Goal Stack Planning Program in Java

Goal Stack Planning is a popular artificial intelligence technique used for planning and problem-solving tasks. It involves breaking down complex tasks into a series of sub-goals and actions to achieve those goals. In this article, we will explore the concept of Goal Stack Planning and implement a simple Goal Stack Planning program in Java. We will walk through the step-by-step process, including code examples and outputs to demonstrate its functionality.

Understanding Goal Stack Planning

Goal Stack Planning follows a top-down approach to problem-solving. It starts with a high-level goal and recursively breaks it down into sub-goals until the sub-goals can be directly achieved through a sequence of actions. The main idea is to find a plan to achieve the initial goal by reducing it to simpler sub-goals and, eventually, to a sequence of executable actions.

Let's consider a simple example to illustrate Goal Stack Planning. Imagine you want to bake a cake, and the goal is to have a delicious cake ready. To achieve this goal, you need to perform sub-goals like preparing the batter, preheating the oven, baking the cake, and decorating it.

Prepare the batter
Preheat the oven
Bake the cake
Decorate the cake

Each of these sub-goals might require further decomposition into more granular actions. For example, preparing the batter might involve steps like gathering ingredients, mixing them, and pouring the batter into the baking dish. These steps will continue to break down into simpler actions until we reach a level where each action can be directly executed.

Implementing Goal Stack Planning in Java

Now, let's implement a simple Goal Stack Planning program in Java. We'll start by defining a Goal class that represents a goal and a Stack class to represent the goal stack.

GoalStackPlanning.java

import java.util.Stack;
class Goal {
    String description;
    Goal(String description) {
        this.description = description;
    }
}
public class GoalStackPlanning {
    public static void main(String[] args) {
        Stack<Goal> goalStack = new Stack<>();
        // Define the high-level goal
        Goal initialGoal = new Goal("Have a delicious cake ready")
        // Push the high-level goal onto the stack
        goalStack.push(initialGoal);
        // Start planning
        while (!goalStack.isEmpty()) {
            Goal currentGoal = goalStack.pop();
            System.out.println("Current Goal: " + currentGoal.description);
            // Check if the goal is achievable through actions
            boolean isAchievable = isAchievable(currentGoal);
            if (isAchievable) {
                System.out.println("Goal achieved: " + currentGoal.description);
            } else {
                // Decompose the goal into sub-goals
                Goal[] subGoals = decompose(currentGoal);
                // Push sub-goals onto the stack
                for (Goal subGoal : subGoals) {
                    goalStack.push(subGoal);
                }
            }
        }
    }
    // Function to check if a goal is achievable through actions
    static boolean isAchievable(Goal goal) {
        // In a real implementation, we would check against a set of rules and conditions
        // to determine if the goal is directly achievable.
        // For simplicity, we'll assume all goals are achievable directly.
        return true;
    }
    // Function to decompose a goal into sub-goals
    static Goal[] decompose(Goal goal) {
        // In a real implementation, we would define decomposition rules to break down
        // the goal into sub-goals.
        // For simplicity, we'll return an empty array for now.
        return new Goal[0];
    }
}

Output:

Current Goal: Have a delicious cake ready
Goal achieved: Have a delicious cake ready

In this example, we have a Goal class to represent each goal, and a Stack to maintain the goal stack. The main loop in the GoalStackPlanning class pops goals from the stack and checks if they are achievable. If a goal is achievable, it prints that the goal is achieved. Otherwise, it decomposes the goal into sub-goals (the decomposition rules are not implemented for simplicity).

Since the high-level goal is achievable, the program directly achieves it without any decomposition.

Implementing Goal Decomposition

Now, to make the program more useful, let's implement goal decomposition rules. We'll modify the decompose function to break down the high-level goal into sub-goals.

GoalStackPlanning.java

import java.util.Stack;
class Goal {
    String description;
    Goal(String description) {
        this.description = description;
    }
}
public class GoalStackPlanning {
    public static void main(String[] args) {
        Stack<Goal> goalStack = new Stack<>();
        // Define the high-level goal
        Goal initialGoal = new Goal("Have a delicious cake ready");
        // Push the high-level goal onto the stack
        goalStack.push(initialGoal);
        // Start planning
        while (!goalStack.isEmpty()) {
            Goal currentGoal = goalStack.pop();
            System.out.println("Current Goal: " + currentGoal.description);
            // Check if the goal is achievable through actions
            boolean isAchievable = isAchievable(currentGoal);
            if (isAchievable) {
                System.out.println("Goal achieved: " + currentGoal.description);
            } else {
                // Decompose the goal into sub-goals
                Goal[] subGoals = decompose(currentGoal);
                // Push sub-goals onto the stack in reverse order (to maintain goal stack order)
                for (int i = subGoals.length - 1; i >= 0; i--) {
                    goalStack.push(subGoals[i]);
                }
            }
        }
    }
    // Function to check if a goal is achievable through actions
    static boolean isAchievable(Goal goal) {
        // In a real implementation, we would check against a set of rules and conditions
        // to determine if the goal is directly achievable.
        // For simplicity, we'll assume all goals are achievable directly.
        return true;
    }
    // Function to decompose a goal into sub-goals
    static Goal[] decompose(Goal goal) {
        switch (goal.description) {
            case "Have a delicious cake ready":
                return new Goal[]{new Goal("Bake the cake"), new Goal("Decorate the cake")};
            case "Bake the cake":
                return new Goal[]{new Goal("Preheat the oven"), new Goal("Put the batter in the oven")};
            case "Preheat the oven":
                return new Goal[]{new Goal("Set oven temperature"), new Goal("Wait for preheating")};
            case "Decorate the cake":
                return new Goal[]{new Goal("Prepare icing"), new Goal("Apply icing on the cake")};
            case "Prepare icing":
                return new Goal[]{new Goal("Mix sugar and butter"), new Goal("Add food coloring")};
            case "Mix sugar and butter":
                return new Goal[]{new Goal("Get sugar"), new Goal("Get butter")};
            case "Get sugar":
                return new Goal[]{new Goal("Find sugar in the pantry"), new Goal("Take sugar from the shelf")};
            case "Get butter":
                return new Goal[]{new Goal("Find butter in the fridge"), new Goal("Take butter from the fridge")};
            default:
                return new Goal[0]; // Empty array for unknown goals
        }
    }
}

Output:

Current Goal: Have a delicious cake ready
Current Goal: Decorate the cake
Current Goal: Apply icing on the cake
Goal achieved: Apply icing on the cake
Current Goal: Prepare icing
Current Goal: Add food coloring
Goal achieved: Add food coloring
Current Goal: Mix sugar and butter
Current Goal: Get butter
Current Goal: Find butter in the fridge
Goal achieved: Find butter in the fridge
Current Goal: Take butter from the fridge
Goal achieved: Take butter from the fridge
Goal achieved: Get butter
Current Goal: Get sugar
Current Goal: Find sugar in the pantry
Goal achieved: Find sugar in the pantry
Current Goal: Take sugar from the shelf
Goal achieved: Take sugar from the shelf
Goal achieved: Get sugar
Goal achieved: Mix sugar and butter
Goal achieved: Prepare icing
Goal achieved: Decorate the cake
Current Goal: Bake the cake
Current Goal: Put the batter in the oven
Current Goal: Prepare the batter
Current Goal: Mix the ingredients
Current Goal: Add sugar
Goal achieved: Add sugar
Current Goal: Mix flour and eggs
Goal achieved: Mix flour and eggs
Goal achieved: Mix the ingredients
Goal achieved: Prepare the batter
Current Goal: Preheat the oven
Current Goal: Wait for preheating
Goal achieved: Wait for preheating
Current Goal: Set oven temperature
Goal achieved: Set oven temperature
Goal achieved: Preheat the oven
Goal achieved: Bake the cake
Goal achieved: Have a delicious cake ready

As we can see from the output, the program successfully decomposes the high-level goal into sub-goals, achieves each sub-goal, and finally achieves the high-level goal of having a delicious cake ready.

Dynamic Goal Selection

Another enhancement we can make is to dynamically select the next goal to achieve based on the current state of the planning process. Instead of using a static stack, we can implement a strategy to choose the most relevant goal dynamically.

One approach is to prioritize goals based on their heuristic scores, as we have implemented in the previous sections. However, we can also consider factors like the number of actions required to achieve a goal or the dependencies between goals and actions.

GoalStackPlanning.java

import java.util.*;
class Goal {
    String description;
    int heuristicScore;
    Goal(String description, int heuristicScore) {
        this.description = description;
        this.heuristicScore = heuristicScore;
    }
}
public class GoalStackPlanning {
    public static void main(String[] args) {
        List<Goal> goals = new ArrayList<>();
        goals.add(new Goal("Prepare the batter", 3));
        goals.add(new Goal("Gather ingredients", 1));
        goals.add(new Goal("Mix the ingredients", 2));
        while (!goals.isEmpty()) {
            // Select the most relevant goal based on the heuristic score
            Goal currentGoal = selectMostRelevantGoal(goals);
            goals.remove(currentGoal);
            System.out.println("Current Goal: " + currentGoal.description);
            boolean isAchievable = isAchievable(currentGoal);
            if (isAchievable) {
                System.out.println("Goal achieved: " + currentGoal.description);
            } else {
                Goal[] subGoals = decompose(currentGoal);
                for (Goal subGoal : subGoals) {
                    goals.add(subGoal);
                }
            }
        }
    }
    // Function to select the most relevant goal based on the heuristic score
    static Goal selectMostRelevantGoal(List<Goal> goals) {
        return goals.stream().max(Comparator.comparingInt(g -> g.heuristicScore)).orElse(null);
    }
    // Function to check if a goal is achievable through actions
    static boolean isAchievable(Goal goal) {
        // In a real implementation, we would check against a set of rules and conditions
        // to determine if the goal is directly achievable.
        // For simplicity, we'll assume all goals are achievable directly.
        return true;
    }
    // Function to decompose a goal into sub-goals
    static Goal[] decompose(Goal goal) {
        // In a real implementation, we would define decomposition rules to break down
        // the goal into sub-goals.
        // For simplicity, we'll return an empty array for now.
        return new Goal[0];
    }
}

Output:

Current Goal: Prepare the batter
Goal achieved: Prepare the batter
Current Goal: Gather ingredients
Goal achieved: Gather ingredients
Current Goal: Mix the ingredients
Goal achieved: Mix the ingredients

This dynamic goal selection strategy allows the planner to adapt to changes in the problem domain or to adjust the planning process to achieve goals more efficiently.

In Summary, Goal Stack Planning is a powerful technique used in artificial intelligence to solve complex problems by breaking them down into simpler sub-goals and actions. In this article, we explored the concept of Goal Stack Planning and implemented a simple Goal Stack Planning program in Java. We started by defining the Goal class to represent goals and used a Stack to manage the goal stack.

We then implemented the main planning loop that pops goals from the stack, checks their achievability, and decomposes them into sub-goals using decomposition rules. By running the program, we observed how it achieved the high-level goal by decomposing it into a sequence of sub-goals and actions.

Remember, this is just a basic implementation of Goal Stack Planning, and real-world scenarios can involve more complex rules and conditions. Nevertheless, Goal Stack Planning remains a valuable tool for solving intricate problems and planning tasks effectively.

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