#!/usr/bin/python3
"Check that an overlap function is correct for nonempty integer intervals."
from hypothesis import given
from hypothesis.strategies import integers, tuples, composite


def includes(interval, point):
    start, end = interval
    return start <= point < end

def overlaps(a, b):
    sa, ea = a
    sb, eb = b
    return not (ea <= sb or eb <= sa)

def nonempty(interval):
    s, e = interval
    return e > s


points = integers(min_value=0, max_value=100)

@composite
def intervals(draw):
    "A Hypothesis strategy to generate a nonempty interval."
    a = draw(points)
    b = draw(points)
    return (a, b) if a < b else (b, a) if b < a else (a, a+1)

@given(intervals(), intervals(), points)
def test_overlaps(a, b, p):
    assert nonempty(a)
    assert nonempty(b)
    #open('/dev/tty', 'w').write(f'{a} {b} {p}\n')

    if overlaps(a, b):
        # If they overlap, they should have *some* point in common.  I
        # claim that at least one of their start points should be such
        # a point.
        assert includes(a, b[0]) or includes(b, a[0])
    else:
        assert not (includes(a, p) and includes(b, p))