The signal that comes back

Problem

Up at the Mount Vela station, the radio telescope logs one signal an hour, as a whole-number code, in the order they arrive. The astronomers are looking for the code that came back after the longest time.

Write a function that takes the list of codes (whole numbers, possibly empty) and returns, as a whole number, the largest gap between two entries with the same code: the position of the later one minus the position of the earlier one. If a code shows up three or more times, you measure from its first time to its last. An entry never pairs with itself: if no code repeats, or the list is empty, you return 0.

With [7, 3, 9, 3, 7, 5, 3]: the 7 is at positions 0 and 4, a gap of 4. The 3 is at 1, 3 and 6, and from 1 to 6 is 5. You return 5.

Examples

• The example

  [7, 3, 9, 3, 7, 5, 3] → 5

• Two in a row

  [4, 4] → 1

• Nothing repeats

  [1, 2, 3] → 0

• Three appearances

  [8, 1, 2, 8, 3, 8] → 5

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def longest_return(codes):
    pass

function longestReturn(codes) {
}