Computing distances between two empty PersistenceDiagrams (#54)

* Added unit test for distances between two empty persistence diagrams

* Distances between empty persistence diagrams return zero instantaneously

* Update test/matching.jl

Co-authored-by: mtsch <matijacufar@gmail.com>

* Update src/matching.jl

Co-authored-by: mtsch <matijacufar@gmail.com>

* Update src/matching.jl

Co-authored-by: mtsch <matijacufar@gmail.com>

Co-authored-by: mtsch <matijacufar@gmail.com>

Author: mbudisic

src/matching.jl

@@ -402,6 +402,14 @@ function (::Bottleneck)(left::PersistenceDiagram, right::PersistenceDiagram; mat
         end
     end
 
+    if length(left) == 0 & length(right) == 0 
+        if matching
+            return Matching(left, right, 0, Pair{Int,Int}[], true)
+        else
+            return 0.0
+        end
+    end 
+
     graph = BottleneckGraph(left, right)
     edges = graph.edges
 
@@ -494,6 +502,15 @@ end
 function (w::Wasserstein)(
     left::PersistenceDiagram, right::PersistenceDiagram; matching=false
 )
+
+    if length(left) == 0 & length(right) == 0 
+        if matching
+            return Matching(left, right, 0, Pair{Int,Int}[], false)
+        else
+            return 0.0
+        end
+    end 
+
     if count(!isfinite, left) == count(!isfinite, right)
         adj = _adjacency_matrix(right, left, w.q)
         match = collect(i => j for (i, j) in enumerate(hungarian(adj)[1]))

test/matching.jl

@@ -79,6 +79,14 @@ end
     @test weight(Wasserstein(), diag1, diag1) ≡ 0.0
 end
 
+@testset "Empty diagrams" begin
+    diag = PersistenceDiagram([])
+
+    @test Wasserstein()(diag,diag; matching = false) ≡ 0.0
+    @test Bottleneck()(diag,diag; matching = false) ≡ 0.0
+
+end
+
 @testset "Infinite intervals" begin
     diag1 = PersistenceDiagram([(1, 2), (5, 8), (1, Inf)])
     diag2 = PersistenceDiagram([(1, 2), (3, 4), (5, 10)])