From 548654eb5f10f95813bd265e1d297744d23143ca Mon Sep 17 00:00:00 2001
From: Jack Dunham <jdunham@flatironinstitute.org>
Date: Tue, 28 Oct 2025 10:37:57 -0400
Subject: [PATCH 1/3] Fix bug with AbstractNetworkIterator types of length 1
 not executing their single `compute!` call

---
 src/solvers/iterators.jl       |  5 ++++-
 test/solvers/test_iterators.jl | 13 +++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/solvers/iterators.jl b/src/solvers/iterators.jl
index 990d86d2..b11a86c1 100644
--- a/src/solvers/iterators.jl
+++ b/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.
diff --git a/test/solvers/test_iterators.jl b/test/solvers/test_iterators.jl
index 730eee93..7f616d68 100644
--- a/test/solvers/test_iterators.jl
+++ b/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))

From 89c787ba5118c5b65ba88bf8dcf96db272efff16 Mon Sep 17 00:00:00 2001
From: Jack Dunham <jdunham@flatironinstitute.org>
Date: Tue, 28 Oct 2025 10:39:06 -0400
Subject: [PATCH 2/3] Constructing a `SweepIterator` and `RegionIterator` of
 length 0 now gives `BoundsError`.

Constructing `RegionIterator` of length 0 is now undefined.

Adjust region iterator `BoundsError` message to align with
`SweepIterator`.
---
 src/solvers/iterators.jl       | 12 +++++++++++-
 test/solvers/test_iterators.jl |  9 +++++++--
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/solvers/iterators.jl b/src/solvers/iterators.jl
index b11a86c1..1fe48449 100644
--- a/src/solvers/iterators.jl
+++ b/src/solvers/iterators.jl
@@ -47,6 +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
@@ -122,8 +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
diff --git a/test/solvers/test_iterators.jl b/test/solvers/test_iterators.jl
index 7f616d68..b88b1202 100644
--- a/test/solvers/test_iterators.jl
+++ b/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
 
@@ -61,7 +61,12 @@ 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))

From 9d05d683d8606813f841ff27b2b73ca7aeedc585 Mon Sep 17 00:00:00 2001
From: Jack Dunham <jdunham@flatironinstitute.org>
Date: Tue, 28 Oct 2025 12:50:02 -0400
Subject: [PATCH 3/3] Add test for single element `EachRegion` types

---
 test/solvers/test_iterators.jl | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/test/solvers/test_iterators.jl b/test/solvers/test_iterators.jl
index b88b1202..b73e2189 100644
--- a/test/solvers/test_iterators.jl
+++ b/test/solvers/test_iterators.jl
@@ -189,6 +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