing
--------------------------------------------------------------------------------
.. function:: int n_divides(mp_limb_t * q, mp_limb_t n, mp_limb_t p)
Returns ``1`` if ``p`` divides ``n`` and sets ``q`` to the quotient,
otherwise return ``0`` and sets ``q`` to ``0``.
Prime number generation and counting
--------------------------------------------------------------------------------
.. function:: void n_primes_init(n_primes_t iter)
Initialises the prime number iterator ``iter`` for use.
.. function:: void n_primes_clear(n_primes_t iter)
Clears memory allocated by the prime number iterator ``iter``.
.. function:: ulong n_primes_next(n_primes_t iter)
Returns the next prime number and advances the state of ``iter``.
The first call returns 2.
Small primes are looked up from ``flint_small_primes``.
When this table is exhausted, primes are generated in blocks
by calling :func:`n_primes_sieve_range`.
.. function:: void n_primes_jump_after(n_primes_t iter, ulong n)
Changes the state of ``iter`` to start generating primes
after `n` (excluding `n` itself).
.. function:: void n_primes_extend_small(n_primes_t iter, ulong bound)
Extends the table of small primes in ``iter`` to contain
at least two primes larger than or equal to ``bound``.
.. function:: void n_primes_sieve_range(n_primes_t iter, ulong a, ulong b)
Sets the block endpoints of ``iter`` to the smallest and
largest odd numbers between `a` and `b` inclusive, and
sieves to mark all odd primes in this range.
The iterator state is changed to point to the first
number in the sieved range.
.. function:: void n_compute_primes(ulong num_primes)
Precomputes at least ``num_primes`` primes and their ``double``
precomputed inverses and stores them in an internal cache.
Assuming that FLINT has been built with support for thread-local storage,
each thread has its own cache.
.. function:: const ulong * n_primes_arr_readonly(ulong num_primes)
Returns a pointer to a read-only array of the first ``num_primes``
prime numbers. The computed primes are cached for repeated calls.
The pointer is valid until the user calls :func:`n_cleanup_primes`
in the same thread.
.. function:: const double * n_prime_inverses_arr_readonly(ulong n)
Returns a pointer to a read-only array of inverses of the first
``num_primes`` prime numbers. The computed primes are cached for
repeated calls. The pointer is valid until the user calls
:func:`n_cleanup_primes` in the same thread.
.. function:: void n_cleanup_primes()
Frees the internal cache of prime numbers used by the current thread.
This will invalidate any pointers returned by
:func:`n_primes_arr_readonly` or :func:`n_prime_inverses_arr_readonly`.
.. function:: ulong n_nextprime(ulong n, int proved)
Returns the next prime after `n`. Assumes the result will fit in an
``ulong``. If proved is `0`, i.e.\ false, the prime is not
proven prime, otherwise it is.
.. function:: ulong n_prime_pi(ulong n)
Returns the value of the prime counting function `\pi(n)`, i.e.\ the
number of primes less than or equal to `n`. The invariant
``n_prime_pi(n_nth_prime(n)) == n``.
Currently, this function simply extends the table of cached primes up to
an upper limit and then performs a binary search.
.. function:: void n_prime_pi_bounds(ulong *lo, ulong *hi, ulong n)
Calculates lower and upper bounds for the value of the prime counting
function ``lo <= pi(n) <= hi``. If ``lo`` and ``hi`` point to
the same location, the high value will be stored.
This does a table lookup for small values, then switches over to some
proven bounds.
The upper approximation is `1.25506 n / \ln n`, and the
lower is `n / \ln n`. These bounds are due to Rosser and
Schoenfeld [RosSch1962]_ and valid for `n \geq 17`.
We use the number of bits in `n` (or one less) to form an
approximation to `\ln n`, taking care to use a value too
small or too large to maintain the inequality.
.. function:: ulong n_nth_prime(ulong n)
Returns the `n`th prime number `p_n`, using the mathematical indexing
convention `p_1 = 2, p_2 = 3, \dotsc`.
This function simply ensures that the table of cached primes is large
enough and then looks up the entry.
.. function:: void n_nth_prime_bounds(ulong *lo, ulong *hi, ulong n)
Calculates lower and upper bounds for the `n` th prime number `p_n` ,
``lo <= p_n <= hi``. If ``lo`` and ``hi`` point to the same
location, the high value will be stored. Note that this function will
overflow for sufficiently large `n`.
We use the following estimates, valid for `n > 5` :
.. math ::
p_n & > n (\ln n + \ln \ln n - 1) \\
p_n & < n (\ln n + \ln \ln n) \\
p_n & < n (\ln n + \ln \ln n - 0.9427) \quad (n \geq 15985)
The first inequality was proved by Dusart [Dus1999]_, and the last
is due to Massias and Robin [MasRob1996]_. For a further overview,
see http://primes.utm.edu/howmany.shtml .
We bound `\ln n` using the number of bits in `n` as in
``n_prime_pi_bounds()``, and estimate `\ln \ln n` to the nearest
integer; this function is nearly constant.
Primality testing
--------------------------------------------------------------------------------
.. function:: int n_is_oddprime_small(ulong n)
Returns `1` if `n` is an odd prime smaller than
``FLINT_ODDPRIME_SMALL_CUTOFF``. Expects `n`
to be odd and smaller than the cutoff.
This function merely uses a lookup table with one bit allocated for each
odd number up to the cutoff.
.. function:: int n_is_oddprime_binary(ulong n)
This function performs a simple binary search through
the table of cached primes for `n`. If it exists in the array it returns
`1`, otherwise `0`. For the algorithm to operate correctly
`n` should be odd and at least `17`.
Lower and upper bounds are computed with :func:`n_prime_pi_bounds`.
Once we have bounds on where to look in the table, we
refine our search with a simple binary algorithm, taking
the top or bottom of the current interval as necessary.
.. function:: int n_is_prime_pocklington(ulong n, ulong iterations)
Tests if `n` is a prime using the Pocklington--Lehmer primality
test. If `1` is returned `n` has been proved prime. If `0` is returned
`n` is composite. However `-1` may be returned if nothing was proved
either way due to the number of iterations being too small.
The most time consuming part of the algorithm is factoring
`n - 1`. For this reason :func:`n_factor_partial` is used,
which uses a combination of trial factoring and Hart's one
line factor algorithm [Har2012]_ to try to quickly factor `n - 1`.
Additionally if the cofactor is less than the square root of
`n - 1` the algorithm can still proceed.
One can also specify a number of iterations if less time
should be taken. Simply set this to ``WORD(0)`` if this is irrelevant.
In most cases a greater number of iterations will not
significantly affect timings as most of the time is spent
factoring.
See
https://mathworld.wolfram.com/PocklingtonsTheorem.html
for a description of the algorithm.
.. function:: int n_is_prime_pseudosquare(ulong n)
Tests if `n` is a prime according to Theorem 2.7 [LukPatWil1996]_.
We first factor `N` using trial division up to some limit `B`.
In fact, the number of primes used in the trial factoring is at
most ``FLINT_PSEUDOSQUARES_CUTOFF``.
Next we compute `N/B` and find the next pseudosquare `L_p` above
this value, using a static table as per
https://oeis.org/A002189/b002189.txt .
As noted in the text, if `p` is prime then Step 3 will pass. This
test rejects many composites, and so by this time we suspect
that `p` is prime. If `N` is `3` or `7` modulo `8`, we are done,
and `N` is prime.
We now run a probable prime test, for which no known
counterexamples are known, to reject any composites. We then
proceed to prove `N` prime by executing Step 4. In the case that
`N` is `1` modulo `8`, if Step 4 fails, we extend the number of primes
`p_i` at Step 3 and hope to find one which passes Step 4. We take
the test one past the largest `p` for which we have pseudosquares
`L_p` tabulated, as this already corresponds to the next `L_p` which
is bigger than `2^{64}` and hence larger than any prime we might be
testing.
As explained in the text, Condition 4 cannot fail if `N` is prime.
The possibility exists that the probable prime test declares a
composite prime. However in that case an error is printed, as
that would be of independent interest.
.. function:: int n_is_prime(ulong n)
Tests if `n` is a prime. This first sieves for small prime factors,
then simply calls :func:`n_is_probabprime`. This has been checked
against the tables of Feitsma and Galway
http://www.cecm.sfu.ca/Pseudoprimes/index-2-to-64.html and thus
constitutes a check for primality (rather than just pseudoprimality)
up to `2^{64}`.
In future, this test may produce and check a certificate of
primality. This is likely to be significantly slower for prime
inputs.
.. function:: int n_is_strong_probabprime_precomp(ulong n, double npre, ulong a, ulong d)
Tests if `n` is a strong probable prime to the base `a`. We
require that `d` is set to the largest odd factor of `n - 1` and
``npre`` is a precomputed inverse of `n` computed with
:func:`n_precompute_inverse`. We also require that `n < 2^{53}`,
`a` to be reduced modulo `n` and not `0` and `n` to be odd.
If we write `n - 1 = 2^s d` where `d` is odd then `n` is a strong
probable prime to the base `a`, i.e.\ an `a`-SPRP, if either
`a^d = 1 \pmod n` or `(a^d)^{2^r} = -1 \pmod n` for some `r` less
than `s`.
A description of strong probable primes is given here:
https://mathworld.wolfram.com/StrongPseudoprime.html
.. function:: int n_is_strong_probabprime2_preinv(ulong n, ulong ninv, ulong a, ulong d)
Tests if `n` is a strong probable prime to the base `a`. We require
that `d` is set to the largest odd factor of `n - 1` and ``npre``
is a precomputed inverse of `n` computed with :func:`n_preinvert_limb`.
We require a to be reduced modulo `n` and not `0` and `n` to be odd.
If we write `n - 1 = 2^s d` where `d` is odd then `n` is a strong
probable prime to the base `a` (an `a`-SPRP) if either `a^d = 1 \pmod n`
or `(a^d)^{2^r} = -1 \pmod n` for some `r` less than `s`.
A description of strong probable primes is given here:
https://mathworld.wolfram.com/StrongPseudoprime.html
.. function:: int n_is_probabprime_fermat(ulong n, ulong i)
Returns `1` if `n` is a base `i` Fermat probable prime. Requires
`1 < i < n` and that `i` does not divide `n`.
By Fermat's Little Theorem if `i^{n-1}` is not congruent to `1`
then `n` is not prime.
.. function:: int n_is_probabprime_fibonacci(ulong n)
Let `F_j` be the `j`th element of the Fibonacci sequence
`0, 1, 1, 2, 3, 5, \dotsc`, starting at `j = 0`. Then if `n` is prime
we have `F_{n - (n/5)} = 0 \pmod n`, where `(n/5)` is the Jacobi
symbol.
For further details, see pp. 142 [CraPom2005]_.
We require that `n` is not divisible by `2` or `5`.
.. function:: int n_is_probabprime_BPSW(ulong n)
Implements a Baillie--Pomerance--Selfridge--Wagstaff probable primality
test. This is a variant of the usual BPSW test (which only uses strong
base-2 probable prime and Lucas-Selfridge tests, see Baillie and
Wagstaff [BaiWag1980]_).
This implementation makes use of a weakening of the usual Baillie-PSW
test given in [Chen2003]_, namely replacing the Lucas test with a
Fibonacci test when `n \equiv 2, 3 \pmod{5}`, (see also the comment on
page 143 of [CraPom2005]_) regarding Fibonacci pseudoprimes.
There are no known counterexamples to this being a primality test.
Up to `2^{64}` the test we use has been checked against tables of
pseudoprimes. Thus it is a primality test up to this limit.
.. function:: int n_is_probabprime_lucas(ulong n)
For details on Lucas pseudoprimes, see [pp. 143] [CraPom2005]_.
We implement a variant of the Lucas pseudoprime test similar to that
described by Baillie and Wagstaff [BaiWag1980]_.
.. function:: int n_is_probabprime(ulong n)
Tests if `n` is a probable prime. Up to ``FLINT_ODDPRIME_SMALL_CUTOFF``
this algorithm uses :func:`n_is_oddprime_small` which uses a lookup table.
Next it calls :func:`n_compute_primes` with the maximum table size and
uses this table to perform a binary search for `n` up to the table limit.
Then up to `1050535501` it uses a number of strong probable prime tests,
:func:`n_is_strong_probabprime_preinv`, etc., for various bases. The
output of the algorithm is guaranteed to be correct up to this bound due
to exhaustive tables, described at
http://uucode.com/obf/dalbec/alg.html .
Beyond that point the BPSW probabilistic primality test is used, by
calling the function :func:`n_is_probabprime_BPSW`. There are no known
counterexamples, and it has been checked against the tables of Feitsma
and Galway and up to the accuracy of those tables, this is an exhaustive
check up to `2^{64}`, i.e. there are no counterexamples.
Chinese remaindering
--------------------------------------------------------------------------------
.. function:: ulong n_CRT(ulong r1, ulong m1, ulong r2, ulong m2)
Use the Chinese Remainder Theorem to set return the unique value
`0 \le x < M` congruent to `r_1` modulo `m_1` and `r_2` modulo `m_2`,
where `M = m_1 \times m_2` is assumed to fit a ulong.
It is assumed that `m_1` and `m_2` are positive integers greater
than `1` and coprime. It is assumed that `0 \le r_1 < m_1` and `0 \le r_2 < m_2`.
Square root and perfect power testing
--------------------------------------------------------------------------------
.. function:: ulong n_sqrt(ulong a)
Computes the integer truncation of the square root of `a`.
The implementation uses a call to the IEEE floating point sqrt function.
The integer itself is represented by the nearest double and its square
root is computed to the nearest place. If `a` is one below a square, the
rounding may be up, whereas if it is one above a square, the rounding
will be down. Thus the square root may be one too large in some
instances which we then adjust by checking if we have the right value.
We also have to be careful when the square of this too large
value causes an overflow. The same assumptions hold for a single
precision float provided the square root itself can be represented
in a single float, i.e.\ for `a < 281474976710656 = 2^{46}`.
.. function:: ulong n_sqrtrem(ulong * r, ulong a)
Computes the integer truncation of the square root of `a`.
The integer itself is represented by the nearest double and its square
root is computed to the nearest place. If `a` is one below a square, the
rounding may be up, whereas if it is one above a square, the rounding
will be down. Thus the square root may be one too large in some
instances which we then adjust by checking if we have the right value.
We also have to be careful when the square of this too
large value causes an overflow. The same assumptions hold for a
single precision float provided the square root itself can be
represented in a single float, i.e. for \
`a < 281474976710656 = 2^{46}`.
The remainder is computed by subtracting the square of the computed square
root from `a`.
.. function:: int n_is_square(ulong x)
Returns `1` if `x` is a square, otherwise `0`.
This code first checks if `x` is a square modulo `64`,
`63 = 3 \times 3 \times 7` and `65 = 5 \times 13`, using lookup tables,
and if so it then takes a square root and checks that the square of this
equals the original value.
.. function:: int n_is_perfect_power235(ulong n)
Returns `1` if `n` is a perfect square, cube or fifth power.
This function uses a series of modular tests to reject most
non 235-powers. Each modular test returns a value from 0 to 7
whose bits respectively indicate whether the value is a square,
cube or fifth power modulo the given modulus. When these are
logically ``AND``-ed together, this gives a powerful test which will
reject most non-235 powers.
If a bit remains set indicating it may be a square, a standard
square root test is performed. Similarly a cube root or fifth
root can be taken, if indicated, to determine whether the power
of that root is exactly equal to `n`.
.. function:: int n_is_perfect_power(ulong * root, ulong n)
If `n = r^k`, return `k` and set ``root`` to `r`. Note that `0` and
`1` are considered squares. No guarantees are made about `r` or `k`
being the minimum possible value.
.. function:: ulong n_rootrem(ulong* remainder, ulong n, ulong root)
This function uses the Newton iteration method to calculate the nth root of
a number.
First approximation is calculated by an algorithm mentioned in this
article : https://en.wikipedia.org/wiki/Fast_inverse_square_root .
Instead of the inverse square root, the nth root is calculated.
Returns the integer part of ``n ^ 1/root``. Remainder is set as
``n - base^root``. In case `n < 1` or ``root < 1``, `0` is returned.
.. function:: ulong n_cbrt(ulong n)
This function returns the integer truncation of the cube root of `n`.
First approximation is calculated by an algorithm mentioned in this
article : https://en.wikipedia.org/wiki/Fast_inverse_square_root .
Instead of the inverse sqare root, the cube root is calculated.
This functions uses different algorithms to calculate the cube root,
depending upon the size of `n`. For numbers greater than `2^46`, it uses
:func:`n_cbrt_chebyshev_approx`. Otherwise, it makes use of the iteration,
`x \leftarrow x - (x*x*x - a)*x/(2*x*x*x + a)` for getting a good estimate,
as mentioned in the paper by W. Kahan [Kahan1991]_ .
.. function:: ulong n_cbrt_newton_iteration(ulong n)
This function returns the integer truncation of the cube root of `n`.
Makes use of Newton iterations to get a close value, and then adjusts the
estimate so as to get the correct value.
.. function:: ulong n_cbrt_binary_search(ulong n)
This function returns the integer truncation of the cube root of `n`.
Uses binary search to get the correct value.
.. function:: ulong n_cbrt_chebyshev_approx(ulong n)
This function returns the integer truncation of the cube root of `n`.
The number is first expressed in the form ``x * 2^exp``. This ensures
`x` is in the range [0.5, 1]. Cube root of x is calculated using
Chebyshev's approximation polynomial for the function `y = x^1/3`. The
values of the coefficient are calculated from the python module mpmath,
http://mpmath.org, using the function chebyfit. x is multiplied
by ``2^exp`` and the cube root of 1, 2 or 4 (according to ``exp%3``).
.. function:: ulong n_cbrtrem(ulong* remainder, ulong n)
This function returns the integer truncation of the cube root of `n`.
Remainder is set as `n` minus the cube of the value returned.
Factorisation
--------------------------------------------------------------------------------
.. function:: int n_remove(ulong * n, ulong p)
Removes the highest possible power of `p` from `n`, replacing
`n` with the quotient. The return value is that highest
power of `p` that divided `n`. Assumes `n` is not `0`.
For `p = 2` trailing zeroes are counted. For other primes
`p` is repeatedly squared and stored in a table of powers
with the current highest power of `p` removed at each step
until no higher power can be removed. The algorithm then
proceeds down the power tree again removing powers of `p`
until none remain.
.. function:: int n_remove2_precomp(ulong * n, ulong p, double ppre)
Removes the highest possible power of `p` from `n`, replacing
`n` with the quotient. The return value is that highest
power of `p` that divided `n`. Assumes `n` is not `0`. We require
``ppre`` to be set to a precomputed inverse of `p` computed
with :func:`n_precompute_inverse`.
For `p = 2` trailing zeroes are counted. For other primes
`p` we make repeated use of :func:`n_divrem2_precomp` until division
by `p` is no longer possible.
.. function:: void n_factor_insert(n_factor_t * factors, ulong p, ulong exp)
Inserts the given prime power factor ``p^exp`` into
the ``n_factor_t`` ``factors``. See the documentation for
:func:`n_factor_trial` for a description of the ``n_factor_t`` type.
The algorithm performs a simple search to see if `p` already
exists as a prime factor in the structure. If so the exponent
there is increased by the supplied exponent. Otherwise a new
factor ``p^exp`` is added to the end of the structure.
There is no test code for this function other than its use by
the various factoring functions, which have test code.
.. function:: ulong n_factor_trial_range(n_factor_t * factors, ulong n, ulong start, ulong num_primes)
Trial factor `n` with the first ``num_primes`` primes, but
starting at the prime with index start (counting from zero).
One requires an initialised ``n_factor_t`` structure, but factors
will be added by default to an already used ``n_factor_t``. Use
the function :func:`n_factor_init` defined in ``ulong_extras`` if
initialisation has not already been completed on factors.
Once completed, ``num`` will contain the number of distinct
prime factors found. The field `p` is an array of ``ulong``'s
containing the distinct prime factors, ``exp`` an array
containing the corresponding exponents.
The return value is the unfactored cofactor after trial
factoring is done.
The function calls :func:`n_compute_primes` automatically. See
the documentation for that function regarding limits.
The algorithm stops when the current prime has a square
exceeding `n`, as no prime factor of `n` can exceed this
unless `n` is prime.
The precomputed inverses of all the primes computed by
:func:`n_compute_primes` are utilised with the :func:`n_remove2_precomp`
function.
.. function:: ulong n_factor_trial(n_factor_t * factors, ulong n, ulong num_primes)
This function calls :func:`n_factor_trial_range`, with the value of
`0` for ``start``. By default this adds factors to an already existing
``n_factor_t`` or to a newly initialised one.
.. function:: ulong n_factor_power235(ulong *exp, ulong n)
Returns `0` if `n` is not a perfect square, cube or fifth power.
Otherwise it returns the root and sets ``exp`` to either `2`,
`3` or `5` appropriately.
This function uses a series of modular tests to reject most
non 235-powers. Each modular test returns a value from 0 to 7
whose bits respectively indicate whether the value is a square,
cube or fifth power modulo the given modulus. When these are
logically ``AND``-ed together, this gives a powerful test which will
reject most non-235 powers.
If a bit remains set indicating it may be a square, a standard
square root test is performed. Similarly a cube root or fifth
root can be taken, if indicated, to determine whether the power
of that root is exactly equal to `n`.
.. function:: ulong n_factor_one_line(ulong n, ulong iters)
This implements Bill Hart's one line factoring algorithm [Har2012]_.
It is a variant of Fermat's algorithm which cycles through a large number
of multipliers instead of incrementing the square root. It is faster than
SQUFOF for `n` less than about `2^{40}`.
.. function:: ulong n_factor_lehman(ulong n)
Lehman's factoring algorithm. Currently works up to `10^{16}`, but is
not particularly efficient and so is not used in the general factor
function. Always returns a factor of `n`.
.. function:: ulong n_factor_SQUFOF(ulong n, ulong iters)
Attempts to split `n` using the given number of iterations
of SQUFOF. Simply set ``iters`` to `` WORD(0)`` for maximum
persistence.
The version of SQUFOF implemented here is as described by Gower
and Wagstaff [GowWag2008]_.
We start by trying SQUFOF directly on `n`. If that fails we
multiply it by each of the primes in ``flint_primes_small`` in
turn. As this multiplication may result in a two limb value
we allow this in our implementation of SQUFOF. As SQUFOF
works with values about half the size of `n` it only needs
single limb arithmetic internally.
If SQUFOF fails to factor `n` we return `0`, however with
``iters`` large enough this should never happen.
.. function:: void n_factor(n_factor_t * factors, ulong n, int proved)
Factors `n` with no restrictions on `n`. If the prime factors are
required to be checked with a primality test, one may set
``proved`` to `1`, otherwise set it to `0`, and they will only be
probable primes. N.B: at the present there is no difference because
the probable prime tests have been exhaustively tested up to `2^{64}`.
However, in future, this flag may produce and separately check
a primality certificate. This may be quite slow (and probably no
less reliable in practice).
For details on the ``n_factor_t`` structure, see
:func:`n_factor_trial`.
This function first tries trial factoring with a number of primes
specified by the constant ``FLINT_FACTOR_TRIAL_PRIMES``. If the
cofactor is `1` or prime the function returns with all the factors.
Otherwise, the cofactor is placed in the array ``factor_arr``. Whilst
there are factors remaining in there which have not been split, the
algorithm continues. At each step each factor is first checked to
determine if it is a perfect power. If so it is replaced by the power
that has been found. Next if the factor is small enough and composite,
in particular, less than ``FLINT_FACTOR_ONE_LINE_MAX`` then
:func:`n_factor_one_line` is called with
``FLINT_FACTOR_ONE_LINE_ITERS`` to try and split the factor. If
that fails or the factor is too large for :func:`n_factor_one_line`
then :func:`n_factor_SQUFOF` is called, with
``FLINT_FACTOR_SQUFOF_ITERS``. If that fails an error results and
the program aborts. However this should not happen in practice.
.. function:: ulong n_factor_trial_partial(n_factor_t * factors, ulong n, ulong * prod, ulong num_primes, ulong limit)
Attempts trial factoring of `n` with the first ``num_primes primes``,
but stops when the product of prime factors so far exceeds ``limit``.
One requires an initialised ``n_factor_t`` structure, but factors
will be added by default to an already used ``n_factor_t``. Use
the function :func:`n_factor_init` defined in ``ulong_extras`` if
initialisation has not already been completed on ``factors``.
Once completed, ``num`` will contain the number of distinct
prime factors found. The field `p` is an array of ``ulong``'s
containing the distinct prime factors, ``exp`` an array
containing the corresponding exponents.
The return value is the unfactored cofactor after trial
factoring is done. The value ``prod`` will be set to the product
of the factors found.
The function calls :func:`n_compute_primes` automatically. See
the documentation for that function regarding limits.
The algorithm stops when the current prime has a square
exceeding `n`, as no prime factor of `n` can exceed this
unless `n` is prime.
The precomputed inverses of all the primes computed by
:func:`n_compute_primes` are utilised with the :func:`n_remove2_precomp`
function.
.. function:: ulong n_factor_partial(n_factor_t * factors, ulong n, ulong limit, int proved)
Factors `n`, but stops when the product of prime factors so far
exceeds ``limit``.
One requires an initialised ``n_factor_t`` structure, but factors
will be added by default to an already used ``n_factor_t``. Use
the function ``n_factor_init()`` defined in ``ulong_extras`` if
initialisation has not already been completed on ``factors``.
On exit, ``num`` will contain the number of distinct prime factors
found. The field `p` is an array of ``ulong``'s containing the
distinct prime factors, ``exp`` an array containing the corresponding
exponents.
The return value is the unfactored cofactor after factoring is done.
The factors are proved prime if ``proved`` is `1`, otherwise
they are merely probably prime.
.. function:: ulong n_factor_pp1(ulong n, ulong B1, ulong c)
Factors `n` using Williams' `p + 1` factoring algorithm, with prime
limit set to `B1`. We require `c` to be set to a random value. Each
trial of the algorithm with a different value of `c` gives another
chance to factor `n`, with roughly exponentially decreasing chance
of finding a missing factor. If `p + 1` (or `p - 1`) is not smooth
for any factor `p` of `n`, the algorithm will never succeed. The
value `c` should be less than `n` and greater than `2`.
If the algorithm succeeds, it returns the factor, otherwise it
returns `0` or `1` (the trivial factors modulo `n`).
.. function:: ulong n_factor_pp1_wrapper(ulong n)
A simple wrapper around ``n_factor_pp1`` which works in the range
`31`-`64` bits. Below this point, trial factoring will always succeed.
This function mainly exists for ``n_factor`` and is tuned to minimise
the time for ``n_factor`` on numbers that reach the ``n_factor_pp1``
stage, i.e. after trial factoring and one line factoring.
.. function:: int n_factor_pollard_brent_single(mp_limb_t *factor, mp_limb_t n, mp_limb_t ninv, mp_limb_t ai, mp_limb_t xi, mp_limb_t normbits, mp_limb_t max_iters)
Pollard Rho algorithm (with Brent modification) for integer factorization.
Assumes that the `n` is not prime. `factor` is set as the factor if found.
It is not assured that the factor found will be prime. Does not compute the complete
factorization, just one factor. Returns 1 if factorization is successful
(non trivial factor is found), else returns 0. Assumes `n` is normalized,
(shifted by normbits bits), and takes as input a precomputed inverse of `n` as
computed by :func:`n_preinvert_limb`. `ai` and `xi` should also be shifted
left by `normbits`.
`ai` is the constant of the polynomial used, `xi` is the initial value.
`max_iters` is the number of iterations tried in process of finding the
cycle.
The algorithm used is a modification of the original Pollard Rho algorithm,
suggested by Richard Brent in the paper, available at
https://maths-people.anu.edu.au/~brent/pd/rpb051i.pdf
.. function:: int n_factor_pollard_brent(mp_limb_t *factor, flint_rand_t state, mp_limb_t n_in, mp_limb_t max_tries, mp_limb_t max_iters)
Pollard Rho algorithm, modified as suggested by Richard Brent. Makes a call to
:func:`n_factor_pollard_brent_single`. The input parameters ai and xi for
:func:`n_factor_pollard_brent_single` are selected at random.
If the algorithm fails to find a non trivial factor in one call, it tries again
(this time with a different set of random values). This process is repeated a
maximum of `max_tries` times.
Assumes `n` is not prime. `factor` is set as the factor found, if factorization
is successful. In such a case, 1 is returned. Otherwise, 0 is returned. Factor
discovered is not necessarily prime.
Arithmetic functions
--------------------------------------------------------------------------------
.. function:: int n_moebius_mu(ulong n)
Computes the Moebius function `\mu(n)`, which is defined as `\mu(n) = 0`
if `n` has a prime factor of multiplicity greater than `1`, `\mu(n) = -1`
if `n` has an odd number of distinct prime factors, and `\mu(n) = 1` if
`n` has an even number of distinct prime factors. By convention,
`\mu(0) = 0`.
For even numbers, we use the identities `\mu(4n) = 0` and
`\mu(2n) = - \mu(n)`. Odd numbers up to a cutoff are then looked up from
a precomputed table storing `\mu(n) + 1` in groups of two bits.
For larger `n`, we first check if `n` is divisible by a small odd square
and otherwise call ``n_factor()`` and count the factors.
.. function:: void n_moebius_mu_vec(int * mu, ulong len)
Computes `\mu(n)` for ``n = 0, 1, ..., len - 1``. This
is done by sieving over each prime in the range, flipping the sign
of `\mu(n)` for every multiple of a prime `p` and setting `\mu(n) = 0`
for every multiple of `p^2`.
.. function:: int n_is_squarefree(ulong n)
Returns `0` if `n` is divisible by some perfect square, and `1` otherwise.
This simply amounts to testing whether `\mu(n) \neq 0`. As special
cases, `1` is considered squarefree and `0` is not considered squarefree.
.. function:: ulong n_euler_phi(ulong n)
Computes the Euler totient function `\phi(n)`, counting the number of
positive integers less than or equal to `n` that are coprime to `n`.
Factorials
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.. function:: ulong n_factorial_fast_mod2_preinv(ulong n, ulong p, ulong pinv)
Returns `n! \bmod p` given a precomputed inverse of `p` as computed
by :func:`n_preinvert_limb`. `p` is not required to be a prime, but
no special optimisations are made for composite `p`.
Uses fast multipoint evaluation, running in about `O(n^{1/2})` time.
.. function:: ulong n_factorial_mod2_preinv(ulong n, ulong p, ulong pinv)
Returns `n! \bmod p` given a precomputed inverse of `p` as computed
by :func:`n_preinvert_limb`. `p` is not required to be a prime, but
no special optimisations are made for composite `p`.
Uses a lookup table for small `n`, otherwise computes the product
if `n` is not too large, and calls the fast algorithm for extremely
large `n`.
Primitive Roots and Discrete Logarithms
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.. function:: ulong n_primitive_root_prime_prefactor(ulong p, n_factor_t * factors)
Returns a primitive root for the multiplicative subgroup of `\mathbb{Z}/p\mathbb{Z}`
where `p` is prime given the factorisation (``factors``) of `p - 1`.
.. function:: ulong n_primitive_root_prime(ulong p)
Returns a primitive root for the multiplicative subgroup of `\mathbb{Z}/p\mathbb{Z}`
where `p` is prime.
.. function:: ulong n_discrete_log_bsgs(ulong b, ulong a, ulong n)
Returns the discrete logarithm of `b` with respect to `a` in the
multiplicative subgroup of `\mathbb{Z}/n\mathbb{Z}` when `\mathbb{Z}/n\mathbb{Z}`
is cyclic That is,
it returns an number `x` such that `a^x = b \bmod n`. The
multiplicative subgroup is only cyclic when `n` is `2`, `4`,
`p^k`, or `2p^k` where `p` is an odd prime and `k` is a positive
integer.
Elliptic curve method for factorization of ``mp_limb_t``
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.. function:: void n_factor_ecm_double(mp_limb_t *x, mp_limb_t *z, mp_limb_t x0, mp_limb_t z0, mp_limb_t n, n_ecm_t n_ecm_inf)
Sets the point `(x : z)` to two times `(x_0 : z_0)` modulo `n` according
to the formula
``x = (x_0 + z_0)^2 \cdot (x_0 - z_0)^2 \mod n,``
``z = 4 x_0 z_0 \left((x_0 - z_0)^2 + 4a_{24}x_0z_0\right) \mod n.``
This group doubling is valid only for points expressed in
Montgomery projective coordinates.
.. function:: void n_factor_ecm_add(mp_limb_t *x, mp_limb_t *z, mp_limb_t x1, mp_limb_t z1, mp_limb_t x2, mp_limb_t z2, mp_limb_t x0, mp_limb_t z0, mp_limb_t n, n_ecm_t n_ecm_inf)
Sets the point `(x : z)` to the sum of `(x_1 : z_1)` and `(x_2 : z_2)`
modulo `n`, given the difference `(x_0 : z_0)` according to the formula
This group doubling is valid only for points expressed in
Montgomery projective coordinates.
.. function:: void n_factor_ecm_mul_montgomery_ladder(mp_limb_t *x, mp_limb_t *z, mp_limb_t x0, mp_limb_t z0, mp_limb_t k, mp_limb_t n, n_ecm_t n_ecm_inf)
Montgomery ladder algorithm for scalar multiplication of elliptic points.
Sets the point `(x : z)` to `k(x_0 : z_0)` modulo `n`.
Valid only for points expressed in Montgomery projective coordinates.
.. function:: int n_factor_ecm_select_curve(mp_limb_t *f, mp_limb_t sigma, mp_limb_t n, n_ecm_t n_ecm_inf)
Selects a random elliptic curve given a random integer ``sigma``,
according to Suyama's parameterization. If the factor is found while
selecting the curve, `1` is returned. In case the curve found is not
suitable, `0` is returned.
Also selects the initial point `x_0`, and the value of `(a + 2)/4`, where `a`
is a curve parameter. Sets `z_0` as `1` (shifted left by
``n_ecm_inf->normbits``. All these are stored in the
``n_ecm_t`` struct.
The curve selected is of Montgomery form, the points selected satisfy the
curve and are projective coordinates.
.. function:: int n_factor_ecm_stage_I(mp_limb_t *f, const mp_limb_t *prime_array, mp_limb_t num, mp_limb_t B1, mp_limb_t n, n_ecm_t n_ecm_inf)
Stage\ I implementation of the ECM algorithm.
``f`` is set as the factor if found. ``num`` is number of prime numbers
`<=` the bound ``B1``. ``prime_array`` is an array of first ``B1``
primes. `n` is the number being factored.
If the factor is found, `1` is returned, otherwise `0`.
.. function:: int n_factor_ecm_stage_II(mp_limb_t *f, mp_limb_t B1, mp_limb_t B2, mp_limb_t P, mp_limb_t n, n_ecm_t n_ecm_inf)
Stage\ II implementation of the ECM algorithm.
``f`` is set as the factor if found. ``B1``, ``B2`` are the two
bounds. ``P`` is the primorial (approximately equal to `\sqrt{B2}`).
`n` is the number being factored.
If the factor is found, `1` is returned, otherwise `0`.
.. function:: int n_factor_ecm(mp_limb_t *f, mp_limb_t curves, mp_limb_t B1, mp_limb_t B2, flint_rand_t state, mp_limb_t n)
Outer wrapper function for the ECM algorithm. It factors `n` which
must fit into a ``mp_limb_t``.
The function calls stage\ I and\ II, and
the precomputations (builds ``prime_array`` for stage\ I,
``GCD_table`` and ``prime_table`` for stage\ II).
``f`` is set as the factor if found. ``curves`` is the number of
random curves being tried. ``B1``, ``B2`` are the two bounds or
stage\ I and stage\ II. `n` is the number being factored.
If a factor is found in stage\ I, `1` is returned.
If a factor is found in stage\ II, `2` is returned.
If a factor is found while selecting the curve, `-1` is returned.
Otherwise `0` is returned.