PBS Core Algorithms
This algorithm is used in conjunction with greedy algorithm to control the open time (the amount of uncovered pairings at each calendar date).
"Stacking" happens during the process of junior awarding when the number of the pairings needed to be covered at a given date is equal to the number of available crewmembers. The "available crewmembers" means those junior crewmembers who have not been awarded yet and are legal to cover a pairing at that date. Each given date in a bid month will have a "stack" to control open time.
When that point is reached, every available junior crewmember has to cover a pairing at that date to resolve the open time issue regardless of his/her preferences. This impacts the satisfaction of junior crewmembers in order to achieve this company goal.
To improve, the core traces back to the awarded senior crewmembers and sees if it is possible to let a senior cover a pairing at that date. This approach is called "un-stacking", because it reduces the stack at the given date. It is only good if the pairing being replaced is something junior crewmembers prefer, and the replacement does not degrade the senior crewmember's preferences and there is no violation of legality.
Greedy algorithm has no ability to constantly monitor the open time, and it keeps awarding lines until it reaches the stacking point, then uses inefficient "un-stacking" to improve the satisfaction of junior crewmembers. The result is questionable.
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