Graph Modeling Patterns - Learning Module

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Implicit Graphs (State Space)

Graphs Too Large to Build

In the previous page, we learned to convert problems into graphs. But what happens when the graph is too large to construct? Consider a Rubik's Cube solver: there are over 43 quintillion (43 × 10¹⁸) possible configurations. You cannot build an adjacency list with 43 quintillion vertices.

Yet we can still solve these problems—by never building the graph at all. Instead, we define the graph implicitly through rules that generate neighbors on demand. This is the power of state space graphs and implicit graph traversal.

What You Will Learn

By the end of this page, you will understand implicit graphs—graphs defined by rules rather than explicit storage. You'll learn to design state representations, implement neighbor generation functions, and apply graph algorithms to enormous state spaces without running out of memory.

What Is an Implicit Graph?

An implicit graph (also called a state space graph) is a graph where:

Vertices are not stored in memory—they represent possible states of a system
Edges are not pre-computed—they are generated by applying transition rules to states
The graph is explored on-demand during traversal algorithms

The key insight is that graph algorithms like BFS and DFS don't actually require the full graph in memory. They only need:

A starting vertex
A function to generate neighbors of any given vertex
A way to track visited vertices

Explicit Graph

•All vertices stored in memory
•Adjacency list/matrix pre-built
•O(V + E) space required upfront
•Best when: graph is small, reused, or fully explored
•Example: Road network for navigation app

Implicit Graph

•Vertices generated on-demand
•Neighbors computed via rules
•Only O(visited vertices) space
•Best when: state space is huge, only partial exploration needed
•Example: Puzzle solver (Rubik's Cube)

The State Space Perspective

Think of the problem as a universe of possible 'states.' Each state is a vertex. Valid transitions between states are edges. You start at an initial state and search for a goal state. This frame—called state space search—is foundational in AI, game solving, and planning algorithms.

Anatomy of an Implicit Graph

Every implicit graph problem requires you to define three components:

Component 1: State Representation

How do you encode a 'state' (vertex) of the system? This must:

Capture all information needed to determine valid transitions
Be hashable/comparable for visited tracking
Be as compact as possible to minimize memory

Component 2: Transition Function (Neighbor Generator)

Given a state, what states can you reach in one step? This function:

Takes a state as input
Returns all valid neighbor states
Encodes the 'rules' of the problem

Component 3: Goal Condition

How do you know when you've found the answer? This is a predicate:

Takes a state as input
Returns true if this state is a solution

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from collections import deque
from typing import TypeVar, Callable, Iterable, Set, Optional
 
State = TypeVar('State')
 
def implicit_bfs(
    start: State,
    get_neighbors: Callable[[State], Iterable[State]],
    is_goal: Callable[[State], bool],
    state_to_hashable: Callable[[State], any] = lambda x: x
) -> Optional[int]:
    """
    Generic BFS on an implicit graph.
    
    Args:
        start: The initial state
        get_neighbors: Function that generates neighboring states
        is_goal: Function that returns True if state is a goal
        state_to_hashable: Convert state to hashable form for visited set
    
    Returns:
        Minimum steps to reach a goal state, or None if unreachable
    """
    if is_goal(start):
        return 0
    
    queue = deque([(start, 0)])  # (state, distance)
    visited: Set = {state_to_hashable(start)}
    
    while queue:
        current_state, distance = queue.popleft()
        
        # Generate neighbors on-demand (implicit graph!)
        for neighbor_state in get_neighbors(current_state):
            neighbor_hash = state_to_hashable(neighbor_state)
            
            if neighbor_hash not in visited:
                if is_goal(neighbor_state):
                    return distance + 1
                
                visited.add(neighbor_hash)
                queue.append((neighbor_state, distance + 1))
    
    return None  # Goal not reachable

The Power of Abstraction

Notice how the BFS template doesn't know anything about the specific problem. It only knows about states, neighbors, and goals. This separation of concerns is powerful—the same template works for puzzles, path finding, game solving, and countless other problems.

Case Study: Sliding Puzzle (8-Puzzle)

The classic 8-puzzle (a 3×3 sliding tile puzzle) is a perfect example of implicit graph search:

The Problem:

Given a 3×3 board with tiles numbered 1-8 and one empty space, slide tiles to reach the goal state. Find the minimum number of moves.

State Space Analysis:

Total possible states: 9! = 362,880
Each state has 2-4 neighbors (depending on empty space position)
Solvable states: 181,440 (exactly half—others are in a different 'parity' component)

This is small enough that we could build the explicit graph, but it's more instructive to treat it implicitly.

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from collections import deque
from typing import List, Tuple, Optional
 
def solve_8_puzzle(initial: List[List[int]]) -> Optional[int]:
    """
    Solve the 8-puzzle using BFS on implicit state space graph.
    
    State Representation: tuple of tuples (hashable)
    Transitions: Swap empty space with adjacent tile
    Goal: [[1,2,3],[4,5,6],[7,8,0]] where 0 is empty
    
    Time Complexity: O(V) where V = reachable states (≤ 181,440)
    Space Complexity: O(V) for visited set and queue
    """
    # Define goal state
    GOAL = ((1, 2, 3), (4, 5, 6), (7, 8, 0))
    
    # Convert initial board to hashable tuple
    def to_tuple(board: List[List[int]]) -> Tuple[Tuple[int, ...], ...]:
        return tuple(tuple(row) for row in board)
    
    # Find position of empty space (0)
    def find_empty(state: Tuple[Tuple[int, ...], ...]) -> Tuple[int, int]:
        for i in range(3):
            for j in range(3):
                if state[i][j] == 0:
                    return (i, j)
        return (-1, -1)  # Should never happen
    
    # Generate all valid neighbor states (THE TRANSITION FUNCTION)
    def get_neighbors(state: Tuple[Tuple[int, ...], ...]):
        """
        Generate neighbor states by moving the empty space.
        This is the core of the implicit graph definition.
        """
        empty_row, empty_col = find_empty(state)
        neighbors = []
        
        # Four possible moves: up, down, left, right
        directions = [(-1, 0), (1, 0), (0, -1), (0, 1)]
        
        for dr, dc in directions:
            new_row, new_col = empty_row + dr, empty_col + dc
            
            # Check bounds
            if 0 <= new_row < 3 and 0 <= new_col < 3:
                # Create new state with swapped tiles
                # Convert to list of lists for mutation
                board = [list(row) for row in state]
                
                # Swap empty with adjacent tile
                board[empty_row][empty_col] = board[new_row][new_col]
                board[new_row][new_col] = 0
                
                # Convert back to tuple for hashing
                neighbors.append(tuple(tuple(row) for row in board))
        
        return neighbors
    
    # BFS on the implicit graph
    start = to_tuple(initial)
    
    if start == GOAL:
        return 0
    
    queue = deque([(start, 0)])
    visited = {start}
    
    while queue:
        current, moves = queue.popleft()
        
        for neighbor in get_neighbors(current):
            if neighbor not in visited:
                if neighbor == GOAL:
                    return moves + 1
                
                visited.add(neighbor)
                queue.append((neighbor, moves + 1))
    
    return None  # Unsolvable (wrong parity)
 
# Example: Solve a shuffled puzzle
initial_board = [
    [1, 2, 3],
    [4, 0, 6],
    [7, 5, 8]
]
result = solve_8_puzzle(initial_board)
print(f"Minimum moves: {result}")  # Output: 2 (move 5 up, then 6 left)

Key Observations:

We never build an adjacency list—neighbors are computed by the get_neighbors function
State representation is crucial—using tuples allows hashing for O(1) visited lookup
The graph exists conceptually but is explored only as needed
Memory usage is proportional to visited states, not total possible states

For harder puzzles (15-puzzle has 10^13 states), implicit graphs become essential—you cannot possibly store the full graph.

Case Study: Open the Lock

Let's examine another classic implicit graph problem:

The Problem:

A lock has 4 circular wheels, each displaying a digit 0-9. Starting from '0000', you can turn any wheel one slot up or down. Given a list of 'deadends' (forbidden combinations) and a target, find the minimum turns to reach the target.

State Space Analysis:

Total states: 10^4 = 10,000 (each wheel has 10 positions)
Each state has exactly 8 neighbors (4 wheels × 2 directions)
Some states are 'blocked' (deadends)

This is a classic BFS on an implicit graph with forbidden vertices.

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from collections import deque
from typing import List, Set
 
def open_lock(deadends: List[str], target: str) -> int:
    """
    Find minimum wheel turns to reach target from '0000'.
    
    State: 4-character string representing wheel positions
    Transitions: Turn any wheel up or down by one
    Forbidden states: deadends
    
    Time Complexity: O(10^4 × 8) = O(80,000) worst case
    Space Complexity: O(10^4) for visited set
    """
    # Convert deadends to set for O(1) lookup
    forbidden: Set[str] = set(deadends)
    
    # Edge case: start or target is a deadend
    if '0000' in forbidden:
        return -1
    if target == '0000':
        return 0
    
    def get_neighbors(state: str) -> List[str]:
        """
        Generate all states reachable by one wheel turn.
        Each wheel can go up or down, wrapping around (9->0, 0->9).
        """
        neighbors = []
        state_list = list(state)
        
        for wheel in range(4):
            original_digit = int(state_list[wheel])
            
            # Turn wheel up (0->1, 1->2, ..., 9->0)
            state_list[wheel] = str((original_digit + 1) % 10)
            neighbors.append(''.join(state_list))
            
            # Turn wheel down (0->9, 1->0, ..., 9->8)
            state_list[wheel] = str((original_digit - 1) % 10)
            neighbors.append(''.join(state_list))
            
            # Restore original digit for next wheel
            state_list[wheel] = str(original_digit)
        
        return neighbors
    
    # BFS from '0000'
    queue = deque([('0000', 0)])
    visited: Set[str] = {'0000'}
    
    while queue:
        current, turns = queue.popleft()
        
        for neighbor in get_neighbors(current):
            # Skip forbidden states and already visited
            if neighbor in forbidden or neighbor in visited:
                continue
            
            # Goal check
            if neighbor == target:
                return turns + 1
            
            visited.add(neighbor)
            queue.append((neighbor, turns + 1))
    
    return -1  # Target unreachable
 
# Example
deadends = ["0201", "0101", "0102", "1212", "2002"]
target = "0202"
result = open_lock(deadends, target)
print(f"Minimum turns: {result}")  # Output: 6

Bidirectional BFS Optimization

For problems like this, bidirectional BFS can dramatically reduce the search space. Instead of searching from start to goal (exploring all states at distance d), you search from both ends simultaneously. When the frontiers meet, you've found the shortest path. This can reduce visited states from O(b^d) to O(2 × b^{d/2}).

Understanding State Space Explosion

A critical challenge with implicit graphs is state space explosion—the state space grows exponentially with problem size:

Problem	State Space Size	Feasibility
8-puzzle (3×3)	9! = 362,880	Easily solvable
15-puzzle (4×4)	16! ≈ 2×10^13	Needs heuristics (A*)
24-puzzle (5×5)	25! ≈ 1.5×10^25	Only with advanced techniques
Rubik's Cube	~4.3×10^19	Requires pattern databases

Unguided BFS/DFS cannot handle these large spaces. We need smarter search strategies.

Techniques for Large State Spaces

•A Search with Heuristics* — Use a heuristic function to estimate distance to goal. Prioritize states that seem closer. Guarantees optimal solution if heuristic is admissible (never overestimates).
•Iterative Deepening — Perform depth-limited DFS with increasing depth limits. Combines DFS's memory efficiency with BFS's optimality for unweighted graphs.
•Bidirectional Search — Search from start and goal simultaneously. Meet in the middle. Reduces exponential search to roughly its square root.
•Pattern Databases — Precompute optimal solutions for subproblems. Use as heuristic for full problem. Essential for puzzles like Rubik's Cube.
•State Pruning — Eliminate states early if they can't possibly lead to a solution. Requires domain knowledge about the problem.

Memory vs Optimality Trade-off

BFS guarantees the shortest path but uses O(b^d) memory where b is branching factor and d is solution depth. For deep solutions, you may run out of memory before finding the answer. Iterative deepening A* (IDA*) trades time for memory—it uses only O(d) memory but may revisit states.

Designing Efficient State Representations

The efficiency of implicit graph search heavily depends on state representation. A well-designed state:

1. Is Minimal

Include only information needed for transitions and goal checking. Extra data wastes memory and slows hashing.

2. Is Efficiently Hashable

States must be stored in a visited set. Prefer immutable types (tuples, frozensets, strings) over mutable ones (lists, dicts).

3. Enables Fast Neighbor Generation

Operations on the state should be efficient. If generating neighbors requires expensive computation, the search slows dramatically.

4. Canonicalizes Equivalent States

If multiple representations encode the same 'real' state, you'll revisit unnecessarily. Use canonical forms.

State Representation Strategies
Problem	Naive Representation	Optimized Representation	Improvement
Puzzle board	List of lists	Tuple of tuples	Hashable, immutable
Set of visited nodes	Set of node objects	Frozen set of node IDs	Memory efficient
Path taken	List of steps	Just current position + distance	Constant space state
Resource tracking	Separate variables	Single bitmask integer	Fast comparison, less memory
Board position	2D array	1D tuple (row-major)	Simpler hashing

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# Example: Representing a set of collected keys in a maze
 
# NAIVE: Using a set (mutable, needs conversion for hashing)
class NaiveState:
    def __init__(self, pos, keys):
        self.pos = pos
        self.keys = set(keys)  # Mutable set
    
    def __hash__(self):
        return hash((self.pos, frozenset(self.keys)))  # Conversion needed
    
    def __eq__(self, other):
        return self.pos == other.pos and self.keys == other.keys
 
# OPTIMIZED: Using a bitmask (immutable integer)
class OptimizedState:
    def __init__(self, pos, keys_bitmask):
        self.pos = pos
        self.keys = keys_bitmask  # Integer: bit i = 1 if key i collected
    
    def __hash__(self):
        return hash((self.pos, self.keys))  # Direct hash, no conversion
    
    def __eq__(self, other):
        return self.pos == other.pos and self.keys == other.keys
    
    def has_key(self, key_id):
        return (self.keys >> key_id) & 1 == 1
    
    def with_key(self, key_id):
        return OptimizedState(self.pos, self.keys | (1 << key_id))
 
# The bitmask version is:
# - Faster to hash (single integer vs set conversion)
# - Uses less memory (one int vs set object overhead)
# - Faster to copy (just copy the int)

Real-World Applications of Implicit Graphs

Implicit graph search isn't just for puzzles—it's a fundamental technique in computer science and engineering:

Industry Applications

•AI Game Playing — Chess, Go, and other games have astronomical state spaces. Alpha-Beta pruning, Monte Carlo Tree Search, and learned heuristics enable play in these implicit graphs.
•Robotic Motion Planning — A robot's configuration space (joint angles, positions) is a continuous state space. Discretized or sampled, it becomes an implicit graph for path planning.
•Compiler Optimization — Finding optimal instruction sequences is state space search where states are register/memory configurations.
•Automated Theorem Proving — Proving mathematical theorems involves searching a state space of logical deductions.
•Drug Discovery — Exploring chemical compound variations to find promising drug candidates is implicit graph search in molecule space.
•Network Protocol Verification — Checking that protocols handle all possible message sequences requires exploring the state space of protocol states.

The AI Connection

Much of classical AI is implicit graph search. The field evolved techniques like A*, minimax, and planning algorithms—all operating on implicitly defined state spaces. Understanding implicit graphs gives you a foundation in AI problem-solving.

Summary: Mastering Implicit Graphs

Implicit graphs unlock the ability to solve problems with enormous—even infinite—state spaces. Let's consolidate the key concepts:

Key Takeaways

•Implicit graphs are defined by rules, not storage — You never build the full graph; you generate neighbors on demand.
•Three components define an implicit graph — State representation, transition function (neighbor generator), and goal condition.
•State representation is critical — Use compact, hashable, efficiently-operable representations. Bitmasks, tuples, and canonical forms help.
•BFS and DFS work unchanged — The same algorithms apply; only the 'get neighbors' step changes from lookup to computation.
•State space explosion is the enemy — Large branching factors and deep solutions require heuristics, pruning, or bidirectional search.
•Real-world applications abound — Game AI, robotics, compilers, verification—implicit graphs are everywhere.

What's Next:

One of the most common implicit graph patterns is the grid as graph—treating 2D grids (mazes, game boards, images) as graphs for traversal. The next page explores this pervasive pattern in depth.

Page Complete

You now understand implicit graphs and state space search. This technique handles problems with state spaces far too large for explicit storage, enabling solutions to puzzles, planning problems, and optimization tasks. Next, we'll explore the specific pattern of modeling grids as graphs.

Implicit Graphs (State Space)

Graphs Too Large to Build

What You Will Learn

What Is an Implicit Graph?

An implicit graph (also called a state space graph) is a graph where:

Vertices are not stored in memory—they represent possible states of a system
Edges are not pre-computed—they are generated by applying transition rules to states
The graph is explored on-demand during traversal algorithms

The key insight is that graph algorithms like BFS and DFS don't actually require the full graph in memory. They only need:

A starting vertex
A function to generate neighbors of any given vertex
A way to track visited vertices

Explicit Graph

•All vertices stored in memory
•Adjacency list/matrix pre-built
•O(V + E) space required upfront
•Best when: graph is small, reused, or fully explored
•Example: Road network for navigation app

Implicit Graph

•Vertices generated on-demand
•Neighbors computed via rules
•Only O(visited vertices) space
•Best when: state space is huge, only partial exploration needed
•Example: Puzzle solver (Rubik's Cube)

The State Space Perspective

Anatomy of an Implicit Graph

Every implicit graph problem requires you to define three components:

Component 1: State Representation

How do you encode a 'state' (vertex) of the system? This must:

Capture all information needed to determine valid transitions
Be hashable/comparable for visited tracking
Be as compact as possible to minimize memory

Component 2: Transition Function (Neighbor Generator)

Given a state, what states can you reach in one step? This function:

Takes a state as input
Returns all valid neighbor states
Encodes the 'rules' of the problem

Component 3: Goal Condition

How do you know when you've found the answer? This is a predicate:

Takes a state as input
Returns true if this state is a solution

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from collections import deque
from typing import TypeVar, Callable, Iterable, Set, Optional
 
State = TypeVar('State')
 
def implicit_bfs(
    start: State,
    get_neighbors: Callable[[State], Iterable[State]],
    is_goal: Callable[[State], bool],
    state_to_hashable: Callable[[State], any] = lambda x: x
) -> Optional[int]:
    """
    Generic BFS on an implicit graph.
    
    Args:
        start: The initial state
        get_neighbors: Function that generates neighboring states
        is_goal: Function that returns True if state is a goal
        state_to_hashable: Convert state to hashable form for visited set
    
    Returns:
        Minimum steps to reach a goal state, or None if unreachable
    """
    if is_goal(start):
        return 0
    
    queue = deque([(start, 0)])  # (state, distance)
    visited: Set = {state_to_hashable(start)}
    
    while queue:
        current_state, distance = queue.popleft()
        
        # Generate neighbors on-demand (implicit graph!)
        for neighbor_state in get_neighbors(current_state):
            neighbor_hash = state_to_hashable(neighbor_state)
            
            if neighbor_hash not in visited:
                if is_goal(neighbor_state):
                    return distance + 1
                
                visited.add(neighbor_hash)
                queue.append((neighbor_state, distance + 1))
    
    return None  # Goal not reachable

The Power of Abstraction

Case Study: Sliding Puzzle (8-Puzzle)

The classic 8-puzzle (a 3×3 sliding tile puzzle) is a perfect example of implicit graph search:

The Problem:

Given a 3×3 board with tiles numbered 1-8 and one empty space, slide tiles to reach the goal state. Find the minimum number of moves.

State Space Analysis:

Total possible states: 9! = 362,880
Each state has 2-4 neighbors (depending on empty space position)
Solvable states: 181,440 (exactly half—others are in a different 'parity' component)

This is small enough that we could build the explicit graph, but it's more instructive to treat it implicitly.

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from collections import deque
from typing import List, Tuple, Optional
 
def solve_8_puzzle(initial: List[List[int]]) -> Optional[int]:
    """
    Solve the 8-puzzle using BFS on implicit state space graph.
    
    State Representation: tuple of tuples (hashable)
    Transitions: Swap empty space with adjacent tile
    Goal: [[1,2,3],[4,5,6],[7,8,0]] where 0 is empty
    
    Time Complexity: O(V) where V = reachable states (≤ 181,440)
    Space Complexity: O(V) for visited set and queue
    """
    # Define goal state
    GOAL = ((1, 2, 3), (4, 5, 6), (7, 8, 0))
    
    # Convert initial board to hashable tuple
    def to_tuple(board: List[List[int]]) -> Tuple[Tuple[int, ...], ...]:
        return tuple(tuple(row) for row in board)
    
    # Find position of empty space (0)
    def find_empty(state: Tuple[Tuple[int, ...], ...]) -> Tuple[int, int]:
        for i in range(3):
            for j in range(3):
                if state[i][j] == 0:
                    return (i, j)
        return (-1, -1)  # Should never happen
    
    # Generate all valid neighbor states (THE TRANSITION FUNCTION)
    def get_neighbors(state: Tuple[Tuple[int, ...], ...]):
        """
        Generate neighbor states by moving the empty space.
        This is the core of the implicit graph definition.
        """
        empty_row, empty_col = find_empty(state)
        neighbors = []
        
        # Four possible moves: up, down, left, right
        directions = [(-1, 0), (1, 0), (0, -1), (0, 1)]
        
        for dr, dc in directions:
            new_row, new_col = empty_row + dr, empty_col + dc
            
            # Check bounds
            if 0 <= new_row < 3 and 0 <= new_col < 3:
                # Create new state with swapped tiles
                # Convert to list of lists for mutation
                board = [list(row) for row in state]
                
                # Swap empty with adjacent tile
                board[empty_row][empty_col] = board[new_row][new_col]
                board[new_row][new_col] = 0
                
                # Convert back to tuple for hashing
                neighbors.append(tuple(tuple(row) for row in board))
        
        return neighbors
    
    # BFS on the implicit graph
    start = to_tuple(initial)
    
    if start == GOAL:
        return 0
    
    queue = deque([(start, 0)])
    visited = {start}
    
    while queue:
        current, moves = queue.popleft()
        
        for neighbor in get_neighbors(current):
            if neighbor not in visited:
                if neighbor == GOAL:
                    return moves + 1
                
                visited.add(neighbor)
                queue.append((neighbor, moves + 1))
    
    return None  # Unsolvable (wrong parity)
 
# Example: Solve a shuffled puzzle
initial_board = [
    [1, 2, 3],
    [4, 0, 6],
    [7, 5, 8]
]
result = solve_8_puzzle(initial_board)
print(f"Minimum moves: {result}")  # Output: 2 (move 5 up, then 6 left)

Key Observations:

We never build an adjacency list—neighbors are computed by the get_neighbors function
State representation is crucial—using tuples allows hashing for O(1) visited lookup
The graph exists conceptually but is explored only as needed
Memory usage is proportional to visited states, not total possible states

For harder puzzles (15-puzzle has 10^13 states), implicit graphs become essential—you cannot possibly store the full graph.

Case Study: Open the Lock

Let's examine another classic implicit graph problem:

The Problem:

A lock has 4 circular wheels, each displaying a digit 0-9. Starting from '0000', you can turn any wheel one slot up or down. Given a list of 'deadends' (forbidden combinations) and a target, find the minimum turns to reach the target.

State Space Analysis:

Total states: 10^4 = 10,000 (each wheel has 10 positions)
Each state has exactly 8 neighbors (4 wheels × 2 directions)
Some states are 'blocked' (deadends)

This is a classic BFS on an implicit graph with forbidden vertices.

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from collections import deque
from typing import List, Set
 
def open_lock(deadends: List[str], target: str) -> int:
    """
    Find minimum wheel turns to reach target from '0000'.
    
    State: 4-character string representing wheel positions
    Transitions: Turn any wheel up or down by one
    Forbidden states: deadends
    
    Time Complexity: O(10^4 × 8) = O(80,000) worst case
    Space Complexity: O(10^4) for visited set
    """
    # Convert deadends to set for O(1) lookup
    forbidden: Set[str] = set(deadends)
    
    # Edge case: start or target is a deadend
    if '0000' in forbidden:
        return -1
    if target == '0000':
        return 0
    
    def get_neighbors(state: str) -> List[str]:
        """
        Generate all states reachable by one wheel turn.
        Each wheel can go up or down, wrapping around (9->0, 0->9).
        """
        neighbors = []
        state_list = list(state)
        
        for wheel in range(4):
            original_digit = int(state_list[wheel])
            
            # Turn wheel up (0->1, 1->2, ..., 9->0)
            state_list[wheel] = str((original_digit + 1) % 10)
            neighbors.append(''.join(state_list))
            
            # Turn wheel down (0->9, 1->0, ..., 9->8)
            state_list[wheel] = str((original_digit - 1) % 10)
            neighbors.append(''.join(state_list))
            
            # Restore original digit for next wheel
            state_list[wheel] = str(original_digit)
        
        return neighbors
    
    # BFS from '0000'
    queue = deque([('0000', 0)])
    visited: Set[str] = {'0000'}
    
    while queue:
        current, turns = queue.popleft()
        
        for neighbor in get_neighbors(current):
            # Skip forbidden states and already visited
            if neighbor in forbidden or neighbor in visited:
                continue
            
            # Goal check
            if neighbor == target:
                return turns + 1
            
            visited.add(neighbor)
            queue.append((neighbor, turns + 1))
    
    return -1  # Target unreachable
 
# Example
deadends = ["0201", "0101", "0102", "1212", "2002"]
target = "0202"
result = open_lock(deadends, target)
print(f"Minimum turns: {result}")  # Output: 6

Bidirectional BFS Optimization

Understanding State Space Explosion

A critical challenge with implicit graphs is state space explosion—the state space grows exponentially with problem size:

Problem	State Space Size	Feasibility
8-puzzle (3×3)	9! = 362,880	Easily solvable
15-puzzle (4×4)	16! ≈ 2×10^13	Needs heuristics (A*)
24-puzzle (5×5)	25! ≈ 1.5×10^25	Only with advanced techniques
Rubik's Cube	~4.3×10^19	Requires pattern databases

Unguided BFS/DFS cannot handle these large spaces. We need smarter search strategies.

Techniques for Large State Spaces

•A Search with Heuristics* — Use a heuristic function to estimate distance to goal. Prioritize states that seem closer. Guarantees optimal solution if heuristic is admissible (never overestimates).
•Iterative Deepening — Perform depth-limited DFS with increasing depth limits. Combines DFS's memory efficiency with BFS's optimality for unweighted graphs.
•Bidirectional Search — Search from start and goal simultaneously. Meet in the middle. Reduces exponential search to roughly its square root.
•Pattern Databases — Precompute optimal solutions for subproblems. Use as heuristic for full problem. Essential for puzzles like Rubik's Cube.
•State Pruning — Eliminate states early if they can't possibly lead to a solution. Requires domain knowledge about the problem.

Memory vs Optimality Trade-off

Designing Efficient State Representations

The efficiency of implicit graph search heavily depends on state representation. A well-designed state:

1. Is Minimal

Include only information needed for transitions and goal checking. Extra data wastes memory and slows hashing.

2. Is Efficiently Hashable

States must be stored in a visited set. Prefer immutable types (tuples, frozensets, strings) over mutable ones (lists, dicts).

3. Enables Fast Neighbor Generation

Operations on the state should be efficient. If generating neighbors requires expensive computation, the search slows dramatically.

4. Canonicalizes Equivalent States

If multiple representations encode the same 'real' state, you'll revisit unnecessarily. Use canonical forms.

State Representation Strategies
Problem	Naive Representation	Optimized Representation	Improvement
Puzzle board	List of lists	Tuple of tuples	Hashable, immutable
Set of visited nodes	Set of node objects	Frozen set of node IDs	Memory efficient
Path taken	List of steps	Just current position + distance	Constant space state
Resource tracking	Separate variables	Single bitmask integer	Fast comparison, less memory
Board position	2D array	1D tuple (row-major)	Simpler hashing

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# Example: Representing a set of collected keys in a maze
 
# NAIVE: Using a set (mutable, needs conversion for hashing)
class NaiveState:
    def __init__(self, pos, keys):
        self.pos = pos
        self.keys = set(keys)  # Mutable set
    
    def __hash__(self):
        return hash((self.pos, frozenset(self.keys)))  # Conversion needed
    
    def __eq__(self, other):
        return self.pos == other.pos and self.keys == other.keys
 
# OPTIMIZED: Using a bitmask (immutable integer)
class OptimizedState:
    def __init__(self, pos, keys_bitmask):
        self.pos = pos
        self.keys = keys_bitmask  # Integer: bit i = 1 if key i collected
    
    def __hash__(self):
        return hash((self.pos, self.keys))  # Direct hash, no conversion
    
    def __eq__(self, other):
        return self.pos == other.pos and self.keys == other.keys
    
    def has_key(self, key_id):
        return (self.keys >> key_id) & 1 == 1
    
    def with_key(self, key_id):
        return OptimizedState(self.pos, self.keys | (1 << key_id))
 
# The bitmask version is:
# - Faster to hash (single integer vs set conversion)
# - Uses less memory (one int vs set object overhead)
# - Faster to copy (just copy the int)

Real-World Applications of Implicit Graphs

Implicit graph search isn't just for puzzles—it's a fundamental technique in computer science and engineering:

Industry Applications

•AI Game Playing — Chess, Go, and other games have astronomical state spaces. Alpha-Beta pruning, Monte Carlo Tree Search, and learned heuristics enable play in these implicit graphs.
•Robotic Motion Planning — A robot's configuration space (joint angles, positions) is a continuous state space. Discretized or sampled, it becomes an implicit graph for path planning.
•Compiler Optimization — Finding optimal instruction sequences is state space search where states are register/memory configurations.
•Automated Theorem Proving — Proving mathematical theorems involves searching a state space of logical deductions.
•Drug Discovery — Exploring chemical compound variations to find promising drug candidates is implicit graph search in molecule space.
•Network Protocol Verification — Checking that protocols handle all possible message sequences requires exploring the state space of protocol states.

The AI Connection

Summary: Mastering Implicit Graphs

Implicit graphs unlock the ability to solve problems with enormous—even infinite—state spaces. Let's consolidate the key concepts:

Key Takeaways

•Implicit graphs are defined by rules, not storage — You never build the full graph; you generate neighbors on demand.
•Three components define an implicit graph — State representation, transition function (neighbor generator), and goal condition.
•State representation is critical — Use compact, hashable, efficiently-operable representations. Bitmasks, tuples, and canonical forms help.
•BFS and DFS work unchanged — The same algorithms apply; only the 'get neighbors' step changes from lookup to computation.
•State space explosion is the enemy — Large branching factors and deep solutions require heuristics, pruning, or bidirectional search.
•Real-world applications abound — Game AI, robotics, compilers, verification—implicit graphs are everywhere.

What's Next:

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