A solution

elevator.py

@@ -1,5 +1,6 @@
 UP = 1
 DOWN = 2
+DESTINATION = 4
 FLOOR_COUNT = 6
 
 class ElevatorLogic(object):
@@ -20,6 +21,50 @@ def __init__(self):
         # Feel free to add any instance variables you want.
         self.destination_floor = None
         self.callbacks = None
+        self.state = [0] * (FLOOR_COUNT + 1)  # UP/DOWN/DESTINATION bitmask per floor (1-based)
+        self.trend = None  # current trend (UP/DOWN/None)
+
+    def find(self, start, stop, step, trend):
+        for floor in range(start, stop, step):
+            if self.state[floor] & (trend | DESTINATION):
+                return floor
+        return None
+
+    def update_state(self, floor, set_mask, unset_mask=0):
+        # set/unset state bitmask for given floor
+        self.state[floor] = (self.state[floor] | set_mask) & ~unset_mask
+        # find the next potential up/down stops
+        # (closest stop in same direction or farthest stop in opposite direction)
+        count = len(self.state)
+        curr = self.callbacks.current_floor
+        next_up = self.find(curr + 1, count, 1, UP) or self.find(count - 1, curr, -1, DOWN)
+        next_down = self.find(curr - 1, 0, -1, DOWN) or self.find(1, curr, 1, UP)
+        # select the destination floor (taking current trend into account)
+        # and set new trend if there is none
+        if next_up and (self.trend == UP or not next_down):
+            self.destination_floor = next_up
+            self.trend = self.trend or UP
+        elif next_down and (self.trend == DOWN or not next_up):
+            self.destination_floor = next_down
+            self.trend = self.trend or DOWN
+        else:
+            self.destination_floor = None
+
+    def prepare_for_departure(self):
+        curr = self.callbacks.current_floor
+        # change trend direction if we were called here to go to opposite direction
+        opposite_trend = self.trend == UP and DOWN or self.trend == DOWN and UP or None
+        if self.state[curr] & ~DESTINATION == opposite_trend:
+            self.trend = opposite_trend
+        # clear (ignore) destinations that are not in current trend direction
+        for f in range(1, len(self.state)):
+            if self.trend == UP and f <= curr or self.trend == DOWN and f >= curr:
+                self.state[f] &= ~DESTINATION
+        # clear used state of current floor and update destination using updated trend
+        self.update_state(curr, 0, self.trend or 0)
+        # update trend according to actual destination
+        dest = self.destination_floor or curr
+        self.trend = dest > curr and UP or dest < curr and DOWN or None
 
     def on_called(self, floor, direction):
         """
@@ -30,7 +75,7 @@ def on_called(self, floor, direction):
         floor: the floor that the elevator is being called to
         direction: the direction the caller wants to go, up or down
         """
-        self.destination_floor = floor
+        self.update_state(floor, direction)
 
     def on_floor_selected(self, floor):
         """
@@ -40,7 +85,7 @@ def on_floor_selected(self, floor):
 
         floor: the floor that was requested
         """
-        self.destination_floor = floor
+        self.update_state(floor, DESTINATION)
 
     def on_floor_changed(self):
         """
@@ -56,7 +101,5 @@ def on_ready(self):
         Maybe passengers have embarked and disembarked. The doors are closed,
         time to actually move, if necessary.
         """
-        if self.destination_floor > self.callbacks.current_floor:
-            self.callbacks.motor_direction = UP
-        elif self.destination_floor < self.callbacks.current_floor:
-            self.callbacks.motor_direction = DOWN
+        self.prepare_for_departure()
+        self.callbacks.motor_direction = self.trend