.. _mpn-extras:

**mpn_extras.h** -- support functions for limb arrays
===============================================================================

Macros
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.. function:: MACRO MPN_NORM(a, an)

    Normalise ``(a, an)`` so that either ``an`` is zero or 
    ``a[an - 1]`` is nonzero.

.. function:: MACRO MPN_SWAP(a, an, b, bn)

    Swap ``(a, an)`` and ``(b, bn)``, i.e. swap pointers and sizes.


Utility functions
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.. function:: void flint_mpn_debug(mp_srcptr x, mp_size_t xsize)

    Prints debug information about ``(x, xsize)`` to ``stdout``. 
    In particular, this will print binary representations of all the limbs.

.. function:: int flint_mpn_zero_p(mp_srcptr x, mp_size_t xsize)

    Returns `1` if all limbs of ``(x, xsize)`` are zero, otherwise `0`.


Divisibility
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.. function:: int flint_mpn_divisible_1_p(x, xsize, d) (macro)

    Expression determining whether ``(x, xsize)`` is divisible by the
    ``mp_limb_t d`` which is assumed to be odd-valued and at least~`3`.

    This function is implemented as a macro.

.. function:: mp_size_t flint_mpn_divexact_1(mp_ptr x, mp_size_t xsize, mp_limb_t d)

    Divides `x` once by a known single-limb divisor, returns the new size.

.. function:: mp_size_t flint_mpn_remove_2exp(mp_ptr x, mp_size_t xsize, mp_bitcnt_t *bits)

    Divides ``(x, xsize)`` by `2^n` where `n` is the number of trailing 
    zero bits in `x`. The new size of `x` is returned, and `n` is stored in 
    the bits argument. `x` may not be zero.

.. function:: mp_size_t flint_mpn_remove_power_ascending(mp_ptr x, mp_size_t xsize, mp_ptr p, mp_size_t psize, ulong *exp)

    Divides ``(x, xsize)`` by the largest power `n` of ``(p, psize)`` 
    that is an exact divisor of `x`. The new size of `x` is returned, and 
    `n` is stored in the ``exp`` argument. `x` may not be zero, and `p` 
    must be greater than `2`.

    This function works by testing divisibility by ascending squares
    `p, p^2, p^4, p^8, \dotsc`, making it efficient for removing potentially
    large powers. Because of its high overhead, it should not be used as
    the first stage of trial division.

.. function:: int flint_mpn_factor_trial(mp_srcptr x, mp_size_t xsize, slong start, slong stop)

    Searches for a factor of ``(x, xsize)`` among the primes in positions 
    ``start, ..., stop-1`` of ``flint_primes``. Returns `i` if 
    ``flint_primes[i]`` is a factor, otherwise returns `0` if no factor 
    is found. It is assumed that ``start >= 1``.


Division
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.. function:: int flint_mpn_divides(mp_ptr q, mp_srcptr array1, mp_size_t limbs1, mp_srcptr arrayg, mp_size_t limbsg, mp_ptr temp)

    If ``(arrayg, limbsg)`` divides ``(array1, limbs1)`` then
    ``(q, limbs1 - limbsg + 1)`` is set to the quotient and 1 is 
    returned, otherwise 0 is returned. The temporary space ``temp``
    must have space for ``limbsg`` limbs.

    Assumes limbs1 ``limbs1 >= limbsg > 0``.

.. function:: mp_limb_t flint_mpn_preinv1(mp_limb_t d, mp_limb_t d2)

    Computes a precomputed inverse from the leading two limbs of the
    divisor ``b, n`` to be used with the ``preinv1`` functions.
    We require the most significant bit of ``b, n`` to be 1.

.. function:: mp_limb_t flint_mpn_divrem_preinv1(mp_ptr q, mp_ptr a, mp_size_t m, mp_srcptr b, mp_size_t n, mp_limb_t dinv)

    Divide ``a, m`` by ``b, n``, returning the high limb of the 
    quotient (which will either be 0 or 1), storing the remainder in-place 
    in ``a, n`` and the rest of the quotient in ``q, m - n``.
    We require the most significant bit of ``b, n`` to be 1.
    dinv must be computed from ``b[n - 1]``, ``b[n - 2]`` by 
    ``flint_mpn_preinv1``. We also require ``m >= n >= 2``.

.. function:: void flint_mpn_mulmod_preinv1(mp_ptr r, mp_srcptr a, mp_srcptr b, mp_size_t n, mp_srcptr d, mp_limb_t dinv, ulong norm)

    Given a normalised integer `d` with precomputed inverse ``dinv`` 
    provided by ``flint_mpn_preinv1``, computes `ab \pmod{d}` and
    stores the result in `r`. Each of `a`, `b` and `r` is expected to 
    have `n` limbs of space, with zero padding if necessary. 

    The value ``norm`` is provided for convenience. If `a`, `b` and
    `d` have been shifted left by ``norm`` bits so that `d` is
    normalised, then `r` will be shifted right by ``norm`` bits
    so that it has the same shift as all the inputs.

    We require `a` and `b` to be reduced modulo `n` before calling the
    function. 

.. function:: void flint_mpn_preinvn(mp_ptr dinv, mp_srcptr d, mp_size_t n)

    Compute an `n` limb precomputed inverse ``dinv`` of the `n` limb
    integer `d`.

    We require that `d` is normalised, i.e. with the most significant
    bit of the most significant limb set.

.. function:: void flint_mpn_mod_preinvn(mp_ptr r, mp_srcptr a, mp_size_t m, mp_srcptr d, mp_size_t n, mp_srcptr dinv)

    Given a normalised integer `d` of `n` limbs, with precomputed inverse
    ``dinv`` provided by ``flint_mpn_preinvn`` and integer `a` of `m`
    limbs, computes `a \pmod{d}` and stores the result in-place in the lower
    `n` limbs of `a`. The remaining limbs of `a` are destroyed.

    We require `m \geq n`. No aliasing of `a` with any of the other operands
    is permitted.

    Note that this function is not always as fast as ordinary division.

.. function:: mp_limb_t flint_mpn_divrem_preinvn(mp_ptr q, mp_ptr r, mp_srcptr a, mp_size_t m, mp_srcptr d, mp_size_t n, mp_srcptr dinv)

    Given a normalised integer `d` with precomputed inverse ``dinv`` 
    provided by ``flint_mpn_preinvn``, computes the quotient of `a` by `d` 
    and stores the result in `q` and the remainder in the lower `n` limbs of
    `a`. The remaining limbs of `a` are destroyed.

    The value `q` is expected to have space for `m - n` limbs and we require
    `m >= n`. No aliasing is permitted between `q` and `a` or between these
    and any of the other operands. 

    Note that this function is not always as fast as ordinary division.

.. function:: void flint_mpn_mulmod_preinvn(mp_ptr r, mp_srcptr a, mp_srcptr b, mp_size_t n, mp_srcptr d, mp_srcptr dinv, ulong norm)

    Given a normalised integer `d` with precomputed inverse ``dinv`` 
    provided by ``flint_mpn_preinvn``, computes `ab \pmod{d}` and
    stores the result in `r`. Each of `a`, `b` and `r` is expected to 
    have `n` limbs of space, with zero padding if necessary. 

    The value ``norm`` is provided for convenience. If `a`, `b` and
    `d` have been shifted left by ``norm`` bits so that `d` is
    normalised, then `r` will be shifted right by ``norm`` bits
    so that it has the same shift as all the inputs.

    We require `a` and `b` to be reduced modulo `n` before calling the
    function. 

    Note that this function is not always as fast as ordinary division.


GCD
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.. function:: mp_size_t flint_mpn_gcd_full(mp_ptr arrayg, mp_ptr array1, mp_size_t limbs1, mp_ptr array2, mp_size_t limbs2)

    Sets ``(arrayg, retvalue)`` to the gcd of ``(array1, limbs1)`` and
    ``(array2, limbs2)``. 

    The only assumption is that neither ``limbs1`` or ``limbs2`` is
    zero.


Random Number Generation
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.. function:: void flint_mpn_rrandom(mp_limb_t *rp, gmp_randstate_t state, mp_size_t n)

    Generates a random number with ``n`` limbs and stores 
    it on ``rp``. The number it generates will tend to have
    long strings of zeros and ones in the binary representation.
    
    Useful for testing functions and algorithms, since this kind of random
    numbers have proven to be more likely to trigger corner-case bugs.
    

.. function:: void flint_mpn_urandomb(mp_limb_t *rp, gmp_randstate_t state, mp_bitcnt_t n)

    Generates a uniform random number ``n`` bits and stores 
    it on ``rp``.