Build Mobile API Request Cache

A mobile app at PulseFit calls several REST endpoints repeatedly. To reduce unnecessary network requests, implement an in-memory cache that stores API responses by endpoint key and expires entries after a fixed time-to-live.

Write a function that processes a sequence of cache operations and returns the result of each get operation.

Formal Specification

Implement process_api_cache(operations, ttl) where:

• operations is a list of operations.
• Each operation is one of:
  • ["set", key, value, timestamp]
  • ["get", key, timestamp]
  • ["invalidate", key, timestamp]
• key and value are strings.
• timestamp is a non-negative integer.
• ttl is a positive integer.

For a get at time t, return the cached value only if the key exists and t - set_timestamp < ttl. Otherwise return "MISS". An invalidate removes the key immediately if present. Return a list containing the outputs of all get operations in order.