From 940177a44eb1b9cb421594254bfadca0ef12dae4 Mon Sep 17 00:00:00 2001
From: David Gustavsson <david.e.gustavsson@gmail.com>
Date: Sat, 28 Nov 2020 20:05:50 +0100
Subject: [PATCH 1/2] Change tab indentation to space indentation

---
 src/IterTools.jl | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/IterTools.jl b/src/IterTools.jl
index f8f11d2..71fdfc1 100644
--- a/src/IterTools.jl
+++ b/src/IterTools.jl
@@ -590,14 +590,14 @@ iterate(it::StaticSizeBinomial{0}, state=false) = state ? nothing : ((), true)
 pop(t::NTuple) = reverse(tail(reverse(t))), t[end]
 
 function advance(it::StaticSizeBinomial{K}, idx) where {K}
-	xs = it.xs
-	lidx, i = pop(idx)
+    xs = it.xs
+    lidx, i = pop(idx)
     i += 1
-	if i > length(xs) - K + length(idx)
-		lidx = advance(it, lidx)
-		i = lidx[end] + 1
-	end
-	return (lidx..., i)
+    if i > length(xs) - K + length(idx)
+        lidx = advance(it, lidx)
+        i = lidx[end] + 1
+    end
+    return (lidx..., i)
 end
 advance(it::StaticSizeBinomial, idx::NTuple{1}) = (idx[end]+1,)
 

From 6848eedf7e7599d872c4f922fb4bb94205c2e8bf Mon Sep 17 00:00:00 2001
From: David Gustavsson <david.e.gustavsson@gmail.com>
Date: Sat, 28 Nov 2020 21:10:13 +0100
Subject: [PATCH 2/2] Add alternate

---
 src/IterTools.jl | 67 +++++++++++++++++++++++++++++++++++++++++++++++-
 test/runtests.jl | 18 +++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/src/IterTools.jl b/src/IterTools.jl
index 71fdfc1..ac477b2 100644
--- a/src/IterTools.jl
+++ b/src/IterTools.jl
@@ -30,7 +30,9 @@ export
     takewhile,
     properties,
     propertyvalues,
-    fieldvalues
+    fieldvalues,
+    alternate
+
 
 function has_length(it)
     it_size = IteratorSize(it)
@@ -1031,4 +1033,67 @@ function iterate(fs::FieldValues, state=1)
     return (getfield(fs.x, state), state + 1)
 end
 
+# Alternate
+
+struct Alternate{Is<:Tuple}
+    xs::Is
+end
+"""
+    alternate(xs...)
+Alternates through each of the given iterators in order, shuffling their values
+until one of them runs out.
+
+```jldoctest
+julia> collect(alternate(1:5,10:-1:1))
+  1
+ 10
+  2
+  9
+  3
+  8
+  4
+  7
+  5
+  6
+```
+"""
+alternate(xs...) = length(xs) == 1 ? xs[1] : Alternate(xs)
+function Base.iterate(it::Alternate, state=1)
+    if state == 1
+        state=[Array{Any}(nothing,length(it.xs)),1]
+    end
+    if isnothing(state[1][state[2]])
+        itreturn = iterate(it.xs[state[2]])
+    else
+        itreturn = iterate(it.xs[state[2]],state[1][state[2]])
+    end
+    isnothing(itreturn) && return nothing
+    state[1][state[2]] = itreturn[2]
+    state[2] = (state[2] % length(it.xs)) + 1
+    return (itreturn[1],state)
+end
+function IteratorEltype(::Type{Alternate{Is}}) where Is
+    iteratoreltypes = IteratorEltype.(fieldtypes(Is))
+    any(iteratoreltypes .== Ref(EltypeUnknown())) && return EltypeUnknown()
+    return HasEltype()
+end
+eltype(::Type{Alternate{Is}}) where Is = Union{eltype.(fieldtypes(Is))...}
+function IteratorSize(::Type{Alternate{Is}}) where Is
+    iteratorsizes =  IteratorSize.(fieldtypes(Is))
+    any(iteratorsizes .== Ref(SizeUnknown())) && return SizeUnknown()
+    all(iteratorsizes .== Ref(IsInfinite())) && return IsInfinite()
+    return HasLength() # Never returns HasShape
+end
+function length(i::Alternate)
+    m = Int64(minimum(IteratorSize(x) isa IsInfinite ? Inf : length(x) for x in i.xs))
+    l = length(i.xs)*m
+    for x in i.xs
+        if IteratorSize(x) isa IsInfinite || length(x) > m
+            l = l+1
+        else
+            return l
+        end
+    end
+end
+
 end # module IterTools
diff --git a/test/runtests.jl b/test/runtests.jl
index 2fa7de9..c91d786 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -524,6 +524,24 @@ include("testing_macros.jl")
         @test collect(fv3) == Any[]
     end
 
+    @testset "alternate" begin
+        intlist1 = 1:5
+        intlist2 = 2:4:14
+        inflist = countfrom(1)
+        unklist = takewhile(x -> x ≤ 10, countfrom(1))
+        shapedlist = [1 2 3;2 4 6]
+        str = "String"
+        @test eltype(alternate(intlist1,intlist2)) == Int64
+        @test eltype(alternate(intlist1,str)) == Union{Int64,Char}
+        @test length(alternate(intlist1,intlist2)) == 9
+        @test length(alternate(intlist2,intlist1)) == 8
+        @test IteratorSize(alternate(inflist,inflist)) isa IsInfinite
+        @test IteratorSize(alternate(intlist1,unklist)) isa SizeUnknown
+        @test IteratorSize(alternate(shapedlist,shapedlist)) isa HasLength
+        @test alternate(intlist1) == intlist1
+        @test collect(alternate(intlist1,intlist2,inflist)) == [1,2,1,2,6,2,3,10,3,4,14,4,5]
+    end
+
     @testset "traits overriding defaults" begin
         iters = [
             firstrest(1:10),