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missionaries_cannibals_example.py
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missionaries_cannibals_example.py
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#!/usr/bin/env python
"""
Example of using PyDDL to solve the "Missionaries and Cannibals" Problem.
A boat must transport a group of 3 missionaries and 3 cannibals across a river,
but at no time can the cannibals outnumber the missionaries at either side of
the river.
"""
from pyddl import Domain, Problem, Action, neg, planner
def problem(verbose):
domain = Domain((
Action(
'cross-right',
preconditions=(
('at', 'left-bank'),
('>', ('occupants',), 0),
),
effects=(
neg(('at', 'left-bank')),
('at', 'right-bank'),
),
),
Action(
'cross-left',
preconditions=(
('at', 'right-bank'),
('>', ('occupants',), 0),
),
effects=(
neg(('at', 'right-bank')),
('at', 'left-bank'),
),
),
Action(
'onboard-cannibal',
parameters=(
('location', 'l'),
),
preconditions=(
('at', 'l'),
('>', ('cannibals', 'l'), 0),
('<', ('occupants',), 2),
),
effects=(
('-=', ('cannibals', 'l'), 1),
('+=', ('cannibals', 'boat'), 1),
('+=', ('occupants',), 1),
),
),
Action(
'onboard-missionary',
parameters=(
('location', 'l'),
),
preconditions=(
('at', 'l'),
('>', ('missionaries', 'l'), 0),
('>', ('missionaries', 'l'), ('cannibals', 'l')),
('<', ('occupants',), 2),
),
effects=(
('-=', ('missionaries', 'l'), 1),
('+=', ('missionaries', 'boat'), 1),
('+=', ('occupants',), 1),
),
),
Action(
'offboard-cannibal',
parameters=(
('location', 'l'),
),
preconditions=(
('at', 'l'),
('>', ('cannibals', 'boat'), 0),
('>', ('missionaries', 'l'), ('cannibals', 'l')),
),
effects=(
('-=', ('cannibals', 'boat'), 1),
('-=', ('occupants',), 1),
('+=', ('cannibals', 'l'), 1),
),
),
Action(
'offboard-missionary',
parameters=(
('location', 'l'),
),
preconditions=(
('at', 'l'),
('>', ('missionaries', 'boat'), 0),
),
effects=(
('-=', ('missionaries', 'boat'), 1),
('-=', ('occupants',), 1),
('+=', ('missionaries', 'l'), 1),
),
),
))
problem = Problem(
domain,
{
'location': ('left-bank', 'right-bank'),
},
init=(
('at', 'left-bank'),
('=', ('missionaries', 'boat'), 0),
('=', ('cannibals', 'boat'), 0),
('=', ('occupants',), 0),
('=', ('missionaries', 'left-bank'), 3),
('=', ('cannibals', 'left-bank'), 3),
('=', ('missionaries', 'right-bank'), 0),
('=', ('cannibals', 'right-bank'), 0),
),
goal=(
('=', ('missionaries', 'right-bank'), 3),
('=', ('cannibals', 'right-bank'), 3),
)
)
plan = planner(problem, verbose=verbose)
if plan is None:
print('No Plan!')
else:
for action in plan:
print(action)
if __name__ == '__main__':
from optparse import OptionParser
parser = OptionParser(usage="Usage: %prog [options]")
parser.add_option('-q', '--quiet',
action='store_false', dest='verbose', default=True,
help="don't print statistics to stdout")
# Parse arguments
opts, args = parser.parse_args()
problem(opts.verbose)