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speeding up + ctfe question
May 26, 2012
maarten van damme
May 26, 2012
sclytrack
May 26, 2012
jerro
May 26, 2012
maarten van damme
May 27, 2012
jerro
May 27, 2012
maarten van damme
May 27, 2012
sclytrack
May 27, 2012
maarten van damme
May 27, 2012
sclytrack
May 27, 2012
jerro
May 27, 2012
maarten van damme
May 28, 2012
jerro
May 28, 2012
maarten van damme
May 28, 2012
jerro
May 28, 2012
maarten van damme
```I finally made the primerange I wanted to make. I think I'm using a
rather dumb algorithm. The idea is to store the prime we're at in
curPrime and the amount of preceding primes in countPrime. Then
opindex is able to calculate how many primes following or preceding
curPrime we have to find to get the asked value.
I'm not english so some names can be poorly chosen. Code is at
http://dl.dropbox.com/u/15024434/d/primeRange.d

The problem is that it's pretty slow. I wanted to find the first prime number with 10 digits and it literally takes forever (but manages it though). Is there an easy way to speed up or is this the ceiling? (better algorithms, stupidity's I've left,...)

I also have an array of primes I use to "quicktest" to see if something is a prime. You can change it's size using setQuicktestSize at runtime. I want to make it large at compile time though. as I'm new to ctfe I have no idea as to how one can do this.
```
```On 05/26/2012 03:49 PM, maarten van damme wrote:
> I finally made the primerange I wanted to make. I think I'm using a
> rather dumb algorithm. The idea is to store the prime we're at in
> curPrime and the amount of preceding primes in countPrime. Then
> opindex is able to calculate how many primes following or preceding
> curPrime we have to find to get the asked value.
> I'm not english so some names can be poorly chosen. Code is at
> http://dl.dropbox.com/u/15024434/d/primeRange.d
>
> The problem is that it's pretty slow. I wanted to find the first prime
> number with 10 digits and it literally takes forever (but manages it
> though). Is there an easy way to speed up or is this the ceiling?
> (better algorithms, stupidity's I've left,...)
>
> I also have an array of primes I use to "quicktest" to see if
> something is a prime. You can change it's size using setQuicktestSize
> at runtime. I want to make it large at compile time though. as I'm new
> to ctfe I have no idea as to how one can do this.

See below for a rather dumb algorithm. The idea is to list
a list of prime numbers. The problem is that it is pretty
slow and you get a ...

//src/main.d(57): Error: template instance main.isDivisable!(499,498) recursive expansion

error when the number is too large. I'm not English so
the names ARE poorly chosen.

//writeln(primeList!(200)); //<---don't pick over 200 or it goes nuts
//writeln(decompose!(49,2));

template primeList(int index)
{
static if (index > 1)
{
static if (isPrime!(index))
{
enum int [] primeList =  primeList!(index-1) ~ [index];
} else
{
enum int [] primeList = primeList!(index-1);
}
}
else
{
enum int [] primeList = [];
}
}

template isDivisable( int number, int divisor)
{
enum bool isDivisable = (number % divisor) == 0;
}
//	10 ---> [2,5]

template decompose(int number, int d = 2)
{
static if (d < number)
{
static if (isDivisable!(number, d))
{
enum int [] decompose = [d] ~ decompose!(number/d, d);
}
else
{
enum int [] decompose = decompose!(number, d+1);
}
} else
{
enum int [] decompose = [number];
}
}

template isPrimeDecompose(int number, int d = 2)
{
static if (d < number)
{
static if (isDivisable!(number, d))
{
enum bool isPrimeDecompose = false;
} else
{
enum bool isPrimeDecompose = isPrimeDecompose!(number, d+1);
}
} else
{
enum bool isPrimeDecompose = true;
}
}

template isPrime(int number)
{
static if (number > 1)
enum bool isPrime = isPrimeDecompose!(number, 2);
else
enum bool isPrime = false;
}
```
```On Saturday, 26 May 2012 at 13:49:53 UTC, maarten van damme wrote:

Is there an easy way to speed up or is this the
> ceiling?

I got a 30 percent speedup by replacing this line:

if(&& canFind(quicktestPrimes, possiblePrime))

with this:

if(quicktestPrimes.back >= possiblePrime &&
canFind(quicktestPrimes, possiblePrime))

You could probably get a much better speedup by using some
kind of a prime sieve.

BTW you shouldn't be using opIndex like that. Calling opIndex
shouldn't change observable state of the object and it should
run in O(log(n)) or better. People expect an index operator to
behave similarly to the index operator on arrays. If the
function does something completely different, it is very
confusing to make it an opIndex.
```
```well, I was thinking about using a sieve but when you were to request prime 400_000 you're sieve would blow up in size. That's why I opted for something else (and I don't know if it was the right thing to do though). (Ab)using opIndex like that is indeed a bit wrong but what would be the alternative? remove slicing and introduce getPrime? Wouldn't that be the same but without the great syntactic sugar?
```
```On Saturday, 26 May 2012 at 18:40:53 UTC, maarten van damme wrote:
> well, I was thinking about using a sieve but when you were to request
> prime 400_000 you're sieve would blow up in size.

Because you only need primes up to sqrt(n) to check if n is prime,
you can make a sieve based range that only uses O(sqrt(n)) memory.
I've put an example at https://gist.github.com/2795954 .It could
be optimized further (skipping even numbers, keeping track of the
multiples of small primes...), but it is already much faster
than a range that does not use a sieve.

>That's why I
> opted
> for something else (and I don't know if it was the right thing to do
> though). (Ab)using opIndex like that is indeed a bit wrong but what
> would be the alternative? remove slicing and introduce getPrime?
> Wouldn't that be the same but without the great syntactic sugar?

Syntax sugar isn't great if it confuses people and causes bugs.
You can use r.drop(n).front to get nth element of a range.
If you need to do it often, you could just write a nth() function.
```
```well, maybe, looking back at it, a range isn't the ideal way to go about generating primes easily. I'm going to implement the sieve of Atkin and make it return an array of primes up to a given number. This might be a bit more useful and fast.

Is there somewhere I can catch up on ctfe? after reading andrei's book and the documentation it's still not clear to me how I could make my setquicktestsize function run at compile time.
```
```On 05/27/2012 01:18 PM, maarten van damme wrote:
> well, maybe, looking back at it, a range isn't the ideal way to go
> about generating primes easily. I'm going to implement the sieve of
> Atkin and make it return an array of primes up to a given number. This
> might be a bit more useful and fast.
>
> Is there somewhere I can catch up on ctfe? after reading andrei's book
> and the documentation it's still not clear to me how I could make my
> setquicktestsize function run at compile time.

https://github.com/PhilippeSigaud/D-templates-tutorial
```
```thank you :)

I wrote a sieve of aristotle because atkin's sieve needed waay to many optimizations to beat aristotle's sieve by even a little bit so I wanted to see if aristotle's was good enough.

I ran in two problems. It was extremely fast when sieving a byte array of 1 million entries (without optimizations at all) but when trying with 10_000_000 entries the program crashes before it even starts to execute (main isn't reached).

Then I decided to benchmark the code and dmd fails to compile with "Internal error: ..\ztc\cgcs.s 354".

I assume the first problem has to do with memory limits and that the second is a little dmd bug. code is at http://dl.dropbox.com/u/15024434/d/aristotleSieve.d
```
```On 05/27/2012 02:10 PM, maarten van damme wrote:
> thank you :)
>
> I wrote a sieve of aristotle because atkin's sieve needed waay to many
> optimizations to beat aristotle's sieve by even a little bit so I
> wanted to see if aristotle's was good enough.
>
> I ran in two problems. It was extremely fast when sieving a byte array
> of 1 million entries (without optimizations at all) but when trying
> with 10_000_000 entries the program crashes before it even starts to
> execute (main isn't reached).
>
> Then I decided to benchmark the code and dmd fails to compile with
> "Internal error: ..\ztc\cgcs.s 354".
>
> I assume the first problem has to do with memory limits and that the
> second is a little dmd bug. code is at
> http://dl.dropbox.com/u/15024434/d/aristotleSieve.d

Yeah I get the same errors. And I'm not acquainted with the benchmark code.
--
http://dlang.org/concepts.html
isprime there says that 2 is not a prime.
enum result = isprime(2);
compile time enum "result" says false.
--
http://erdani.com/d/web/phobos-prerelease/std_benchmark.html
(the future)
--
Er is zon buiten. Zonnige zondag namiddag met priemgetallen in de plaats van buiten zitten. Tss tss. :-)
```
```> I ran in two problems. It was extremely fast when sieving a byte array
> of 1 million entries (without optimizations at all) but when trying
> with 10_000_000 entries the program crashes before it even starts to
> execute (main isn't reached).

You say it does compile, but then crashes immediatly? I can't
reproduce that. For me it is a compile time error if toSieve
is 16MB or more, but otherwise it runs fine. Anyway you should
use a dynamic array instead of a static one if you want really
large arrays.

> Then I decided to benchmark the code and dmd fails to compile with
> "Internal error: ..\ztc\cgcs.s 354".
>
> I assume the first problem has to do with memory limits and that the
> second is a little dmd bug.

It is a bug, all internal errors are. It looks like you can
avoid like this:

auto tmp=(benchmark!(f0)(1000));
int time=tmp[0].to!("msecs", int);
```
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