Foo.bar zombit monitoring

zombit.py

def intersect(P, i):
    ps = []
    for p in P:
        if p[0] <= i[0] and p[1] >= i[0]:
            ps.append(p)
        elif p[0] <= i[1] and p[1] >= i[1]:
            ps.append(p)
        elif p[0] >= i[0] and p[1] <= i[1]:
            ps.append(p)
    return ps

def add(*ps):
    n_min = None
    n_max = None
    for p in ps:
        if n_min is None or n_min > p[0]:
            n_min = p[0]
        if n_max is None or n_max < p[1]:
            n_max = p[1]
    return [n_min, n_max]

def reduce(I):
    P = []
    for i in I:
        ps = intersect(P, i)
        if len(ps):
            new_p = add(i, *ps)
            for p in ps:
                P.remove(p)
            P.append(new_p)
        else:
            P.append(i)
    return P

def answer(I):
    P = reduce(I)
    s = 0
    for p in P:
        s+= p[1] - p[0]
    return s