from ctypes import c_float, c_int32, cast, byref, POINTER
import sys
from random import randint
import time
import struct
'''
from: https://github.com/ajcr/ajcr.github.io/blob/master/_posts/2016-04-01-fast-inverse-square-root-python.md
Using ctypes, not the fastest version. 
'''
def ctypes_isqrt(number):
    threehalfs = 1.5
    x2 = number * 0.5
    y = c_float(number)

    i = cast(byref(y), POINTER(c_int32)).contents.value
    i = c_int32(0x5f3759df - (i >> 1))
    y = cast(byref(i), POINTER(c_float)).contents.value

    y = y * (1.5 - (x2 * y * y))
    return y

def struct_isqrt(number):
    threehalfs = 1.5
    x2 = number * 0.5
    y = number
    
    packed_y = struct.pack('f', y)       
    i = struct.unpack('i', packed_y)[0]  # treat float's bytes as int 
    i = 0x5f3759df - (i >> 1)            # arithmetic with magic number
    packed_i = struct.pack('i', i)
    y = struct.unpack('f', packed_i)[0]  # treat int's bytes as float
    
    y = y * (threehalfs - (x2 * y * y))  # Newton's method
    return y

def norm_isq(number):
	return 1.0/(number**0.5)

def main(argv):
	numSamples = long(argv[0])
	samples = [randint(1,10000) for i in range(0, numSamples)]
	startTime = time.time()
	[norm_isq(n) for n in samples]
	print("normal functions ran by python: %.6f seconds." % (time.time() - startTime))
	startTime = time.time()
	# calling fast inverse square root:
	[struct_isqrt(n) for n in samples]
	print("fast inverse square root struct: %.6f seconds." % (time.time() - startTime))
	# calling fast inverse square root:
	[ctypes_isqrt(n) for n in samples]
	print("fast inverse square root ctypes: %.6f seconds." % (time.time() - startTime))

if __name__ == "__main__":
    main(sys.argv[1:])