Calculators

Calculators compute derived values before guards and actions run. They're useful for preparing data that guards need to evaluate or that actions need to use.

Execution Order

Calculators run first, so guards and actions can use the calculated values.

Basic Usage

Inline Calculator

MachineDefinition::define(
    config: [
        'states' => [
            'cart' => [
                'on' => [
                    'CHECKOUT' => [
                        'target' => 'checkout',
                        'calculators' => 'calculateTotal',
                        'guards' => 'hasMinimumTotal',
                    ],
                ],
            ],
        ],
    ],
    behavior: [
        'calculators' => [
            'calculateTotal' => function (ContextManager $context) {
                $context->total = collect($context->items)->sum('price');
            },
        ],
        'guards' => [
            'hasMinimumTotal' => fn($ctx) => $ctx->total >= 10,
        ],
    ],
);

Class-Based Calculator

use Tarfinlabs\EventMachine\Behavior\CalculatorBehavior;

class CalculateTotalCalculator extends CalculatorBehavior
{
    public function __invoke(ContextManager $context): void
    {
        $subtotal = collect($context->items)->sum('price');
        $tax = $subtotal * 0.1;
        $shipping = $this->calculateShipping($context);

        $context->subtotal = $subtotal;
        $context->tax = $tax;
        $context->shipping = $shipping;
        $context->total = $subtotal + $tax + $shipping;
    }

    private function calculateShipping(ContextManager $context): float
    {
        $itemCount = count($context->items);
        return $itemCount > 5 ? 0 : 5.99;
    }
}

Multiple Calculators

Chain calculators for complex computations:

'on' => [
    'CHECKOUT' => [
        'target' => 'processing',
        'calculators' => [
            'calculateSubtotal',
            'applyDiscounts',
            'calculateTax',
            'calculateShipping',
            'calculateFinalTotal',
        ],
        'guards' => 'hasValidTotal',
    ],
],

Calculator Parameters

class DiscountCalculator extends CalculatorBehavior
{
    public function __invoke(
        ContextManager $context,
        EventBehavior $event,
    ): void {
        $discountCode = $event->payload['discountCode'] ?? null;

        if ($discountCode) {
            $discount = $this->lookupDiscount($discountCode);
            $context->discount = $discount;
            $context->total -= $discount;
        }
    }
}

Dependency Injection

class CalculateTaxCalculator extends CalculatorBehavior
{
    public function __construct(
        private readonly TaxService $taxService,
    ) {}

    public function __invoke(ContextManager $context): void
    {
        $taxRate = $this->taxService->getRateForAddress(
            $context->shippingAddress
        );

        $context->taxRate = $taxRate;
        $context->tax = $context->subtotal * $taxRate;
    }
}

Practical Examples

Order Total Calculation

class CalculateOrderTotalCalculator extends CalculatorBehavior
{
    public function __invoke(ContextManager $context): void
    {
        $items = collect($context->items);

        // Calculate subtotal
        $subtotal = $items->sum(fn($item) =>
            $item['price'] * $item['quantity']
        );

        // Apply discount
        $discount = $this->calculateDiscount($context, $subtotal);

        // Calculate tax (after discount)
        $taxable = $subtotal - $discount;
        $tax = $taxable * 0.1;

        // Calculate shipping
        $shipping = $this->calculateShipping($items);

        // Set all values
        $context->subtotal = $subtotal;
        $context->discount = $discount;
        $context->tax = $tax;
        $context->shipping = $shipping;
        $context->total = $taxable + $tax + $shipping;
    }

    private function calculateDiscount($context, $subtotal): float
    {
        if ($context->discountCode === 'SAVE20') {
            return $subtotal * 0.2;
        }
        return 0;
    }

    private function calculateShipping($items): float
    {
        $weight = $items->sum('weight');
        return $weight > 10 ? 15.99 : 5.99;
    }
}

User Eligibility Calculation

class CalculateEligibilityCalculator extends CalculatorBehavior
{
    public function __construct(
        private readonly UserService $users,
        private readonly CreditService $credit,
    ) {}

    public function __invoke(ContextManager $context): void
    {
        $user = $this->users->find($context->userId);

        // Calculate credit score
        $creditScore = $this->credit->getScore($user);

        // Calculate debt ratio
        $debtRatio = $user->total_debt / $user->annual_income;

        // Calculate eligibility
        $context->creditScore = $creditScore;
        $context->debtRatio = $debtRatio;
        $context->isEligible = $creditScore >= 650 && $debtRatio < 0.4;
        $context->maxLoanAmount = $this->calculateMaxLoan($creditScore, $user);
    }
}

Pricing Calculation

class CalculatePricingCalculator extends CalculatorBehavior
{
    public function __invoke(
        ContextManager $context,
        EventBehavior $event,
    ): void {
        $plan = $event->payload['plan'];
        $period = $event->payload['period'] ?? 'monthly';

        $pricing = $this->getPricing($plan, $period);

        $context->plan = $plan;
        $context->period = $period;
        $context->basePrice = $pricing['base'];
        $context->discount = $pricing['discount'];
        $context->finalPrice = $pricing['final'];
    }

    private function getPricing(string $plan, string $period): array
    {
        $prices = [
            'basic' => ['monthly' => 9.99, 'yearly' => 99.99],
            'pro' => ['monthly' => 19.99, 'yearly' => 199.99],
            'enterprise' => ['monthly' => 49.99, 'yearly' => 499.99],
        ];

        $base = $prices[$plan][$period] ?? 0;
        $discount = $period === 'yearly' ? $base * 0.17 : 0;

        return [
            'base' => $base,
            'discount' => $discount,
            'final' => $base - $discount,
        ];
    }
}

Date/Time Calculations

class CalculateDeliveryDateCalculator extends CalculatorBehavior
{
    public function __invoke(ContextManager $context): void
    {
        $shippingMethod = $context->shippingMethod ?? 'standard';

        $deliveryDays = match ($shippingMethod) {
            'express' => 2,
            'priority' => 5,
            'standard' => 7,
            default => 10,
        };

        $orderDate = now();
        $deliveryDate = $orderDate->addBusinessDays($deliveryDays);

        $context->orderDate = $orderDate->toDateString();
        $context->estimatedDelivery = $deliveryDate->toDateString();
        $context->deliveryDays = $deliveryDays;
    }
}

Calculator Failure Behavior

Critical

If a calculator throws an exception, the entire transition is aborted. The exception is caught, a machine.calculator.{name}.fail internal event is recorded, and the transition returns false.

class RiskyCalculator extends CalculatorBehavior
{
    public function __invoke(ContextManager $context): void
    {
        // If this throws, the transition is aborted
        $result = $this->externalService->calculate($context->data);
        $context->calculatedValue = $result;
    }
}

Handle potential failures gracefully:

class SafeCalculator extends CalculatorBehavior
{
    public function __invoke(ContextManager $context): void
    {
        try {
            $result = $this->externalService->calculate($context->data);
            $context->calculatedValue = $result;
        } catch (ExternalServiceException $e) {
            // Set a default or flag for guards to check
            $context->calculatedValue = null;
            $context->calculationFailed = true;
        }
    }
}

The internal event recorded on failure:

// Event type: {machine}.calculator.{calculatorName}.fail
// Payload: ['error' => 'Calculator failed: {exception message}']

Calculator vs Action

Aspect	Calculator	Action
Runs	Before guards	After guards pass
Purpose	Prepare data	Execute side effects
Typical use	Compute values	Call services, send emails
Can block transition	Yes (if throws exception)	No

When to Use Calculators

• Computing totals, averages, or aggregates
• Looking up data needed by guards
• Preparing context for guards/actions
• Formatting or transforming data

When to Use Actions

• Saving to database
• Sending notifications
• Calling external APIs
• Any side effects

Testing Calculators

it('calculates order total correctly', function () {
    $machine = MachineDefinition::define(
        config: [
            'initial' => 'cart',
            'context' => [
                'items' => [
                    ['price' => 10, 'quantity' => 2],
                    ['price' => 25, 'quantity' => 1],
                ],
            ],
            'states' => [
                'cart' => [
                    'on' => [
                        'CALCULATE' => [
                            'calculators' => 'calculateTotal',
                        ],
                    ],
                ],
            ],
        ],
        behavior: [
            'calculators' => [
                'calculateTotal' => function ($ctx) {
                    $ctx->total = collect($ctx->items)
                        ->sum(fn($i) => $i['price'] * $i['quantity']);
                },
            ],
        ],
    );

    $state = $machine->transition(['type' => 'CALCULATE']);

    expect($state->context->total)->toBe(45);
});

Best Practices

1. Keep Calculators Pure

Calculate values, don't trigger side effects:

// Good - only calculates
$context->total = collect($context->items)->sum('price');

// Avoid - side effect
$this->analytics->track('total_calculated', $context->total);

2. Order Calculators Logically

'calculators' => [
    'calculateSubtotal',    // First: base calculation
    'applyDiscounts',       // Second: depends on subtotal
    'calculateTax',         // Third: depends on discounted amount
    'calculateTotal',       // Last: combines all values
],

3. Use Descriptive Names

// Good
'calculators' => [
    'calculateOrderSubtotal',
    'calculateShippingCost',
    'applyMembershipDiscount',
],

// Avoid
'calculators' => ['calc1', 'calc2', 'doStuff'],

4. Handle Edge Cases

public function __invoke(ContextManager $context): void
{
    $items = $context->items ?? [];

    if (empty($items)) {
        $context->total = 0;
        return;
    }

    $context->total = collect($items)->sum('price');
}