# multiprocessing.py
#  illustration of multiple processes 

# -*- coding: utf-8 -*-
#      diese Zeile legt die Codierung von Umlauten fest
##################################################################

import sys, time, numpy as np
#-->
import multiprocessing as mp #  <--


def worker(id, input, Q):
  '''a simple worker process
     Args:
       * int id            identification number
       * float input       input
       * mp.Queue          multiprocessing Queue
  ''' 

  print ('worker %i started'%id, time.strftime('%y/%m/%d-%H:%M:%S') )

  # code to be executed on worker here --> 
  result = calcF(input)

  # send result to main process
  Q.put( (id, result) )
  print ('worker %i finished'%id, time.strftime('%y/%m/%d-%H:%M:%S') )
  return

# 
def calcF(input):
  '''MC-calculation of volume of a high-dimensional sphere with radius 1
     Args:
       * float input      seed for random generator
  ''' 
  d, npoints = 16, 1000000 # dimension of sphere & number of sampling points
  print('    volume fraction of %i-dimenional sphere and cube'%(d))
  i, inside = 0, 0.  
  np.random.seed(np.int64(1000000*input) ) # seed random generator 
  while i <= npoints:
    # test if vector of d random coordinates in [-1,1] is inside sphere
    if np.sum( (1.-2*np.random.random(d))**2 ) <= 1.: inside += 1
    i += 1
  Vratio = inside / npoints  
  err = np.sqrt(Vratio * (1. - Vratio) / npoints)
  return Vratio, err

# important:
#   this line protects rest of the script from being executed in sub-processes
if __name__ == "__main__": # - - - - - - - - - - - - - - - - - - - - - - - - -

  if len(sys.argv) < 2: 
    nworkers = 4
  else:
    nworkers = int(sys.argv[1])
    
# set up a Queue for data transfer (results from workers)
  Q =  mp.Queue(nworkers) 

# define and start worker processes 
  input = np.random.rand(nworkers)
  procs=[]
  for i in range(nworkers):
    name='worker %i'%(i)
    prc = mp.Process(name=name,
                 target = worker, 
                 args=(i,   input[i],   Q ) ) 
    procs.append(prc) #store in process list
    prc.start()
    print(' -> starting process ', prc.name, ', PID=', prc.pid)

# wait for processes to finish       
  for prc in procs:
    prc.join()

# collect results
  results = []
  print("\nresults from workers:")
  while Q.qsize():
    id, r = Q.get()
    print('  #%i:  %.3g +/- %.2g'%(id, r[0],r[1]) )