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Use queue.
Store top-time in each queue element
Before each operation check died elements and remove it from queue.
Put processed element into end of queue
To speed up searching of free element, you can store in some variable index of least element that can be empty. When you allocating next element, you increase it. When one elemenet if "dying" with index less than that stored value, you assign to that index of died element.
And so on and so on...