collapse overlapping ranges in list

overlapcollapse.groovy

List.metaClass.collapse = {->
    def start,end
    def reverse = delegate[0]?.getAt(0)-delegate[0]?.getAt(-1) > 0
    def expanded = delegate.flatten().unique().sort()

    def result = (reverse ? expanded.reverse() : expanded).inject([]) { l, n ->
        start = start == null ? n : start
        end = end == null ? n : end
        if ((reverse?n+1:n-1)==end) {
          end = n
        } else if (start != end) {
          l << (start..end)
          start=n
          end=n
        } 
        l
    } << (start..end)
    

    result
    
}

assert [ 12..13, 17..19, 18..22, 17..19, 22..23, 19..20 ].collapse() == [ 12..13, 17..23 ]
assert [ -2..3, -5..-2 ].collapse() == [-5..3]
assert [ 3..1, 1..5 ].collapse() == [5..1] // descending order initially, so descend wholly
assert [ 1..5, 3..1 ].collapse() == [1..5] 
assert [ 1..5, 3..1, -1..-4 ].collapse() == [-4..-1, 1..5] // ascending order initially, so ascend across the collection
assert [ 1..5, -6..-4, 3..1, -1..-4 ].collapse() == [-6..-1, 1..5] // mixed-direction provided, ascend in finality
assert [ 1..3, 5..6, 3..5 ].collapse() == [1..6]