implement empty and isempty (#48)

Author: ranocha

src/RootedTrees.jl

@@ -83,6 +83,8 @@ iscanonical(t::RootedTree) = t.iscanonical
 #TODO: Validate rooted tree in constructor?
 
 Base.copy(t::RootedTree) = RootedTree(copy(t.level_sequence), t.iscanonical)
+Base.isempty(t::RootedTree) = isempty(t.level_sequence)
+Base.empty(t::RootedTree) = RootedTree(empty(t.level_sequence), iscanonical(t))
 
 
 #  #function RootedTree(sequence::Vector{T}, valid::Bool)

test/runtests.jl

@@ -67,12 +67,18 @@ Plots.unicodeplots()
   # more tests of the canonical representation
   t = rootedtree([1, 2, 3, 2, 3, 3, 2])
   @test t.level_sequence == [1, 2, 3, 3, 2, 3, 2]
+  @test !isempty(t)
 
   t = rootedtree([1, 2, 3, 2, 3, 4, 2, 3])
   @test t.level_sequence == [1, 2, 3, 4, 2, 3, 2, 3]
+  @test !isempty(t)
 
   t = rootedtree([1, 2, 3, 2, 3, 3, 2, 3])
   @test t.level_sequence == [1, 2, 3, 3, 2, 3, 2, 3]
+  @test !isempty(t)
+
+  @test isempty(rootedtree(Int[]))
+  @test isempty(empty(t))
 
   level_sequence = zeros(Int, RootedTrees.BUFFER_LENGTH + 1)
   level_sequence[1] -= 1