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I am building a custom hash implementation for storing NASDAQ ITCH order messages. Obviously this is keyed on the order reference number and I am wondering if these numbers are sequential, random or exhibit any interesting characteristics in their distribution.

The hash table will be on a relatively low memory device and knowing these characteristics will allow me to efficiently optimise the datastructure.

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This question is very similar to your previous one. – chrisaycock♦ Mar 22 '12 at 1:55
The only thing you're guaranteed is that they are day unique. Anything else would be heuristic, which I'd urge you to try to measure yourself if you think it is important. – Louis Marascio Mar 22 '12 at 3:11
@chrisaycock Similar although the previous one was volume, this one refers to the order IDs. I have added "reference" to the question to try and make this more clear. – Jonathan Evans Mar 22 '12 at 11:17
@LouisMarascio I may have to although this probably wont be possible as its a purely academic project. – Jonathan Evans Mar 22 '12 at 11:18
No offense, but I'll never buy that optimizing the hash map on NASDAQ's order id's is a purely academic project. – LazyCat Mar 22 '12 at 13:03
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I don't work with ITCH, but I would do the following:

1) ask the data provider.

2) whenever a new order arrives, record it's ID separately, and then analyze for a pattern.

Most probably they are sequentially growing numbers per instrument.

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