Fix bug with length 1 AbstractNetworkIterator types not executing their only compute! call.

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.
@@ -44,6 +47,9 @@ mutable struct RegionIterator{Problem, RegionPlan} <: AbstractNetworkIterator
     which_region::Int
     const which_sweep::Int
     function RegionIterator(problem::P, region_plan::R, sweep::Int) where {P, R}
+        if length(region_plan) == 0
+            throw(BoundsError("Cannot construct a region iterator with 0 elements."))
+        end
         return new{P, R}(problem, region_plan, 1, sweep)
     end
 end
@@ -119,8 +125,15 @@ mutable struct SweepIterator{Problem, Iter} <: AbstractNetworkIterator
     which_sweep::Int
     function SweepIterator(problem::Prob, sweep_kwargs::Iter) where {Prob, Iter}
         stateful_sweep_kwargs = Iterators.Stateful(sweep_kwargs)
-        first_kwargs, _ = Iterators.peel(stateful_sweep_kwargs)
+        first_state = Iterators.peel(stateful_sweep_kwargs)
+
+        if isnothing(first_state)
+            throw(BoundsError("Cannot construct a sweep iterator with 0 elements."))
+        end
+
+        first_kwargs, _ = first_state
         region_iter = RegionIterator(problem; sweep = 1, first_kwargs...)
+
         return new{Prob, Iter}(region_iter, stateful_sweep_kwargs, 1)
     end
 end

test/solvers/test_iterators.jl

@@ -1,5 +1,5 @@
-using Test: @test, @testset
-using ITensorNetworks: SweepIterator, islaststep, state, increment!, compute!, eachregion
+using Test: @test, @testset, @test_throws
+using ITensorNetworks: SweepIterator, RegionIterator, islaststep, state, increment!, compute!, eachregion
 
 module TestIteratorUtils
 
@@ -49,6 +49,24 @@ 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
+
+            prob = TestIteratorUtils.TestProblem([])
+            @test_throws BoundsError SweepIterator(prob, 0)
+            @test_throws BoundsError RegionIterator(prob, [], 1)
+        end
+
         TI = TestIteratorUtils.TestIterator(1, 4, [])
 
         @test !islaststep((TI))
@@ -171,6 +189,17 @@ end
             @test prob.data[1:2:end] == fill(1, 5)
             @test prob.data[2:2:end] == fill(2, 5)
 
+
+            let i = 1, prob = TestIteratorUtils.TestProblem([])
+                SI = SweepIterator(prob, 1)
+                cb = []
+                for _ in eachregion(SI)
+                    push!(cb, i)
+                    i += 1
+                end
+                @test length(cb) == 2
+            end
+
         end
     end
 end