#! /usr/bin/env python3
#
# stop_go.py
#
"""
Example of creating a "behavior tree" in the AtomSpace.
Behavior trees can be thought of as long sequences of nested
if-then-else statements; the branch that is taken results in
a "behavior" being performed.
Behavior trees are usually driven by dynamic data; however, to keep
the example simple, the below is static; it simply counts green and
red lights, and halts at the first red light.
The if-then is implemented via a matching clause with the pattern
matcher. When a match is seen, the matcher moves on to the next
clause.
"""
from opencog.atomspace import *
from opencog.atomspace import *
from opencog.logger import Logger, log
# Logging will be written to opencog.log in the current directory.
log.set_level('DEBUG')
log.info("Starting the stop-go demo")
# The callback counts the number of red and green lights.
# It returns True for green lights and False the red lights.
# False is interpreted as a mismatch (failure to satisfy)
# by the query engine, and thus, the query engine will backtrack
# and search for a different solution. Since the example below contains
# no variables, it will just backtrack to the start, and then report
# non-satisfiability (which is what we want, when we get a red light).
green = 0
red = 0
def stop_go(atom):
if atom == ConceptNode("green light"):
print("Got a green light...")
global green
green += 1
return True
elif atom == ConceptNode("red light"):
print("Got a red light!")
global red
red += 1
return False
else:
print("Oh No!! Car wreck!")
assert(false)
return None
# This is the pattern that the pattern matcher attempts to ground.
# It consists of two green lights, which evaluate to true, followed
# by a red light, which halts the satisfiability search. The last
# term is thus never encountered. Which is a good thing.
satisfaction_handle = SatisfactionLink(
SequentialAndLink(
EvaluationLink(
GroundedPredicateNode("py: stop_go"),
ListLink(
ConceptNode("green light")
)
),
EvaluationLink(
GroundedPredicateNode("py: stop_go"),
ListLink(
ConceptNode("green light")
)
),
EvaluationLink(
GroundedPredicateNode("py: stop_go"),
ListLink(
ConceptNode("red light")
)
),
EvaluationLink(
GroundedPredicateNode("py: stop_go"),
ListLink(
ConceptNode("traffic ticket")
)
)
)
)
# Perform the actual satisfiability search.
result = satisfaction_handle.execute()
print("Satisfaction query result:", result)
print("Number of green lights:", green)
print("Number of red lights:", red)