Add multiset_permutations method that doesn't require length (#164)

* Add `multiset_permutations` method that doesn't require length

in analogy to the same case for regular `permuations`

* Add test

* Update src/permutations.jl

Co-authored-by: Thomas Christensen <tchr@mit.edu>

* Update test/permutations.jl

* Update src/permutations.jl

Co-authored-by: Chengyu Han <cyhan.dev@outlook.com>

---------

Co-authored-by: Thomas Christensen <tchr@mit.edu>
Co-authored-by: Chengyu Han <cyhan.dev@outlook.com>

Author: 3 people

src/permutations.jl

@@ -211,10 +211,18 @@ function Base.length(c::MultiSetPermutations)
     return round(Int, p[t+1])
 end
 
+
+"""
+    multiset_permutations(a)
+
+Generate all permutations of an array `a` where `a` may have duplicated elements.
+"""
+multiset_permutations(a) = multiset_permutations(a, length(a))
+
 """
     multiset_permutations(a, t)
 
-Generate all permutations of size `t` from an array `a` with possibly duplicated elements.
+Generate all permutations of size `t` from an array `a` where `a` may have duplicated elements.
 
 # Examples
 ```jldoctest

test/permutations.jl

@@ -59,6 +59,8 @@ end
     @test collect(multiset_permutations("aabc", 5)) == Any[]
     @test collect(multiset_permutations("aabc", 2)) == Any[['a', 'a'], ['a', 'b'], ['a', 'c'], ['b', 'a'],
         ['b', 'c'], ['c', 'a'], ['c', 'b']]
+    @test collect(multiset_permutations("aabcc", 5)) == collect(multiset_permutations("aabcc"))
+    @test collect(multiset_permutations("")) == Any[Char[]]
     @test collect(multiset_permutations("aabc", 0)) == Any[Char[]]
     @test collect(multiset_permutations("aabc", -1)) == Any[]
     @test collect(multiset_permutations("", 1)) == Any[]