//

//  File:       <eGmp.cpp>

//

//  Abstract:   Calculate e using the GMP C++ library for MP floating point arithmetic. 

//

//  Version: <1.0 >

//

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// ****************************************************

#pragma mark -

#pragma mark * includes & imports *

// ----------------------------------------------------

#include <stdlib.h>

#include <strings.h>

#include <stdio.h>

#include <unistd.h>

#include <gmpxx.h>

#include <unistd.h>

#include <mach/mach_time.h> 

// ****************************************************

#pragma mark -

#pragma mark * local ( static ) function prototypes *

// ----------------------------------------------------

static void usage(char **argv);

static double currentTime(void);

static void calculateE( unsigned bits, mpf_class &e);

// ****************************************************

#pragma mark -

#pragma mark * exported function implementations *

// ----------------------------------------------------

int main(int argc, char **argv)

{

    unsigned bits;

    double start, end;

    if(argc < 2) {

        usage(argv);

    }

    bits = atoi(argv[1]);

    if(bits == 0) {

        usage(argv);

    }

    printf("Calculating e to %d bits...", bits );

    fflush(stdout);

    start = currentTime();

    mpf_set_default_prec(bits);

    mpf_class e;

    calculateE(bits, e);

    end = currentTime();

    printf("\ne = ");

    mpf_out_str(stdout, 10, 0, e.get_mpf_t());

    printf("\n");

    printf("total elapsed time : %.2f seconds\n", end - start);

    return 0;

}

// ****************************************************

#pragma mark -

#pragma mark * local ( static ) function implementations *

// ----------------------------------------------------

static void usage(char **argv)

{

    printf("usage: %s bits\n", argv[0]);

    exit(1);

}

static double currentTime(void) 

{ 

    static double scale = 0.0; 

    if (0.0 == scale) { 

        mach_timebase_info_data_t info; 

        mach_timebase_info(&info); 

        scale = info.numer / info.denom * 1e-9; 

    } 

    return mach_absolute_time() * scale; 

} 

/* 

 * Evaluate the series 1/0! + 1/1! + 1/2! + 1/3! + 1/4! ...

 * On input, 'bits' is the desired precision in bits.

 * On output, e' contains the calculated value.

 */

static void calculateE(

    unsigned bits,

    mpf_class &e)

{

    /* initial conditions, including the first two terms */

    mpf_class lastE(0.0);

    mpf_class invFact(1.0);         /* 1/2!, 1/3!, 1/4!, etc. */

    unsigned term = 2;              /* 2, 3, 4... */

    e = 2;

    for(;;) {

        invFact /= term;

        e += invFact;

        /* if e == lastE, within the requested precision, we're done */

        if(mpf_eq(e.get_mpf_t(), lastE.get_mpf_t(), bits)) {

            return;

        }

        lastE = e;

        term++;

    }

    /* NOT REACHED */

}