cat inv.f
/* inv.f May 3, 1987 */
inv(u,v,w) /* This function generates the modular inverse through a process
which was developed using bc and avoids reentrant code. Inverter4 is the
name of the bc script which this function was developed from.      */
char *u , *v, *w ;
{
        char *a[15],*b[15],*x[15],*y[15],*k[15],*z[15] ;
        char *r , *t , *s , *f, *malloc() ;
        int i, j ;
        r = malloc(15) ;
        t = malloc(15) ;
        s = malloc(15) ;
        f = malloc(15) ;
        j = 0 ;
        for( i = 0 ; i < 15 ; i++) a[i] = malloc(15) ;
        for( i = 0 ; i < 15 ; i++) b[i] = malloc(15) ;
        for( i = 0 ; i < 15 ; i++) x[i] = malloc(15) ;
        for( i = 0 ; i < 15 ; i++) y[i] = malloc(15) ;
        for( i = 0 ; i < 15 ; i++) k[i] = malloc(15) ;
        for( i = 0 ; i < 15 ; i++) z[i] = malloc(15) ;
        copy(u,r) ;
        copy(v,t) ;
        while(strcmp(r,one) > 0)
        {
                copy(r,a[j]) ;
                copy(t,b[j]) ;
                copy(b[j],r) ;
                residue(r,a[j]) ;
                copy(r,s) ;
                copy(a[j],r) ;
                sub(r,s) ;
                copy(b[j],x[j]) ;
                divide(x[j],a[j]) ;
                add(x[j],one) ;
                copy(one,y[j]) ;
                copy(b[j],f) ;
                residue(f,r) ;
                while(strcmp(f,xero) > 0 )
                {
                        copy(b[j],t) ;
                        divide(t,r) ;
                        add(t,one) ;
                        multiply(x[j],t,v) ;
                        multiply(y[j],t,v) ;
                        add(y[j],one) ;
                        multiply(y[j],one,v) ;
                        copy(b[j],f) ;
                        residue(f,r) ;
                        sub(r,f) ;
                        copy(b[j],f) ;
                        residue(f,r) ;
                }
                copy(a[j],t) ;
                j = j + 1 ;
        }
















        j = j - 1 ;
        copy(x[j],z[j]) ;
        copy(y[j],k[j]) ;
        if ( j == 0)
        {       copy(z[0],w) ;
                return ;
        }
        while( j > 0 )
        {
                i = j ;
                j = j - 1 ;
                copy(z[i],z[j]) ;
                multiply(z[j],x[j],v);
                while(strcmp(z[j],k[i]) < 0) add(z[j],v) ;
                sub(z[j],k[i] ) ;
                multiply(z[j],one,v) ;
                copy(z[i],k[j] ) ;
                multiply(k[j],y[j],v) ;
        }
copy(z[0],w) ;
}

$