Generate a & ldquo; effective & rdquo; encoding from a list of numbers

Possible Duplicate:
How to convert sequence of numbers in an array to range of numbers

``````1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 19, 21, 22, 23, 999, 1000, 1001
```
```

is there any library or code snipped that will turn the following into something like:

``````1-13, 19, 21-23, 999-1001
```
```

In other words, reduce a full list of numbers to a bunch of ranges or so. I haven't been able to find anything. If no such thing exists, anyone have some ideas for an efficient implementation?

``````def get_groups(lst):
slices = [i+1 for i, v in enumerate(zip(lst, l[1:])) if v[0] != v[1]-1]
slices = [0] + slices + [len(lst)]
for start, end in zip(slices, slices[1:]):
yield lst[start:end]

>>> list(get_groups([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 19, 21, 22, 23, 999, 1000, 1001]))
[[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13], [19], [21, 22, 23], [999, 1000, 1001]]
```
```

or

``````def get_ranges(lst):
slices = [i+1 for i, v in enumerate(zip(lst, l[1:])) if v[0] != v[1]-1]
slices = [0] + slices + [len(lst)]
for start, end in zip(slices, slices[1:]):
yield "%d-%d" % (lst[start], lst[end-1])

>>> list(get_ranges([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 19, 21, 22, 23, 999, 1000, 1001]))
['1-13', '19-19', '21-23', '999-1001']
```
```