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digitalmars.D.learn - Function that calculates in compile time when it can

reply "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
I want to write a fibonacci(n) function that calculates the 
result.
a) if n is known at compile time, use a template
b) if not, use a normal function

I know how to write a template version:
template fib(ulong n)
{
	static if( n < 2 )
		const fib = n;
	else
		const fib = fib!(n-1) + fib!(n-2);
}

But how can I 'know' if n is known at compile time to make it use 
the other version? (which I won't post 'cause it is fairly easy).
Aug 06 2012
next sibling parent "Tobias Pankrath" <tobias pankrath.net> writes:
On Monday, 6 August 2012 at 12:21:38 UTC, Minas Mina wrote:

 I want to write a fibonacci(n) function that calculates the 
 result.
 a) if n is known at compile time, use a template
 b) if not, use a normal function

 I know how to write a template version:
 template fib(ulong n)
 {
 	static if( n < 2 )
 		const fib = n;
 	else
 		const fib = fib!(n-1) + fib!(n-2);
 }

 But how can I 'know' if n is known at compile time to make it 
 use the other version? (which I won't post 'cause it is fairly 
 easy).



What exactly do you try to accomplish? Why can't you use CTFE 
instead of a template?

You can check for compile time with

static if(__ctfe)
Aug 06 2012
prev sibling next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
Something like this:

template fib(ulong n)
{
	static if( n < 2 )
		const fib = n;
	else
		const fib = fib!(n-1) + fib!(n-2);
	
	if( n < 2)
		return n;
	return fib(n-1) + fib(n-2);
}

It doesn't work of course, as I am in a template and trying to 
"return" something.

CTFE? Is that "compile time function evaluation"? If yes, it's 
really slow...
If I try:
static x = fib(40); // fib is a normal function

it takes forever and makes my pc run really slowly.
Aug 06 2012
prev sibling next sibling parent "Eyyub" <eyyub.pangearaion gmail.com> writes:
On Monday, 6 August 2012 at 13:46:14 UTC, Tobias Pankrath wrote:

 You can check for compile time with

 static if(__ctfe)




__ctfe is a "CTFE-ed"(?) value.

But you can do something like that : 
http://dpaste.dzfl.pl/e3f26239

Minas : I don't know if your PC is outdated, but it's weird. :/
Aug 06 2012
prev sibling next sibling parent Artur Skawina <art.08.09 gmail.com> writes:
On 08/06/12 15:46, Tobias Pankrath wrote:

 On Monday, 6 August 2012 at 12:21:38 UTC, Minas Mina wrote:

 I want to write a fibonacci(n) function that calculates the result.
 a) if n is known at compile time, use a template
 b) if not, use a normal function

 I know how to write a template version:
 template fib(ulong n)
 {
     static if( n < 2 )
         const fib = n;
     else
         const fib = fib!(n-1) + fib!(n-2);
 }

 But how can I 'know' if n is known at compile time to make it use the other
version? (which I won't post 'cause it is fairly easy).



 What exactly do you try to accomplish? Why can't you use CTFE instead of a
template?



He wants the function to be evaluated at compile time if that is possible,
but at runtime if that can't be done; and *not* have to handle both cases
in every caller. Which I can't think of any way to do right now.

An "T fib(T)(enum T n)" overload that would enable cases like this (and
bearophiles ranged-integers) to work, has been proposed in the past (only
with 'static' instead of 'enum'; the latter fits better).
IOW a "fib(42)" call would map to more or less:

   enum T c = 42;
   fib(c);

and inside such an overload the argument 'n' would be treated just like if it
had been defined as:

   enum T n = 42;

ie it would be cfte'able and /then/ static-if and friends would be able to 
handle the rest.

artur
Aug 06 2012
prev sibling next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Monday, 6 August 2012 at 14:25:29 UTC, Eyyub wrote:

 On Monday, 6 August 2012 at 13:46:14 UTC, Tobias Pankrath wrote:

 You can check for compile time with

 static if(__ctfe)




 __ctfe is a "CTFE-ed"(?) value.

 But you can do something like that : 
 http://dpaste.dzfl.pl/e3f26239

 Minas : I don't know if your PC is outdated, but it's weird. :/



It's Ubuntu 12.04 running on an Intel i5.

That's what I tried to calculate:
static x = fib(40);

ulong fib(ulong n)
{
	if(n < 2)
		return n;
	else
		return fib(n-1) +  fib(n-2);
}

Maybe because it has a lot of recursion?
Aug 06 2012
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Mon, Aug 6, 2012 at 3:54 PM, Minas Mina <minas_mina1990 hotmail.co.uk> wrote:

 Something like this:


 template fib(ulong n)
 {
         static if( n < 2 )
                 const fib = n;
         else
                 const fib = fib!(n-1) + fib!(n-2);

         if( n < 2)
                 return n;
         return fib(n-1) + fib(n-2);
 }

 It doesn't work of course, as I am in a template and trying to "return"
 something.

 CTFE? Is that "compile time function evaluation"? If yes, it's really
 slow...




 If I try:
 static x = fib(40); // fib is a normal function

 it takes forever and makes my pc run really slowly.



Well, you're using the worst possible algorithm to calculate Fibonacci
(exponential time), so it's no wonder it's taking foverer :)
Then, you've to know that CT calculation is far slower than runtime
calculation. My experience on this is about an order of magnitude
slower, and even more. On the machine I'm currently on, fib(30) is
calculated instantaneously at runtime, but it takes 4-6s at CT.
Fib(40) aloready takes 4-6 s at runtime, so I won't test at CT :)

To come back to your question. __ctfe should be used with a standard
(non-static) if.
Here I implement to Fibonacci algos, one linear in n at CT, one
exponential ar RT.
That's just to show that a good algo at CT can run circles around a
bad algo at RT.
At compile-time, getting fib(100) is instantaneous, while getting only
fib(40) at RT takes a few seconds on my machine.

import std.conv: to;
import std.stdio;

long fib(size_t n)
{
	if(__ctfe) // compile-time, linear, sustained development
	{
		long[] temp = new long[](n+1); // dynamic array during CTFE, D rox
		temp[0] = 1;
		temp[1] = 1;
		size_t p = 1;
		while (p < n)
		{
			++p;
			temp[p] = temp[p-1]+temp[p-2];
		}
		return -temp[p]; // '-' as an indication that this indeed took place at CT
	}
	else // runtime, exponential, woohoo baby!
	{	
		if (n == 0 || n == 1)
		        return 1;
		else
			return fib(n-1)+fib(n-2);
	}
}

void main()
{
	enum f1 = fib(100); // CT
	pragma(msg, "At CT, fib(100) = " ~to!string(f1)); // will be < 0 as a flag
	auto f2 = fib(40); // RT
	writeln("At RT, fib(40) = ", f2);
}

Don't try fib(100) at runtime!
Aug 06 2012
prev sibling next sibling parent "Tobias Pankrath" <tobias pankrath.net> writes:
On Monday, 6 August 2012 at 15:18:22 UTC, Minas Mina wrote:

 That's what I tried to calculate:
 static x = fib(40);

 ulong fib(ulong n)
 {
 	if(n < 2)
 		return n;
 	else
 		return fib(n-1) +  fib(n-2);
 }

 Maybe because it has a lot of recursion?



That algorithm makes O(2^n) calls to fib. I think templates get 
only expanded
once for every set of parameters, so you get memoization build in 
and thus it's faster in this case.
Aug 06 2012
prev sibling next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Monday, 6 August 2012 at 15:21:38 UTC, Philippe Sigaud wrote:

 On Mon, Aug 6, 2012 at 3:54 PM, Minas Mina 
 <minas_mina1990 hotmail.co.uk> wrote:

 Something like this:


 template fib(ulong n)
 {
         static if( n < 2 )
                 const fib = n;
         else
                 const fib = fib!(n-1) + fib!(n-2);

         if( n < 2)
                 return n;
         return fib(n-1) + fib(n-2);
 }

 It doesn't work of course, as I am in a template and trying to 
 "return"
 something.

 CTFE? Is that "compile time function evaluation"? If yes, it's 
 really
 slow...




 If I try:
 static x = fib(40); // fib is a normal function

 it takes forever and makes my pc run really slowly.



 Well, you're using the worst possible algorithm to calculate 
 Fibonacci
 (exponential time), so it's no wonder it's taking foverer :)
 Then, you've to know that CT calculation is far slower than 
 runtime
 calculation. My experience on this is about an order of 
 magnitude
 slower, and even more. On the machine I'm currently on, fib(30) 
 is
 calculated instantaneously at runtime, but it takes 4-6s at CT.
 Fib(40) aloready takes 4-6 s at runtime, so I won't test at CT 
 :)

 To come back to your question. __ctfe should be used with a 
 standard
 (non-static) if.
 Here I implement to Fibonacci algos, one linear in n at CT, one
 exponential ar RT.
 That's just to show that a good algo at CT can run circles 
 around a
 bad algo at RT.
 At compile-time, getting fib(100) is instantaneous, while 
 getting only
 fib(40) at RT takes a few seconds on my machine.

 import std.conv: to;
 import std.stdio;

 long fib(size_t n)
 {
 	if(__ctfe) // compile-time, linear, sustained development
 	{
 		long[] temp = new long[](n+1); // dynamic array during CTFE, 
 D rox
 		temp[0] = 1;
 		temp[1] = 1;
 		size_t p = 1;
 		while (p < n)
 		{
 			++p;
 			temp[p] = temp[p-1]+temp[p-2];
 		}
 		return -temp[p]; // '-' as an indication that this indeed 
 took place at CT
 	}
 	else // runtime, exponential, woohoo baby!
 	{	
 		if (n == 0 || n == 1)
 		        return 1;
 		else
 			return fib(n-1)+fib(n-2);
 	}
 }

 void main()
 {
 	enum f1 = fib(100); // CT
 	pragma(msg, "At CT, fib(100) = " ~to!string(f1)); // will be < 
 0 as a flag
 	auto f2 = fib(40); // RT
 	writeln("At RT, fib(40) = ", f2);
 }

 Don't try fib(100) at runtime!



Thank you for your reply!

Haha, yeah, I knew I was using the worst possible algorithm. I
Aug 06 2012
prev sibling next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Monday, 6 August 2012 at 15:21:38 UTC, Philippe Sigaud wrote:

 On Mon, Aug 6, 2012 at 3:54 PM, Minas Mina 
 <minas_mina1990 hotmail.co.uk> wrote:

 Something like this:


 template fib(ulong n)
 {
         static if( n < 2 )
                 const fib = n;
         else
                 const fib = fib!(n-1) + fib!(n-2);

         if( n < 2)
                 return n;
         return fib(n-1) + fib(n-2);
 }

 It doesn't work of course, as I am in a template and trying to 
 "return"
 something.

 CTFE? Is that "compile time function evaluation"? If yes, it's 
 really
 slow...




 If I try:
 static x = fib(40); // fib is a normal function

 it takes forever and makes my pc run really slowly.



 Well, you're using the worst possible algorithm to calculate 
 Fibonacci
 (exponential time), so it's no wonder it's taking foverer :)
 Then, you've to know that CT calculation is far slower than 
 runtime
 calculation. My experience on this is about an order of 
 magnitude
 slower, and even more. On the machine I'm currently on, fib(30) 
 is
 calculated instantaneously at runtime, but it takes 4-6s at CT.
 Fib(40) aloready takes 4-6 s at runtime, so I won't test at CT 
 :)

 To come back to your question. __ctfe should be used with a 
 standard
 (non-static) if.
 Here I implement to Fibonacci algos, one linear in n at CT, one
 exponential ar RT.
 That's just to show that a good algo at CT can run circles 
 around a
 bad algo at RT.
 At compile-time, getting fib(100) is instantaneous, while 
 getting only
 fib(40) at RT takes a few seconds on my machine.

 import std.conv: to;
 import std.stdio;

 long fib(size_t n)
 {
 	if(__ctfe) // compile-time, linear, sustained development
 	{
 		long[] temp = new long[](n+1); // dynamic array during CTFE, 
 D rox
 		temp[0] = 1;
 		temp[1] = 1;
 		size_t p = 1;
 		while (p < n)
 		{
 			++p;
 			temp[p] = temp[p-1]+temp[p-2];
 		}
 		return -temp[p]; // '-' as an indication that this indeed 
 took place at CT
 	}
 	else // runtime, exponential, woohoo baby!
 	{	
 		if (n == 0 || n == 1)
 		        return 1;
 		else
 			return fib(n-1)+fib(n-2);
 	}
 }

 void main()
 {
 	enum f1 = fib(100); // CT
 	pragma(msg, "At CT, fib(100) = " ~to!string(f1)); // will be < 
 0 as a flag
 	auto f2 = fib(40); // RT
 	writeln("At RT, fib(40) = ", f2);
 }

 Don't try fib(100) at runtime!





Thank you for your reply!

Haha, yeah, I knew I was using the worst possible algorithm. It 
was just for testing... I'm never going to use ctfe with 
algorithms of that complexity again!

Ok, so I can use if(__ctfe) to define different behaviour(or not) 
at compile time than at runtime. The way I want to use it, 
however, I don't have to:

import std.stdio;

void main()
{
	ulong f1 = fibonacci(50); // calculated at runtime
	static f2 = fibonacci(50); // calculated at compile time
	
	writeln(f);
}

// calcuated at O(1) woohoo!
ulong fibonacci(ulong n)
{
	import std.math;
	
	double
		r0 = (1 + sqrt(5.0)) / 2.0,
		r1 = (1 - sqrt(5.0)) / 2.0,

		a =  1.0 / sqrt(5.0),
		b = -1.0 / sqrt(5.0);

	// fn = a*r0 + b*r1
	return cast(ulong)(a * pow(r0, cast(double)n) + b * pow(r1, 
cast(double)n));
}


What I was really looking for was to do something like the way 
std.math.sin works. I read somewhere that if the argument is 
available at compile time, it evaluates the result at compile 
time.
So, if I do:
double f = sin(0.5); // calculated at compile time or not?

If it is calculated at compile time, how do I do it for my own 
functions?
If not, I guess the only way is to use static or enum like you 
guys showed me.
Aug 06 2012
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Mon, Aug 6, 2012 at 5:33 PM, Minas Mina <minas_mina1990 hotmail.co.uk> wrote:


 // calcuated at O(1) woohoo!




         return cast(ulong)(a * pow(r0, cast(double)n) + b * pow(r1,



I wonder if there is any rounding error going on with these pow, but
yes, even better this way.


 double f = sin(0.5); // calculated at compile time or not?

 If it is calculated at compile time, how do I do it for my own functions?



sin is a library function, it has no special rights your own function
doesn't as far as I know.

I'd say, put an if(__ctfe) in sin:

if (__ctfe)
{
    pragma(msg, "Yep, CT");
    return 2.0;
}
else
{
    // standard sin code
}

And tell us the result :)
Aug 06 2012
prev sibling next sibling parent "Namespace" <rswhite4 googlemail.com> writes:
Take a look into std.math:
https://github.com/D-Programming-Language/phobos/blob/master/std/math.d
Aug 06 2012
prev sibling next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Monday, 6 August 2012 at 15:56:52 UTC, Namespace wrote:

 Take a look into std.math:
 https://github.com/D-Programming-Language/phobos/blob/master/std/math.d



I found this:

real sin(real x)  safe pure nothrow; /* intrinsic */

And these:

creal sin(creal z)  safe pure nothrow
{
      creal cs = expi(z.re);
      creal csh = coshisinh(z.im);
      return cs.im * csh.re + cs.re * csh.im * 1i;
}

/** ditto */
ireal sin(ireal y)  safe pure nothrow
{
      return cosh(y.im)*1i;
}

I don't see anything about ctfe. Maybe I didn't understand well
and sin is not evaluated at compile time. Can someone clarify
this?
Aug 06 2012
prev sibling next sibling parent "Eyyub" <eyyub.pangearaion gmail.com> writes:
On Monday, 6 August 2012 at 16:17:22 UTC, Minas Mina wrote:

 On Monday, 6 August 2012 at 15:56:52 UTC, Namespace wrote:

 Take a look into std.math:
 https://github.com/D-Programming-Language/phobos/blob/master/std/math.d



 I found this:

 real sin(real x)  safe pure nothrow; /* intrinsic */

 And these:

 creal sin(creal z)  safe pure nothrow
 {
      creal cs = expi(z.re);
      creal csh = coshisinh(z.im);
      return cs.im * csh.re + cs.re * csh.im * 1i;
 }

 /** ditto */
 ireal sin(ireal y)  safe pure nothrow
 {
      return cosh(y.im)*1i;
 }

 I don't see anything about ctfe. Maybe I didn't understand well
 and sin is not evaluated at compile time. Can someone clarify
 this?



Justly, you see a normal runtime function, and a CTFE function is
a normal runtime function(with some restrictions) that can be
interpreted at runtime.

static res = sin(4); //compile time
auto res = sin(4); //runtime
Aug 06 2012
prev sibling next sibling parent "Eyyub" <eyyub.pangearaion gmail.com> writes:
On Monday, 6 August 2012 at 16:21:04 UTC, Eyyub wrote:

 Justly, you see a normal runtime function, and a CTFE function 
 is
 a normal runtime function(with some restrictions) that can be
 interpreted at runtime.
Aug 06 2012
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Mon, Aug 6, 2012 at 6:17 PM, Minas Mina <minas_mina1990 hotmail.co.uk> wrote:


 I found this:

 real sin(real x)  safe pure nothrow; /* intrinsic */




 I don't see anything about ctfe. Maybe I didn't understand well
 and sin is not evaluated at compile time. Can someone clarify
 this?



Oh, I thought they were implemented as polynomial approximations.

Never mind, you can test it with any function:

int foo(int i)
{
    if (__ctfe)
    {
        pragma(msg, "CT");
        return -1;
    }
    else
        return 1;
}

void main()
{
    auto i = foo(0);
    static j = foo(0);
    writeln(i); // 1
    writeln(j); // -1
}

So, auto i doesn't provoke CT-evaluation (else it'd be -1).
Aug 06 2012
prev sibling next sibling parent Russel Winder <russel winder.org.uk> writes:
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Mon, 2012-08-06 at 17:21 +0200, Philippe Sigaud wrote:
[=E2=80=A6]

 Well, you're using the worst possible algorithm to calculate Fibonacci
 (exponential time), so it's no wonder it's taking foverer :)



Memoization is a bit of a help it destroying that problem.

[=E2=80=A6]

 Don't try fib(100) at runtime!



Of course the real problem is that quants and such folk often need
values of Fibonacci and factorial that cannot be held in an hardware
integer.
--=20
Russel.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder ekiga.n=
et
41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel winder.org.uk
London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder
Aug 06 2012
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
On Mon, Aug 6, 2012 at 8:37 PM, Russel Winder <russel winder.org.uk> wrote:

 On Mon, 2012-08-06 at 17:21 +0200, Philippe Sigaud wrote:
 [=E2=80=A6]

 Well, you're using the worst possible algorithm to calculate Fibonacci
 (exponential time), so it's no wonder it's taking foverer :)



 Memoization is a bit of a help it destroying that problem.



Memoization at compile-time is a bit tricky, though.

Which reminds me... What if someone wants Fibonacci values depending
on a runtime value, but would like them to be calculated really fast?
Use the "Sabalausky Trick" (aka Nick's Run-Time to Compile-Time Time
Machine): Pregenerate desired values at compile-time (for example here
in an array), to get O(1) fibonacci access at runtime.
Aug 06 2012
prev sibling next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 8/6/12, Philippe Sigaud <philippe.sigaud gmail.com> wrote:

 Which reminds me... What if someone wants Fibonacci values depending
 on a runtime value, but would like them to be calculated really fast?



I think TDPL had some use of the memoize template and used fibonacci
as sample code. It even used it internally for a recursive call. It's
somewhere in the book.
Aug 06 2012
prev sibling next sibling parent Marco Leise <Marco.Leise gmx.de> writes:
Am Mon, 06 Aug 2012 14:21:37 +0200
schrieb "Minas Mina" <minas_mina1990 hotmail.co.uk>:


 I want to write a fibonacci(n) function that calculates the 
 result.
 a) if n is known at compile time, use a template
 b) if not, use a normal function
 
 I know how to write a template version:
 template fib(ulong n)
 {
 	static if( n < 2 )
 		const fib = n;
 	else
 		const fib = fib!(n-1) + fib!(n-2);
 }
 
 But how can I 'know' if n is known at compile time to make it use 
 the other version? (which I won't post 'cause it is fairly easy).



That's easy: http://dpaste.dzfl.pl/521f47a0

-- 
Marco
Aug 07 2012
prev sibling parent Artur Skawina <art.08.09 gmail.com> writes:
On 08/07/12 19:43, Marco Leise wrote:

 Am Mon, 06 Aug 2012 14:21:37 +0200
 schrieb "Minas Mina" <minas_mina1990 hotmail.co.uk>:
 

 I want to write a fibonacci(n) function that calculates the 
 result.
 a) if n is known at compile time, use a template
 b) if not, use a normal function

 I know how to write a template version:
 template fib(ulong n)
 {
 	static if( n < 2 )
 		const fib = n;
 	else
 		const fib = fib!(n-1) + fib!(n-2);
 }

 But how can I 'know' if n is known at compile time to make it use 
 the other version? (which I won't post 'cause it is fairly easy).



 That's easy: http://dpaste.dzfl.pl/521f47a0



It's not a function, but a template - meaning not only does it require
a different syntax, but also that it can only be used with literals and
symbols. Ie 'fib!(expression_not_evaluable_at_ct)' like 'fib!(variable+1)'
will fail to compile.

artur

PS. Your code does not work with older compilers (like the 2.057 based
gdc that i'm using); this version works and is also more readable:

   template isSymbol(alias S) {
      enum isSymbol = __traits(compiles, __traits(parent, S));
   }
   
   template fib(alias N) {
      static if (isSymbol!N) {
          property typeof(N+N) fib(typeof(N) N = N) {
            return N<2 ? N : fib(N-1)+fib(N-2);
         }
      }
      else {
         static if (N<2)
            alias N fib;
         else
            enum fib = fib!(N-1)+fib!(N-2);
      }
   }
Aug 08 2012