Bugfix: include empty combination (#129)

* bugfix: include empty combination, update tests

---------

Co-authored-by: Stefan Rigger <[email protected]>
Co-authored-by: Chengyu Han <[email protected]>

Author: 3 people

src/combinations.jl

@@ -56,9 +56,7 @@ end
 Generate combinations of the elements of `a` of all orders. Chaining of order iterators
 is eager, but the sequence at each order is lazy.
 """
-combinations(a) = Iterators.flatten([combinations(a, k) for k = 1:length(a)])
-
-
+combinations(a) = Iterators.flatten([combinations(a, k) for k = 0:length(a)])
 
 # cool-lex combinations iterator
 

test/combinations.jl

@@ -1,6 +1,7 @@
 @testset "combinations" begin
-    @test [combinations([])...] == []
-    @test [combinations(['a', 'b', 'c'])...] == [['a'], ['b'], ['c'], ['a', 'b'], ['a', 'c'], ['b', 'c'], ['a', 'b', 'c']]
+    @test [combinations([])...] == [[]]
+    @test [combinations(['a', 'b', 'c'])...] == [[],['a'],['b'],['c'],['a','b'],['a','c'],['b','c'],['a','b','c']]
+    @test length(collect(combinations(1:5))) == 32
 
     @test [combinations("abc", 3)...] == [['a', 'b', 'c']]
     @test [combinations("abc", 2)...] == [['a', 'b'], ['a', 'c'], ['b', 'c']]