def transitionSequence(state1,state2,diff):

	return []

"""Theory:

We have a sequence S of valid board states s_1, ..., s_n.

We search for a picked subsequence S_p of length k and m additional moves M such that:

- S_p augmented by M forms a valid sequence of moves

- k-m is maximal for S_p picked from S

It is relatively cheap to add new items to S.

User can change detector parameters, presumably in case the previous don't fit the (current) situation.

In such a case we assume the new parameters are correct from the current position onwards.

But the change might have been appropriate even earlier (before the user detected and fixed an error).

So we try to find the correct crossover point like this:

- construct a sequence S' = s'_i, ..., s'_n by reanalyzing the positions with a new set of parameters, where s_i is the point of previous user intervention or s_0

- for each s'_j:

	- try to append it to S[:j]

	- try to append it to S'[:j]

	- remember the better variant

- linearize the fork back by discarding s'_j-s preceding the crossover and s_j-s following the crossover

"""

from util import BLACK,WHITE,EMPTY

from go import transitionSequence

## Crude lower bound on edit distance between states.

def estimateDistance(diff):

	additions=sum(1 for d in diff if d[2]=="+")

	deletions=sum(1 for d in diff if d[2]=="-")

	replacements=len(diff)-additions-deletions

	if additions>0: return additions+replacements

	elif replacements==0 and deletions>0: return 2 # take n, return 1

	return replacements # ???

class BoardState:

	def __init__(self,board):

		self._board=tuple(tuple(x for x in row) for row in board)

		self.prev=None

		self._next=None

		self.moves=[]

		self.weight=0

		self.diff2Prev=None

	def __iter__(self): return iter(self._board)

	def __getitem__(self,key): return self._board[key]

	def __sub__(self,x):

		res=[]

		for (r,(row1,row2)) in enumerate(zip(self._board,x)):

			for (c,(item1,item2)) in enumerate(zip(row1,row2)):

				if item1==item2: continue

				elif item2==EMPTY: res.append((r,c,"+",item1))

				elif item1==EMPTY: res.append((r,c,"-",item2))

				else: res.append((r,c,item2,item1)) # from->to

		return res

		sn=BoardState(board)

		for s in reversed(self._states):

			diff=sn-s

			distEst=estimateDistance(diff)

			if distEst>3: continue # we couldn't find every such move sequence anyway without a clever algorithm

			weightEst=s.weight-distEst

			if weightEst<=sn.weight: continue

			moves=transitionSequence(s,sn,diff)

			weight=s.weight-len(moves)

			if weight<=sn.weight: continue

			sn.prev=s

			sn.diff2Prev=diff

			sn.moves=moves

			sn.weight=weight

		self._states.append(sn)

	def merge(self,branch):

		pass