Add iterator methods and length support to SequentialSampler (#130)

* Add iterator methods and length support to SequentialSampler

* Update SamplingReduction.jl

Author: Tortar

src/SamplingInterface.jl

@@ -197,6 +197,10 @@ struct SequentialSampler{S}
 end
 Base.iterate(s::SequentialSampler) = iterate(s.s)
 Base.iterate(s::SequentialSampler, state) = iterate(s.s, state)
+Base.IteratorEltype(::SequentialSampler) = Base.HasEltype()
+Base.eltype(::SequentialSampler) = Int
+Base.IteratorSize(::SequentialSampler) = Base.HasLength()
+Base.length(s::SequentialSampler) = s.s.n
 
 """
     itsample([rng], iter, method = AlgRSWRSKIP())

src/SamplingReduction.jl

@@ -11,11 +11,18 @@ function reduce_samples(t::Union{TypeS,TypeUnion}, ss::BinaryHeap...)
 end
 function reduce_samples(ps::AbstractArray, rngs, t::Union{TypeS,TypeUnion}, ss::AbstractArray...)
     nt = length(ss)
-    v = Vector{Vector{get_type_rs(t, ss...)}}(undef, nt)
+    T = get_type_rs(t, ss...)
+    v = Vector{Vector{T}}(undef, nt)
     n = minimum(length.(ss))
     ns = rand(extract_rng(rngs, 1), Multinomial(n, ps))
     Threads.@threads for i in 1:nt
-        v[i] = sample(extract_rng(rngs, i), ss[i], ns[i]; replace = false)
+        s = ss[i]
+        vi = Vector{T}(undef, ns[i])
+        @inbounds for (q, j) in enumerate(SequentialSampler(extract_rng(rngs, i), 
+                                          ns[i], length(s), AlgHiddenShuffle()))
+            vi[q] = s[j]
+        end
+        v[i] = vi
     end
     return reduce(vcat, v)
 end