Call popcount from Ruby / Python / C #

CodeIQ 2812 "Red and White" Problem. For an overview of the problem, refer to Explanation. In short, you can output 1 << popcount (n). If you solve in C language

n;main(){scanf("%d",&n);n=!printf("%d\n",1<<__builtin_popcount(n));}

However, it is necessary to devise to call this __builtin_popcount from other than C. As a result of the investigation, it was found that __sched_popcount () can be used on Linux and __popcountdi2 () can be used on OS X. The signature is ʻint __sched_popcount (size_t siz, long * a) , but it works for the time being even if you pass (8, long long [1] {…}) . This is because the latter is interpreted as ʻint [2] {…} in a 32-bit environment, and __sched_popcount returns the sum.

In Ruby / Python / C #, to call these functions: Magic number 8 should be avoided if possible. Python is the most accurate because it says ctypes.sizeof (ctypes.c_long).

Ruby

#!/usr/bin/ruby
if RUBY_PLATFORM=~/linux/
	if true
		require 'fiddle'
		__popcount_fn=Fiddle::Function.new(Fiddle::Handle::DEFAULT['__sched_cpucount'],[Fiddle::TYPE_INT,Fiddle::TYPE_VOIDP],Fiddle::TYPE_INT)
		define_method(:popcount){|n|__popcount_fn.call(8,[n].pack('q'))}
	else
		require 'fiddle/import'
		module LibC
			extend Fiddle::Importer
			dlload 'libc.so.6'
			extern 'int __sched_cpucount(int,long long*)'
		end
		def popcount(n) LibC.__popcountdi2(8,[n]) end
	end
elsif RUBY_PLATFORM=~/darwin/
	if true
		require 'fiddle'
		__popcount_fn=Fiddle::Function.new(Fiddle::Handle::DEFAULT['__popcountdi2'],[Fiddle::TYPE_LONG],Fiddle::TYPE_INT)
		define_method(:popcount){|n|__popcount_fn.call(n)}
	else
		require 'fiddle/import'
		module LibC
			extend Fiddle::Importer
			dlload 'libSystem.dylib'
			extern 'int __popcountdi2(long)'
		end
		def popcount(n) LibC.__popcountdi2(n) end
	end
else
	def popcount(n) n==0 ? 0 : popcount(n/2)+n%2 end
end

p 1<<popcount(gets.to_i)

Since Ruby of CodeIQ is 1.9.3, do as follows. It's dirty because it has global variables, but it's unavoidable.

require 'fiddle'
require 'dl'
$__popcount_fn=Fiddle::Function.new(DL::Handle::DEFAULT['__sched_cpucount'],[Fiddle::TYPE_INT,Fiddle::TYPE_VOIDP],Fiddle::TYPE_INT)
def popcount(n) $__popcount_fn.call(8,[n].pack('q')) end

Python

#!/usr/bin/python
import sys,ctypes
if sys.version_info[0]>=3:
	raw_input=input
	xrange=range

if sys.platform.startswith('linux'):
	libc=ctypes.cdll.LoadLibrary('libc.so.6')
	popcount=lambda n:libc.__sched_cpucount(ctypes.sizeof(ctypes.c_long),(ctypes.c_long*1)(n))
elif sys.platform=='darwin':
	libc=ctypes.cdll.LoadLibrary('libSystem.dylib')
	popcount=lambda n:libc.__popcountdi2(n)
else:
	popcount=lambda n:0 if n==0 else popcount(n/2)+n%2

print(1<<popcount(int(raw_input())))

using System;
using System.Runtime.InteropServices;
 
class CodeIQ2812{
	[DllImport("msvcrt",CallingConvention=CallingConvention.Cdecl)]static extern int __popcountdi2(long n);
	[DllImport("msvcrt",CallingConvention=CallingConvention.Cdecl)]static extern int __sched_cpucount(int n, long[] a);

	static int popcount(long n){
		//Comment out appropriately as there is no way to determine at compile time
		//return __popcountdi2(n);
		return __sched_cpucount(8,new long[]{n});
		//return n==0 ? 0 : popcount(n/2)+(int)(n%2);
	}
	static void Main(){
		int n=int.Parse(Console.ReadLine());
		Console.WriteLine(1<<popcount(n));
	}
}

Finally

If you're wondering if the marshalling overhead may be greater, it's probably a hit. Use this method systematically.