============================
V E N D O R    C H O O S E R
============================

The Vendor Chooser algorithm converts pre-orders into complete orders.
For each pre-order, it selects a vendor and item type, computes its
cost, and adds the room, start time, and duration of the event.


-= Input and output =-

Input: Current schedule, vendor description, pre-order list
Output: Order list

Note that the input pre-order list must be for the input schedule;
that is, each pre-order must refer to an event in the schedule.


-= Main steps =-

Create an empty Order-List
For every Pre-Order:
  Best-Cost = +Infinity
  For each Item-Type that matches the service property in Pre-Order:
    For each known Vendor:
      If Vendor offers Item-Type at a known cost:
        Let Cost be the total cost of ordering such items from this vendor,
          for the number of items and event duration specified in Pre-Order
        If Cost < Best-Cost:
          Best-Vendor = Vendor; Best-Item-Type = Item-Type; Best-Cost = Cost
  If Best-Cost == +Infinity, then terminate with an error
  Create Order from Pre-Order,
    by adding Best-Vendor, Best-Item-Type, and Best-Cost,
    as well as the room, start time, and duration of the event
  Add Order to Order-List
Return Order-List