You are given a grid of oranges, some fresh, some rotten. Rotten oranges contaminate adjacent fresh oranges; find the minimum time until all oranges are rotten.

Design a key-value store that supports storing values associated with timestamps. You should be able to retrieve the value for a given key at the latest timestamp less than or equal to a given time.

Implement a function to read characters from a file using a provided read4 API, but the read function can be called multiple times, so you need to handle internal buffering.