Adjacent number algorithm grouper

Question

By which I mean this:

Given the input set of numbers:

1,2,3,4,5 becomes \"1-5\".

1,2,3,5,7,9,10,11,12,14 becomes \"1-3, 5, 7, 9-12, 14\"

This is

Answer 1

Pure functional Python:

#!/bin/env python

def group(nums):
    def collect((acc, i_s, i_e), n):
        if n == i_e + 1: return acc, i_s, n
        return acc + ["%d"%i_s + ("-%d"%i_e)*(i_s!=i_e)], n, n
    s = sorted(nums)+[None]
    acc, _, __ = reduce(collect, s[1:], ([], s[0], s[0]))
    return ", ".join(acc)

assert group([1,2,3,5,7,9,10,11,12,14]) == "1-3, 5, 7, 9-12, 14"