sort a javascript array using a decorate-sort-undecorate pattern; especially useful if the sort comparison is expensive to compute

gistfile1.coffee

# Accepts a list and a key function to apply to each item,
# and then uses a decorate-sort-undecorate pattern to actually
# do the sorting. Returns a new list. Keeps track of the index
# to make sure the sort is stable.
sort_by = (list, key) ->
    cmp = ({key: k_a, index: i_a}, {key: k_b, index: i_b}) ->
        if k_a == k_b then i_a - i_b else if k_a < k_b then -1 else 1
    decorated = ({item, index, key: key item} for item, index in list)
    item for {item} in decorated.sort cmp