From c4c826ae7a3d6542986bffc3fbc04332b4e58e00 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Thu, 7 Jan 2016 18:02:54 -0500
Subject: [PATCH] add efficient product iterator

note: not exported yet
---
 base/iterator.jl   | 68 ++++++++++++++++++++++++++++++++++++++++++++++
 test/functional.jl | 11 ++++++++
 2 files changed, 79 insertions(+)

diff --git a/base/iterator.jl b/base/iterator.jl
index 3d71068d8280d..4d94146dc39c5 100644
--- a/base/iterator.jl
+++ b/base/iterator.jl
@@ -223,3 +223,71 @@ next(it::Repeated, state) = (it.x, nothing)
 done(it::Repeated, state) = false
 
 repeated(x, n::Int) = take(repeated(x), n)
+
+# product
+
+abstract AbstractProdIterator
+
+immutable Prod2{I1, I2} <: AbstractProdIterator
+    a::I1
+    b::I2
+end
+
+"""
+    product(iters...)
+
+Returns an iterator over the product of several iterators. Each generated element is
+a tuple whose `i`th element comes from the `i`th argument iterator. The first iterator
+changes the fastest. Example:
+
+    julia> collect(product(1:2,3:5))
+    6-element Array{Tuple{Int64,Int64},1}:
+     (1,3)
+     (2,3)
+     (1,4)
+     (2,4)
+     (1,5)
+     (2,5)
+"""
+product(a) = Zip1(a)
+product(a, b) = Prod2(a, b)
+eltype{I1,I2}(::Type{Prod2{I1,I2}}) = Tuple{eltype(I1), eltype(I2)}
+length(p::AbstractProdIterator) = length(p.a)*length(p.b)
+
+function start(p::AbstractProdIterator)
+    s1, s2 = start(p.a), start(p.b)
+    s1, s2, Nullable{eltype(p.b)}(), (done(p.a,s1) || done(p.b,s2))
+end
+
+@inline function prod_next(p, st)
+    s1, s2 = st[1], st[2]
+    v1, s1 = next(p.a, s1)
+
+    nv2 = st[3]
+    if isnull(nv2)
+        v2, s2 = next(p.b, s2)
+    else
+        v2 = nv2.value
+    end
+
+    if done(p.a, s1)
+        return (v1,v2), (start(p.a), s2, oftype(nv2,nothing), done(p.b,s2))
+    end
+    return (v1,v2), (s1, s2, Nullable(v2), false)
+end
+
+@inline next(p::Prod2, st) = prod_next(p, st)
+@inline done(p::AbstractProdIterator, st) = st[4]
+
+immutable Prod{I1, I2<:AbstractProdIterator} <: AbstractProdIterator
+    a::I1
+    b::I2
+end
+
+product(a, b, c...) = Prod(a, product(b, c...))
+eltype{I1,I2}(::Type{Prod{I1,I2}}) = tuple_type_cons(eltype(I1), eltype(I2))
+
+@inline function next{I1,I2}(p::Prod{I1,I2}, st)
+    x = prod_next(p, st)
+    ((x[1][1],x[1][2]...), x[2])
+end
diff --git a/test/functional.jl b/test/functional.jl
index 6c60e62a96191..1e63186762b14 100644
--- a/test/functional.jl
+++ b/test/functional.jl
@@ -155,6 +155,17 @@ let i = 0
     end
 end
 
+# product
+# -------
+
+@test isempty(Base.product(1:2,1:0))
+@test isempty(Base.product(1:2,1:0,1:10))
+@test isempty(Base.product(1:2,1:10,1:0))
+@test isempty(Base.product(1:0,1:2,1:10))
+@test collect(Base.product(1:2,3:4)) == [(1,3),(2,3),(1,4),(2,4)]
+@test isempty(collect(Base.product(1:0,1:2)))
+@test length(Base.product(1:2,1:10,4:6)) == 60
+
 # foreach
 let
     a = []