Compress a given string by replacing consecutive repeating characters with the character followed by the count of repetitions. If the compressed string isn't shorter, return the original.

Design an algorithm to encode a list of strings into a single string and then decode the single string back to the original list of strings.

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