8_puzzle_problem_using_A_star_algorithm.py

'''
8 PUZZLE PROBLEM SOLVING USING A* ALGORITHM

An instance of the n-puzzle game consists of a board holding n^{2}-1 
distinct movable tiles, plus an empty space. The tiles are numbers from 
the set 1,..,n^{2}-1. For any such board, the empty space may be legally 
swapped with any tile horizontally or vertically adjacent to it. In this 
assignment, the blank space is going to be represented with the number 0. 
Given an initial state of the board, the combinatorial search problem is 
to find a sequence of moves that transitions this state to the goal state; 
that is, the configuration with all tiles arranged in ascending order 
0,1,..,n^{2}-1.

So, this is the goal state that we want to reach:
[1, 2, 3]
[8, 0, 4]
[7, 6, 5] 

The search space is the set of all possible states reachable from the 
initial state. The blank space may be swapped with a component in one of 
the four directions {‘Up’, ‘Down’, ‘Left’, ‘Right’}, one move at a time.

A* algorithm has 3 parameters:
g: the cost of moving from the initial cell to the current cell.
h: also known as the heuristic value, it is the estimated cost of moving from 
   the current cell to the final cell. The actual cost cannot be calculated until 
   the final cell is reached. Hence, h is the estimated cost. We must make sure 
   that there is never an over estimation of the cost.
f: it is the sum of g and h. So, f = g + h
We always go to the state that has minimum 'f' value.

'''

# importing the necessary libraries
from time import time
from queue import PriorityQueue

# creating a class Puzzle
class Puzzle:
    # setting the goal state of 8-puzzle
    goal_state=[1,2,3,8,0,4,7,6,5]
    # setting up the members of a class
    heuristic=None
    evaluation_function=None
    needs_hueristic=False
    num_of_instances=0
    
    # constructor to initialize the class members
    def __init__(self,state,parent,action,path_cost,needs_hueristic=False):
        self.parent=parent
        self.state=state
        self.action=action
        # calculating the path_cost as the sum of its parent cost and path_cost
        if parent:
            self.path_cost = path_cost + parent.path_cost
        else:
            self.path_cost = path_cost
        if needs_hueristic:
            self.needs_hueristic=True
            self.generate_heuristic()
            # calculating the expression as f = g + h
            self.evaluation_function= path_cost + needs_hueristic
        # incrementing the number of instance by 1
        Puzzle.num_of_instances+=1
    
    # method used to display a state of 8-puzzle
    def __str__(self):
        return str(self.state[0:3])+'\n'+str(self.state[3:6])+'\n'+str(self.state[6:9])

    # method used to generate a heuristic value
    def generate_heuristic(self):
        self.heuristic=0
        for num in range(1,9):
            # calculating the heuristic value as manhattan distance which is the absolute 
            # difference between current state and goal state. 
            # using index() method to get the index of num in state
            distance= abs(self.state.index(num) - self.goal_state.index(num)) 
            i=int(distance/3)
            j=int(distance%3)
            self.heuristic=self.heuristic+i+j

    def goal_test(self):
        # including a condition to compare the current state with the goal state
        if self.state == self.goal_state:
            return True
        return False

    @staticmethod
    def find_legal_actions(i,j):
        # find the legal actions as Up, Down, Left, Right based on each cell of state
        legal_action = ['U', 'D', 'L', 'R']
        if i == 0:  # up is disable
            # if row is 0 in board then up is disable
            legal_action.remove('U')
        elif i == 2:  
            legal_action.remove('D')
        if j == 0:
            legal_action.remove('L')
        elif j == 2:
            legal_action.remove('R')
         # returnig legal_action
        return legal_action

    # method to generate the child of the current state of the board
    def generate_child(self):
        children=[]
        x = self.state.index(0)
        # generating the row (i) & col (j) position based on the current index of 0 on the board 
        i = int(x/3)
        j = int(x%3)
        # calling the method to find the legal actions based on i and j values
        legal_actions=self.find_legal_actions(i,j)

        for action in legal_actions:
            new_state = self.state.copy()
            # if the legal action is UP
            if action is 'U':
                # swapping between current index of 0 with its up element on the board
                new_state[x], new_state[x-3] = new_state[x-3], new_state[x]
            elif action is 'D':
                # swapping between current index of 0 with its down element on the board
                new_state[x], new_state[x+3] = new_state[x+3], new_state[x]
            elif action is 'L':
                # swapping between the current index of 0 with its left element on the board
                new_state[x], new_state[x-1] = new_state[x-1], new_state[x]
            elif action is 'R':
                # swapping between the current index of 0 with its right element on the board
                new_state[x], new_state[x+1] = new_state[x+1], new_state[x]
            # appending the new_state of Puzzle object with parent, action,path_cost is 1, its needs_hueristic flag
            children.append(Puzzle(new_state, self, action, 1, self.needs_hueristic))
        # returning the children
        return children
    
    # method to find the solution
    def find_solution(self):
        solution = []
        all_states = []
        solution.append(self.action)
        all_states.append(self)
        path = self
        while path.parent != None:
            path = path.parent
            solution.append(path.action)
            all_states.append(path)
        solution = solution[:-1]
        solution.reverse

        print('\nAll states from goal to initial: ')
        for i in all_states:
          print(i, '\n')
        return solution
    
# method for A-star search
# passing the initial_state as parameter to the breadth_first_search method
def Astar_search(initial_state):
    count=0
    # creating an empty list of explored nodes
    explored=[]
    # creating a instance of Puzzle as initial_state, None, None, 0, True
    start_node=Puzzle(initial_state, None, None, 0, True)
    q = PriorityQueue()
    # putting a tuple with start_node.evaluation_function, count, start_node into PriorityQueue
    q.put((start_node.evaluation_function, count, start_node))

    while not q.empty():
        # getting the current node of a queue. Use the get() method of Queue
        node=q.get()
        # extracting the current node of a PriorityQueue based on the index of a tuple. 
        # referring a tuple format put in PriorityQueue  
        node=node[2]
        # appending the state of node in the explored list as node.state
        explored.append(node.state)
        if node.goal_test():
            return node.find_solution()
        # calling the generate_child method to generate the child node of current node
        children=node.generate_child()
        for child in children:
            if child.state not in explored:
                count += 1
                # putting a tuple with child.evaluation_function, count, child into PriorityQueue
                q.put((child.evaluation_function, count, child))
    return

# starting executing the 8-puzzle with setting up the initial state
# here we have considered 3 initial state intitalized using state variable
state= [1, 3, 4,
        8, 6, 2,
        7, 0, 5]

# initializing the num_of_instances to zero
Puzzle.num_of_instances = 0
# set t0 to current time
t0 = time()
astar = Astar_search(state)
# getting the time t1 after executing the breadth_first_search method
t1 = time() - t0
print('A*:',astar)
print('space:', Puzzle.num_of_instances)
print('time:', t1)
print()
print('------------------------------------------')
    
'''
Sample working:

All states from goal to initial: 
[1, 2, 3]
[8, 0, 4]
[7, 6, 5] 

[1, 0, 3]
[8, 2, 4]
[7, 6, 5] 

[1, 3, 0]
[8, 2, 4]
[7, 6, 5] 

[1, 3, 4]
[8, 2, 0]
[7, 6, 5] 

[1, 3, 4]
[8, 0, 2]
[7, 6, 5] 

[1, 3, 4]
[8, 6, 2]
[7, 0, 5] 

A*: ['D', 'L', 'U', 'R', 'U']
space: 117
time: 0.0029821395874023438

'''