Compute the adjusted rand_index

rand_index_adjusted.py

def rand_stat_adjusted(C, P):
    """Return Rand statistic.
    C and P are two iterables.
    """
    sz = len(C)
    M = a = b = c= d = 0
    for i in range(sz-1):
        for j in range(i+1,sz):
            M += 1
            if C[i] == C[j]:
                if P[i]==P[j]:
                    a += 1
                else:
                    b += 1
            else:
                if P[i] == P[j]:
                    c += 1
                else:
                    d += 1
    expected = ((a+b)*(a+c))+((c+d)*(b+d))
    RIA = ((M*(a+d))-expected) / float(M**2 - expected)
    return RIA