Re: ProjectEuler problem 35

[Dmitry Olshansky]

On 16.05.2012 13:26, Tiberiu Gal wrote:

hi

many claim their code solves the problem in order of ms (
c/pascal/haskell code)

my D code takes ~1.5 seconds.
Can it be made faster ( without pointers )?
it runs the same with or without caching the primes, and most of the
time it spends finding primes; isCircularPrime can be optimized a bit,
obviously, but it's not the bottleneck.

thanks

===========================================
module te35;

import std.stdio, std.math, std.conv;

const Max = 1_000_000;

byte[Max] primes;

void main() {
primes[] = -1;
int cnt;
foreach(i; 2 .. Max) {
//writefln("is %s prime ? %s ", i, i.isPrime);
if( i.isPrime && i.isCircularPrime) {
cnt++;
//writefln("\t\tis %s, circular prime ? %s ", i, i.isCircularPrime);
}
}

writeln(cnt);

}

bool isPrime(int n) {
byte x = 0;
if( ( x = primes[n]) != -1) return (x == 1);

if( n < 2 && n > 0 ) {
primes[n] = 0;
return true;
}

//int s = cast(int) (sqrt( cast(real) n) ) + 1;
for(int i=2; i*i < n + 1; i++) {
if( n %i == 0 ) {
primes[n] = 0;
return false;
}
}

primes[n] = 1;
return true;

}

bool isCircularPrime( int n) {

auto c = to!string(n);
for(int i ; i < c.length; i++){
c = c[1 .. $] ~ c[0];

Don't ever do that. I mean allocating memory in tight cycle.
Instead use circular buffer.  (just use the same array and wrap indexes)

if( !(to!int(c) ).isPrime )

And since  to!int can't know about circular buffers.
You'd have roll your own. I don't think it's hard.

return false;
}
return true;
}

--
Dmitry Olshansky