Calculate Standard Deviation.

example.txt

π pry
>> handle = Handle.new './libsd.so'
=> #<Fiddle::Handle:0x00005646ea646220>
>> sd = Function.new handle['standard_deviation'], [TYPE_VOIDP, TYPE_INT], TYPE_FLOAT
=> #<Fiddle::Function:0x00005646ea670890 @abi=2, @args=[1, 4], @ptr=140326241698073, @return_type=7>
>> arr = [*1..10]
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>> ptr = arr.pack("i#{arr.size}")
=> "\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04\x00\x00\x00\x05\x00\x00\x00\x06\x00\x00\x00\a\x00\x00\x00\b\x00\x00\x00\t\x00\x00\x00\n\x00\x00\x00"
>> sd.(ptr, arr.size)
=> 2.872281312942505
>> 

standard_deviation.c

#include <math.h>

float standard_deviation(int a[], int size)
{
  float sum = 0.0, sd = 0.0, m;
  int i;

  for(i = 0; i < size; i++)
    sum += a[i];

  m = sum / size;

  for(i = 0; i < size; i++)
    sd += pow(a[i] - m, 2);

  return sqrt(sd / size);
}


// gcc -g -Wall -shared -o libsd.so -fPIC standard_deviation.c

time

>> def ms_time
 |   s = Process.clock_gettime Process::CLOCK_MONOTONIC, :float_millisecond  
 |   yield  
 |   Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_millisecond) - s  
 | end  
=> :ms_time
>> 
>> handle = Handle.new './libsd.so'
=> #<Fiddle::Handle:0x00005610c18d94b0>
>> sd = Function.new handle['standard_deviation'], [TYPE_VOIDP, TYPE_INT], TYPE_FLOAT
=> #<Fiddle::Function:0x00005610c1908f30 @abi=2, @args=[1, 4], @ptr=140166056644889, @return_type=7>
>> arr = [*1..10_000_000];
>> ptr = arr.pack("i#{arr.size}");
>> 
>> ms_time { sd.(ptr, arr.size) }
=> 132.675954002887
>>