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O R D E R   D I F F
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The Order Diff algorithm determines the "delta" between two order
lists; that is, it identifies the placements, cancellations, and
modifications required for converting an old list into a new list.

If it finds an old order "similar" to some new order, it modifies the
old order to obtain the new order. If an old order is not similar to
any new orders, it cancels this order. If a new order is not similar
to any old order, it places this order. 

Currently, we consider two orders "similar" if they are for the same
event, same vendor, and same item type. We may later change the
similarity definition based on the user feedback.


-= Input and output =-

Input: Old order list, new order list
Output: List of order changes

The output does not include utilities of placements, modifications,
and cancellations. The system later calls the Utility Calc algorithm
to compute these utilities.


-= Main steps =-

Create empty Placement-List and Modification-List
Create Old-List, which initially includes all old orders
For every New-Order in the list of new orders:
  Search for Old-Order in Old-List that is identical to New-Order
  If not found, search for Old-Order in Old-List that is similar to New-Order
  If an identical Old-Order is found, then remove it from Old-List
  Else, if a similar Old-Order is found:
    Remove Old-Order from Old-List
    Add modification of Old-Order into New-Order to Modification-List
  Else, add placement of New-Order to Placement-List
Create Cancellation-List that includes all orders from Old-List
Create the list of order changes,
  by combining Placement-List, Cancellation-List, and Modification-List
Return the resulting list