kkck: A solver for Kriss Kross puzzles

# - - -   P u z z l e . _ _ r e d u c e C e l l

    def __reduceCell ( self, cell ):
        '''Eliminate any choices not common to two intersecting slots.

          [ cell is a Cell in self ->
              if cell is related to two slots ->
                  self.__wordBank  :=  self.__wordBank -
                      (choices for those slots that have letters at
                      cell's position not found in the intersecting
                      slot)
              else -> I ]
        '''
        if VERBOSE:
            print "=== Reduce cell %s" % cell.coord
        #-- 1 --
        # [ if two slots intersect at cell ->
        #     slotSet  :=  a list containing those two slots as
        #                  Slot instances
        #   else -> return ]
        slotSet = [ slot
                    for slot in cell.genSlots() ]
        if len(slotSet) < 2:
            return

Next, we interrogate the WordBank instance to find the set of all possible characters that could occur at the given cell; see Section 72, “WordBank.cellChoices(): What characters are choices at a given coordinate?”.

        #-- 2 --
        #     letterSet  :=  a set of one-character strings representing
        #         possible choices at cell ]
        letterSet = self.__wordBank.cellChoices ( cell.coord )
        if len(letterSet) == 0:
            print "@@@ Empty choices at %s" % cell.coord
            self.showAllChoices()
            fatal("No choices during Puzzle-reduceSlot")
        if VERBOSE: print "=== Choices:", ''.join(list(letterSet))

Finally, eliminate any choices that have letters at this cell's position that are not in letterSet; see Section 41, “Puzzle.__reduceSlot(): Eliminate choices for one slot”.

        #-- 3 --
        # [ self.__wordBank  :=  self.__wordBank - (choices for
        #       slots in slotSet whose characters at cell's position
        #       are not in letterSet ]
        for slot in slotSet:
            #-- 3 body --
            # [ self.__wordBank  :=  self.__wordBank - (choices for
            #       slot whose characters at cell's position
            #       are not in letterSet ]
            self.__reduceSlot ( cell, slot, letterSet )