[Jprogramming] Equilibrium index

[Sherlock, Ric]

Was solving a recent Rosetta Code task
http://rosettacode.org/wiki/Equilibrium_index

"An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices."

My first idea was the most obvious:
eq0=: +/\ I.@:= +/\.

The idea of calculating the running sums twice seemed non-optimal so I came up with the following:
eq1=: I.@(= (+/ - 2 * +/\ - ]))
eq2=: I.@(+/ = +:@(+/\) - ])
eq3=: I.@(+/ = (2 * +/\) - ])

I was pretty happy with that, but then decided to test how much better it was.

ts=: 6!:2 , 7!:2@]
seq=: _25 + 1e6 ?@$ 50

   10 ts 'eq0 tst'
0.0133672715 8913728
   10 ts 'eq1 tst'
0.0283096334 8389504
   10 ts 'eq2 tst'
0.0228435712 8389440
   10 ts 'eq3 tst'
0.0217780194 8389440

Very nice to see that the simplest, most intuitive answer is also the fastest. Thanks Roger/Ken!