Fix bug with AbstractNetworkIterator types of length 1 not executing their single compute! call

Author: Jack Dunham

src/solvers/iterators.jl

@@ -12,7 +12,10 @@ abstract type AbstractNetworkIterator end
 islaststep(iterator::AbstractNetworkIterator) = state(iterator) >= length(iterator)
 
 function Base.iterate(iterator::AbstractNetworkIterator, init = true)
-    islaststep(iterator) && return nothing
+    # The assumption is that first "increment!" is implicit, therefore we must skip the
+    # the termination check for the first iteration, i.e. `AbstractNetworkIterator` is not
+    # defined when length < 1,
+    init || islaststep(iterator) && return nothing
     # We seperate increment! from step! and demand that any AbstractNetworkIterator *must*
     # define a method for increment! This way we avoid cases where one may wish to nest
     # calls to different step! methods accidentaly incrementing multiple times.

test/solvers/test_iterators.jl

@@ -49,6 +49,19 @@ end
     import .TestIteratorUtils
 
     @testset "`AbstractNetworkIterator` Interface" begin
+
+        @testset "Edge cases" begin
+            TI = TestIteratorUtils.TestIterator(1, 1, [])
+            cb = []
+            @test islaststep(TI)
+            for _ in TI
+                @test islaststep(TI)
+                push!(cb, state(TI))
+            end
+            @test length(cb) == 1
+            @test length(TI.output) == 1
+            @test only(cb) == 1
+        end
         TI = TestIteratorUtils.TestIterator(1, 4, [])
 
         @test !islaststep((TI))