def _get_nmer(self, n):
"""Get the string of the last N bases on the path stack.
n: aninteger or integer-castable value; nonnegative
Returns None if n is greater than the number of searchnodes on the
path stack. Otherwise, returns a string of the values of those
nodes, in the order they were pushed on the stack
"""
nmer = []
result = None
n = makeNonnegInt(n)
#only try to get the nmer if there are at least n items on stack
if n <= len(self._path_stack):
#for n to 0 -- start with the furthest-back entry
for temp_length in xrange(n, 0, -1):
#Note that we add one bc temp length is base 1, while
#top index isn't. Ex: a stack with length 5 has top
#index 4. If you want the last 4, you want to start
#at index 1. Thus, 4 - 4 + 1 = 1
temp_index = self._top_index - temp_length +1
#get value of node at that index and add to list
nmer.append(self._path_stack[temp_index].Value)
#next intermediate length
#join list to get current nmer and return
result = "".join(nmer)
#end if
return result
def _get_alphabet(self, position):
"""Return the alphabet for input position. If none, return default
position: a nonnegative integer or something castable to it.
Positions are assumed to be ZERO based.
"""
position = makeNonnegInt(position)
if position in self._alphabets:
result = self._alphabets[position]
else:
result = self._alphabets[self.DEFAULT_KEY]
#end if
return result
def generate(self, path_length):
"""Generate a valid path of required length and return its value.
Returns None if no path is possible.
path_length: a nonnegative integer or castable to it. Indicates
length of desired valid path.
"""
path_length = makeNonnegInt(path_length)
#while length of path stack < path_length
while len(self._path_stack) < path_length:
#always have to get the alphabet based on the current
#top index plus 1. This is because, if we have to remove
#a node because it contributes to a forbidden sequence,
#we need to make sure that the position used to generate
#the next node is the same
position = self._top_index
if position is None: position = -1
position += 1
#get the alphabet for the next node
curr_alphabet = self._get_alphabet(position)
#make new SearchNode and push it onto path
new_node = SearchNode(curr_alphabet)
self._add_node(new_node)
#select the next available allowed option
option_exists = self.findAllowedOption()
#if no more options, no valid searchpath is possible
#break out and return None
if not option_exists: return None
#end while
#return path as string
return self.Value
