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`.

Author: Jack Dunham

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

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))