fox_hen_grain.py

fox_hen_grain.py

from hypothesis import note, settings
from hypothesis.stateful import RuleBasedStateMachine, rule, invariant, precondition
from hypothesis import strategies as st


class Item():
    def __init__(self, name):
        self.crossed = False
        self.name = name

class FoxHenGrainProblem(RuleBasedStateMachine):
    solutions = {}

    def __init__(self):
        super().__init__()
        self.start()

    def start(self):
        self.items = []
        self.items.append(Item('Fox'))
        self.items.append(Item('Grain'))
        self.items.append(Item('Hen'))
        self.farmer = Item('Farmer')
        self.moves = []

    @precondition(lambda self: self.farmer.crossed)
    @rule(choice=st.choices())
    def make_return_move(self, choice):
        items = [x for x in self.items if x.crossed]
        if len(items) > 1:
            if choice([True, False]):
                item = choice(items)
                self.moves.append([item, 'back'])
                item.crossed = False
        self.farmer.crossed = False

    @precondition(lambda self: not self.farmer.crossed)
    @rule(choice=st.choices())
    def make_forward_move(self, choice):
        items = [x for x in self.items if not x.crossed]
        item1 = choice(items)
        self.moves.append([item1, 'fwd'])
        item1.crossed = True
        self.farmer.crossed = True

    @invariant()
    def items_safe(self):
        if self.farmer.crossed:
            items = [x for x in self.items if not x.crossed]
            if len(items) == 0:
                self.write_notes()
                assert False
        else:
            items = [x for x in self.items if x.crossed]
        if len(items) == 2:
            items.sort(key=lambda x: x.name)
            if items[0].name == 'Fox' and items[1].name == 'Hen':
                self.start()
            elif items[0].name == 'Grain' and items[1].name == 'Hen':
                self.start()

    def write_notes(self):
        for x in self.moves:
            note("%s" % [x[0].name, x[1]])


# The default of 200 is sometimes not enough for Hypothesis to find
# a falsifying example.
with settings(max_examples=700):
    DieHardTest = FoxHenGrainProblem.TestCase