feat: add TaxRule base class

src/Tax/TaxRule.php

@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wearepixel\Cart\Tax;
+
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Contracts\CartConditionable;
+use Wearepixel\Cart\Exceptions\InvalidTaxRuleException;
+
+abstract class TaxRule implements CartConditionable
+{
+    protected string $name;
+
+    protected string $value;
+
+    protected string $target = 'subtotal';
+
+    protected ?float $minimum = null;
+
+    protected ?float $maximum = null;
+
+    abstract public function isApplicable(): bool;
+
+    public function toCondition(): CartCondition
+    {
+        if (! $this->isApplicable()) {
+            throw new InvalidTaxRuleException("Tax rule [{$this->name}] is not applicable.");
+        }
+
+        $data = [
+            'name'   => $this->name,
+            'type'   => 'tax',
+            'value'  => $this->value,
+            'target' => $this->target,
+        ];
+
+        if ($this->minimum !== null) {
+            $data['minimum'] = $this->minimum;
+        }
+
+        if ($this->maximum !== null) {
+            $data['maximum'] = $this->maximum;
+        }
+
+        return new CartCondition($data);
+    }
+}