import math
import random
import numpy as np

def produce_list_and_its_sum(semilength=100, valmax=1000.):
    valmax=abs(valmax)
    valmin=-valmax
    ret = np.zeros(2* semilength +1)
    for i in range(semilength):
        x = random.uniform(valmin, valmax)
        ret[2*i], ret[2*i +1] = x, -x
    y = random.uniform(valmin, valmax)
    ret[2*semilength] = y
    np.random.shuffle(ret)
    return ret, y

def produce_list_and_its_sum2(semilength=100, valmax=1000.):
    length=2*semilength
    valmax=abs(valmax)
    ret = np.zeros(2* semilength +1)
    totret = tot = random.uniform(0, valmax)
    for i in range(length):
        x = random.uniform(0, tot)
        ret[i] = x
        tot -= x
    np.random.shuffle(ret)
    return ret, totret

def do_my_sum(list_):
    ret = 0.0
    for real in list_:
        ret += real
    return ret
#    return math.fsum(list_)
#    return sum(list_)
        
def compare_sums(size=100, valmax=1000., print_liste=False):
    (list_, result) = produce_list_and_its_sum2(size//2, valmax)
    if print_liste:
        print(list_)
    max_ = list_.max()
    min_ = list_.min()
    realsize = len(list_)
    sum1 = do_my_sum(list_)
    list_.sort()
    if print_liste:
        print(list_)
    sum2 = do_my_sum(list_)
    list_[:] = list_[::-1]
    if print_liste:
        print(list_)
    sum3 = do_my_sum(list_)
    err1 = abs(result - sum1)
    err2 = abs(result - sum2)
    err3 = abs(result - sum3)
    fmt="""Taille de la liste : {}
    Max el : {}, min el : {}
    Erreurs dans les sommes : {}, {}, {}
    """
    print(fmt.format(realsize, max_, min_, err1, err2, err3))
    return (err1, err2, err3)

if __name__ == '__main__':
    errors = np.zeros((3,100))
    for i in range(100):
        e1, e2, e3 = compare_sums(50, 1000.0)
        errors[0,i] = e1
        errors[1,i] = e2
        errors[2,i] = e3
    avg1=np.average(errors[0])
    avg2=np.average(errors[1])
    avg3=np.average(errors[2])
    print(avg1, avg2, avg3)


### Remarques
#    Si on utilise des nombres negatifs et postifs alors mieux 
#    ne pas trie le tableu
#    Si on utilise des nombres positifs, 
#    mieux trier le tableau en ordre ascendant
#    Si on utilise math.fsum, les resultats sont egaux