Jihanki: Vending Machine State Machine Implementation

Question

## Problem

Implement a vending machine (`Jihanki` is Japanese for vending machine). The machine accepts coins, displays available items with prices, dispenses an item if enough coins have been inserted, and returns change.

```python
class Jihanki:
    def __init__(self, inventory: dict[str, tuple[int, int]]):
        """inventory: {item_name: (price_cents, quantity)}"""
        pass

def insert_coin(self, amount: int) -> None:
        """Insert a coin of given denomination."""
        pass

def select_item(self, item: str) -> tuple[str, int]:
        """Attempt to purchase item. Return (status, change_returned).
        status: 'dispensed', 'insufficient_funds', 'out_of_stock', 'invalid_item'
        """
        pass

def cancel(self) -> int:
        """Cancel and return all inserted coins as total cents."""
        pass

def get_balance(self) -> int:
        pass
```

```
m = Jihanki({"cola": (150, 5), "water": (100, 2)})
m.insert_coin(100)
m.insert_coin(50)
m.select_item("cola")  -> ("dispensed", 0)
m.insert_coin(200)
m.select_item("cola")  -> ("dispensed", 50)
```

## Follow-ups

1. How would you enforce valid coin denominations (e.g., only 5, 10, 25, 50, 100 cents)?
2. How do you handle concurrent access if multiple users interact simultaneously?
3. Extend to support a maintenance mode that blocks purchases and allows restocking.
4. How would you model this as a formal state machine with defined states and transitions?

Jihanki: Vending Machine State Machine Implementation

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