[OT] The horizon of a stream

[Andrei Alexandrescu]

Consider some text stream. We define the horizon of the stream as the 
longest number of lines between two identical lines minus one, or zero 
if the stream consists of unique lines.

For example, this stream has horizon 9:

lorem
ipsum
dolor
habeas
corpus
ipsum
sit
amet
rigor
mortis
consectetur
coitus
interruptus
corpus
adipisicing
elit

This is because the word "corpus" occurs on line 5 and line 14. This is 
the farthest two lines in the file are; notice that "ipsum" also occurs 
twice, but at a smaller distance.

The challenge, should you blah blah, is to compute the horizon of a 
potentially large file. Of course theoretical and practical speed are as 
important as always.

This is a task that's interesting and subtle in quite a number of ways, 
and it allows for many creative solutions. Therefore I intentionally did 
not restrict it much. Let ideas flow!


Andrei