实现订单状态机。

订单状态包括：
```
pending
paid
shipped
completed
cancelled
```
允许流转：
```
$transitions = [
    'pending' => ['paid', 'cancelled'],
    'paid' => ['shipped', 'cancelled'],
    'shipped' => ['completed'],
    'completed' => [],
    'cancelled' => [],
];
```
要求：
```
实现 OrderStateMachine
判断是否可以从当前状态切换到目标状态
执行状态切换
非法切换抛出异常
状态配置可以从外部传入
```
练习点：
```
状态模式
声明式状态配置
异常处理
业务规则集中管理
```

这是声明式状态机。

代码
```
<?php

class OrderStateMachine
{
    public function __construct(
        private array $transitions
    ) {}

/**
     * 判断是否可以流转
     */
    public function canTransition(string $from, string $to): bool
    {
        return in_array($to, $this->transitions[$from] ?? [], true);
    }

/**
     * 执行流转
     */
    public function transition(OrderEntity $order, string $to): void
    {
        $from = $order->status;

if (!$this->canTransition($from, $to)) {
            throw new RuntimeException("订单状态不能从 {$from} 切换到 {$to}");
        }

$order->status = $to;
    }
}

/**
 * 简单订单实体
 */
class OrderEntity
{
    public function __construct(
        public int $id,
        public string $status = 'pending'
    ) {}
}

$transitions = [
    'pending' => ['paid', 'cancelled'],
    'paid' => ['shipped', 'cancelled'],
    'shipped' => ['completed'],
    'completed' => [],
    'cancelled' => [],
];

$order = new OrderEntity(1);

$machine = new OrderStateMachine($transitions);

$machine->transition($order, 'paid');
echo $order->status . PHP_EOL;

$machine->transition($order, 'shipped');
echo $order->status . PHP_EOL;

```

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