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Customers need empty or full containers picked up or delivered
from a central facility\depot\yard\ramp.
We have the following customer types who need only and no more:
1 TFE) a full container from the ramp\depot\yard delivered to the customer and an empty container picked up,
2 TEF) an empty container brought to the customer and a full
container picked-up and brought to the ramp,
3 TEN) an empty container brought to the customer,
4 TNE) an empty container picked-up from customer,
5 TFN) a full container delivered from the ramp to the customer, no empty to pickup,
6 TNF) a full container picked-up from the customer and brought to the ramp, no empty needed,
7 TFF) a full container delivered to the customer and a full container
picked-up from same customer and brought to the ramp.
A truck-tractor can move only one container at a time.
The ramp has enough empty containers to satisfy any customer needs.
The ramp has sufficient storage for any empty containers that customers wish to get rid of.
A simple "out-and-back" way of satisfying each customer is to simply send one truck independently to each customer to do whatever delivery or pickup is required.
A notable feature is that empty containers are generic. A customer does not care where an empty container goes or where it came from.
This provides an opportunity to do better than the simple one truck out-and-back to each customer.
Thus, we can see that the following pairings are possible for one trip from the ramp and back:
We do not consider:
1) the more general pick-up and delivery problem in which
a truck may visit more than two customers, picking up an empty container at one,
delivering it to the next customer, proceeding with no container to a third
customer and picking up an empty and taking it to a fourth customer, etc.
2) different container types, e.g., different sizes, or
some customers might need regular, others an empty "reefer" trailer,