Design Time-Based Key-Value Store

Design an in-memory key-value store that supports time-based lookups.

Operations

Implement a class or module with the following methods:

1. set(key, value, timestamp)

   • Stores the string value for the string key at the integer timestamp.
   • Multiple values may be stored for the same key, each associated with a different timestamp.

2. get(key, timestamp)

   • Returns the value associated with key for the largest stored timestamp t such that t <= timestamp.
   • If no such timestamp exists for the given key, return an empty string. Ensure consistency in the return type (e.g., always empty string or always null).

Notes and Constraints

• Timestamps for set() calls on the same key are guaranteed to be non-decreasing.
• The primary goal is to achieve efficient get() query performance.
• Keys and values are ASCII strings.
• The store is entirely in-memory.

Example

Consider the following sequence of operations:

• set("room", "A", 10)
• set("room", "B", 20)
• get("room", 5) -> ""
• get("room", 10) -> "A"
• get("room", 15) -> "A"
• get("room", 20) -> "B"
• get("room", 25) -> "B"