345449.vhj5l3oj7.asia: ChainOfResponsibility.py

Wed Dec 26 16:00:00 GMT 2001

ChainOfResponsibility

#: c06:ChainOfResponsibility.py

# Carry the information into the strategy:
class Messenger: pass

# The Result object carries the result data and
# whether the strategy was successful:
class Result:
  def __init__(self):
    self.succeeded = 0
  def isSuccessful(self): 
    return self.succeeded 
  def setSuccessful(self, succeeded): 
    self.succeeded = succeeded

class Strategy:
  def __call__(messenger): pass
  def __str__(self): 
    return "Trying " + self.__class__.__name__ \
      + " algorithm"

# Manage the movement through the chain and
# find a successful result:
class ChainLink:
  def __init__(self, chain, strategy):
    self.strategy = strategy
    self.chain = chain
    self.chain.append(self)

def next(self):
    # Where this link is in the chain:
    location = self.chain.index(self)
    if not self.end():
      return self.chain[location + 1]

def end(self):
    return (self.chain.index(self) + 1 >= 
            len(self.chain))

def __call__(self, messenger):
    r = self.strategy(messenger)
    if r.isSuccessful() or self.end(): return r
    return self.next()(messenger)

# For this example, the Messenger
# and Result can be the same type:
class LineData(Result, Messenger):
  def __init__(self, data):
    self.data = data
  def __str__(self): return `self.data`

class LeastSquares(Strategy):
  def __call__(self, messenger):
    print self
    linedata = messenger
    # [ Actual test/calculation here ]
    result = LineData([1.1, 2.2]) # Dummy data
    result.setSuccessful(0)
    return result

class NewtonsMethod(Strategy):
  def __call__(self, messenger):
    print self
    linedata = messenger
    # [ Actual test/calculation here ]
    result = LineData([3.3, 4.4]) # Dummy data
    result.setSuccessful(0)
    return result

class Bisection(Strategy):
  def __call__(self, messenger):
    print self
    linedata = messenger
    # [ Actual test/calculation here ]
    result = LineData([5.5, 6.6]) # Dummy data
    result.setSuccessful(1)
    return result

class ConjugateGradient(Strategy):
  def __call__(self, messenger):
    print self
    linedata = messenger
    # [ Actual test/calculation here ]
    result = LineData([7.7, 8.8]) # Dummy data
    result.setSuccessful(1)
    return result

solutions = []
solutions = [
  ChainLink(solutions, LeastSquares()),
  ChainLink(solutions, NewtonsMethod()),
  ChainLink(solutions, Bisection()),
  ChainLink(solutions, ConjugateGradient())
]

line = LineData([ 
  1.0, 2.0, 1.0, 2.0, -1.0, 
  3.0, 4.0, 5.0, 4.0 
])

print solutions[0](line)
#:~

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