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GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it

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#include "typedef.h"

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/**************************************************************************\

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@---------------------------------------------------------------------------

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@---------------------------------------------------------------------------

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@ FILE: chin_remainder.c

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@---------------------------------------------------------------------------

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@---------------------------------------------------------------------------

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@

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\**************************************************************************/

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/**************************************************************************\

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@-------------------------------------------------------------------------

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@ chinrest(x1, x2, p1, p2) calculates for positive primes p1,p2

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@ and integers x1, x2 an integer x with

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@   -(p1*p2)/2 < x < (p1*p2)/2 and

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@ x kongruent x1 modulo p1 and x konguent x2 modulo p2

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@-------------------------------------------------------------------------

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\**************************************************************************/

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int chin_remainder(x1, x2, p1, p2)

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int x1, x2, p1, p2;

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{

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  int a1, a2;

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  double q, q1, q2, res, waste;

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  int y1, y2;

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  a1 = p_inv(p2, p1);

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  a2 = p_inv(p1, p2);

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  q = ((double) p1) * ((double) p2);

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  q1 = ((double) p2) * ((double) a1);

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  q2 = ((double) p1) * ((double) a2);

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    y1 = x1%p1;

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    if(2*y1 <= -p1)

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      y1 +=p1;

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    if(2*y1 > p1)

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      y1 -=p1;

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    y2 = x2%p2;

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    if(2*y2 <= -p2)

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      y2 +=p2;

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    if(2*y2 > p2)

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      y2 -=p2;

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    if(y1 == y2)

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      return(y1);

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    res = ((double) y1) * q1 + ((double) y2) * q2;

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    modf(res/q, &waste);

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    res = res - waste * q;

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    while( (res/q) > 0.5)

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       res = res -q;

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    while( (res/q) < -0.5)

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       res = res + q;

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    return(((int) res));

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}

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