Implementing Skill Logic: MP & Targeting

Hello! Welcome back to our journey into building a classic JRPG.

In the last lesson, we laid the architectural groundwork for all combat strategy by designing a comprehensive data structure for skills. We defined how a skill's cost, target type (scope), power, and effects can be stored as clean, manageable JSON data.

Today, we'll bring that design to life. This lesson focuses on the learning outcome: Implement logic for MP consumption and target selection for skills. We'll write the code that reads the cost and scope properties from our skill data and makes them functional within the battle system. You'll see how the engine checks if a character can afford a skill, deducts the cost, and presents the correct targeting options to the player.

1. The Targeting Pipeline: From Selection to Execution

Before we write code, let's establish the architectural flow. When a player selects a skill, the game needs to determine who or what can be targeted. This is dictated by the skill's scope. Your front-end development experience with state machines and user interaction flows will be very relevant here.

A robust way to think about this is to understand how professional game engines handle it.

As the article outlines, the general workflow is:

1. The player selects a skill from the command menu.
2. The engine reads the skill's scope (e.g., one_enemy, all_allies, self).
3. The battle UI enters a "targeting mode".
4. The engine determines the list of valid targets based on the scope.
5. The player selects a target from this list (or the system auto-selects if the scope is 'all' or 'self').
6. The chosen action (user, skill, and target) is stored, ready for execution.

2. Implementing Target Selection

Let's translate this into code. We need a function that takes the skill's scope and the current battle state to return an array of valid targets.

Assuming you have a battleState object that contains partyMembers and enemies arrays, the logic could look like this:

function getValidTargets(scope, user, battleState) {
    const { partyMembers, enemies } = battleState;

    switch (scope) {
        case 'one_enemy':
        case 'all_enemies':
            // Return all living enemies
            return enemies.filter(e => e.stats.hp > 0);

        case 'one_ally':
        case 'all_allies':
            // Return all living party members
            return partyMembers.filter(p => p.stats.hp > 0);
        
        case 'one_dead_ally':
             // For revival skills
            return partyMembers.filter(p => p.stats.hp <= 0);

        case 'self':
            // The target is the user themself
            return [user];

        default:
            return [];
    }
}

This function is the core of our targeting logic. In your UI code, you would:

1. Call getValidTargets when the player chooses a skill.
2. If the scope is for a single target (e.g., one_enemy), you would activate a targeting cursor and allow the player to cycle through the returned list of valid targets.
3. If the scope is for multiple targets (all_enemies) or self, you can often skip the selection step and confirm the action immediately.

To see what this looks like in practice, the following video segment shows how the RPG Maker UI handles different scopes.

3. Implementing Resource Consumption

A character can't use a skill if they can't pay the cost. This involves two steps: checking if the skill is usable and then paying the cost upon execution.

Step 1: The canUse Check

A clean architectural pattern is to create a "gatekeeper" function that checks all requirements for using a skill. For now, this will just be MP, but it can be expanded easily.

/**
 * Checks if a character can use a given skill.
 * @param {object} user The character attempting to use the skill.
 * @param {object} skill The skill data object.
 * @returns {boolean} True if the skill can be used, false otherwise.
 */
function canUse(user, skill) {
    // If there's no cost, it's always usable.
    if (!skill.cost || skill.cost.value === 0) {
        return true;
    }

    // Check for MP cost
    if (skill.cost.type === 'mp') {
        return user.stats.mp >= skill.cost.value;
    }

    // Future-proofing: we can add checks for HP, TP, items, etc. here.
    // if (skill.cost.type === 'hp') { ... }

    return true; // Default to true if cost type is unknown for now
}

You would call this function when drawing the skill list in the battle menu. If canUse(character, skill) returns false, you would render that skill's name in a disabled state (e.g., greyed out) and prevent the player from selecting it. This is analogous to disabling a button in a web UI when a form is invalid.

Step 2: Paying the Cost

Once the player has selected a valid skill and target, and the action is about to be executed, you need to deduct the resource. This is a straightforward state update.

/**
 * Deducts the resource cost for using a skill from the user.
 * @param {object} user The character using the skill.
 * @param {object} skill The skill data object.
 */
function payCost(user, skill) {
    if (!skill.cost || skill.cost.value === 0) {
        return; // No cost to pay
    }

    if (skill.cost.type === 'mp') {
        user.stats.mp -= skill.cost.value;
        // Ensure MP doesn't drop below 0
        if (user.stats.mp < 0) user.stats.mp = 0; 
    }
    
    // ... logic for other cost types
}

This function would be called as part of the action execution sequence, right before the skill's damage or effects are calculated and applied.

function renderSkillMenu(user, skills) {
  clearMenu();
  
  skills.forEach(skill => {
    const isEnabled = canUse(user, skill);
    
    const skillButton = createButton(skill.name);
    skillButton.disabled = !isEnabled;
    
    if (isEnabled) {
      skillButton.onClick(() => {
        // Transition to targeting mode
        startTargeting(user, skill); 
      });
    }
    
    add(skillButton);
  });
}

4. Expanding the System: Advanced Costs

Classic JRPGs feature a variety of skill costs beyond simple MP deduction—HP for "dark knight" abilities, consuming items for archers, or even Gold for special moves. Our system should be flexible enough to accommodate this.

RPG Maker handles this through a powerful plugin system that extends the base engine. We can learn from this to evolve our own data structure.

Inspired by this, we can make our cost property more robust. Instead of a single object, it could be an array of costs.

New skill data structure:

{
    "id": "power_shot_1",
    "name": "Power Shot",
    // ...
    "cost": [
        { "type": "tp", "value": 15 },
        { "type": "item", "itemId": "arrow", "value": 1 }
    ],
    "scope": "one_enemy"
}

With this structure, we can refactor our canUse and payCost functions to iterate through the array, checking and paying each cost in sequence.

Refactored canUse function:

function canUse(user, skill) {
    if (!skill.cost) return true;

    // Check every cost in the array. If any check fails, return false.
    return skill.cost.every(c => {
        switch (c.type) {
            case 'mp':
                return user.stats.mp >= c.value;
            case 'hp':
                // Note: a skill shouldn't kill you just by its cost.
                return user.stats.hp > c.value; 
            case 'item':
                return user.inventory.hasItem(c.itemId, c.value);
            // ... more cost types
            default:
                return true;
        }
    });
}

This data-driven and extensible approach is a hallmark of good game engine design. It empowers you to create rich mechanics without having to rewrite core engine code for every new idea.

Conclusion

In this lesson, you've bridged the gap between data design and implementation for two critical combat systems. You now have the logic to handle both resource consumption and target selection in a way that is clean, scalable, and directly driven by the skill data we designed previously.

Key Takeaways:

• Targeting is a State Flow: Selecting a skill triggers a process that reads its scope to determine valid targets and guide the player's choice.
• Gatekeeper Functions are Key: A function like canUse cleanly separates the rules for skill eligibility (like affording the cost) from the UI that displays it.
• Decouple Checking from Paying: The check (canUse) happens during the command selection phase, while the payment (payCost) happens during the action execution phase.
• Data Structures Evolve: By observing professional tools, we saw how to evolve our cost data from a single object to an array, allowing for complex, multi-resource costs without complicating the core logic.

Preview of the next lesson:
Now that we can successfully target a combatant and pay the price for a skill, we need to broaden the range of things that can happen to the target. In the next lesson, we will implement the logic that was defined in our skill data's effects array, covering: Implement status effects (e.g., Poison, Sleep) and elemental properties.