From 4b839aac408709daed4f0ef8195305fb8f7f8e6d Mon Sep 17 00:00:00 2001
From: Stefan Karpinski <stefan@karpinski.org>
Date: Tue, 15 Oct 2019 13:18:41 -0400
Subject: [PATCH] unzip: now with widening recursion trick

---
 base/iterators.jl | 54 +++++++++++++++++++++++++++++++----------------
 1 file changed, 36 insertions(+), 18 deletions(-)

diff --git a/base/iterators.jl b/base/iterators.jl
index 9091430cfd894..1660c7ab1a0c4 100644
--- a/base/iterators.jl
+++ b/base/iterators.jl
@@ -426,28 +426,46 @@ julia> unzip([[1, "apple"], [2.5, "orange"], [0, "mango"]])
 ```
 """
 function unzip(itrs)
-    n = Base.haslength(itrs) ? length(itrs) : -1
-    vecs = Vector[]
-    for itr in itrs
-        for (j, x) in enumerate(itr)
-            if length(vecs) < j
-                v = [x]
-                push!(vecs, v)
-                n ≥ 0 && sizehint!(v, n)
-            else
-                v = vecs[j]
-                if !(x isa eltype(v))
-                    T = Base.promote_typejoin(typeof(x), eltype(v))
-                    v = vecs[j] = copyto!(similar(v, T), v)
-                    n ≥ 0 && sizehint!(v, n)
+    n = Base.haslength(itrs) ? length(itrs) : nothing
+    outer = iterate(itrs)
+    outer === nothing && return ()
+    vals, state = outer
+    vecs = ntuple(length(vals)) do i
+        x = vals[i]
+        v = Vector{typeof(x)}(undef, something(n, 1))
+        @inbounds v[1] = x
+        return v
+    end
+    unzip_rest(vecs, typeof(vals), n isa Int ? 1 : nothing, itrs, state)
+end
+
+function unzip_rest(vecs, eltypes, i, itrs, state)
+    while true
+        i isa Int && (i += 1)
+        outer = iterate(itrs, state)
+        outer === nothing && return vecs
+        itr, state = outer
+        vals = Tuple(itr)
+        if vals isa eltypes
+            for (v, x) in zip(vecs, vals)
+                if i isa Int
+                    @inbounds v[i] = x
+                else
+                    push!(v, x)
                 end
-                push!(v, x)
             end
+        else
+            vecs′ = map(vecs, vals) do v, x
+                T = Base.promote_typejoin(eltype(v), typeof(x))
+                v′ = Vector{T}(undef, length(v) + !(i isa Int))
+                copyto!(v′, v)
+                @inbounds v′[something(i, end)] = x
+                return v′
+            end
+            eltypes′ = Tuple{map(eltype, vecs′)...}
+            return unzip_rest(Tuple(vecs′), eltypes′, i, itrs, state)
         end
-        length(first(vecs)) == length(last(vecs)) ||
-            throw(ArgumentError("unzip called with uneven iterators"))
     end
-    return Tuple(vecs)
 end
 
 # filter