iterators.jl

# This file is a part of Julia. License is MIT: https://julialang.org/license

using Base.Iterators
using Random
using Base: IdentityUnitRange
using Dates: Date, Day

@test Base.IteratorSize(Any) isa Base.SizeUnknown

# zip and filter iterators
# issue #4718
@test collect(Iterators.filter(x->x[1], zip([true, false, true, false],"abcd"))) == [(true,'a'),(true,'c')]

let z = zip(1:2)
    @test size(z) == (2,)
    @test collect(z) == [(1,), (2,)]
    # Issue #13979
    @test eltype(z) == Tuple{Int}
end

for z in (zip(1:2, 3:4), zip(1:2, 3:5))
    @test collect(z) == [(1,3), (2,4)]
    @test eltype(z) == Tuple{Int,Int}
    @test size(z) == (2,)
    @test axes(z) == (Base.OneTo(2),)
    @test length(z) == 2
end

let z = zip(1:2, Iterators.countfrom(3))
    @test collect(z) == [(1,3), (2,4)]
    @test eltype(z) == Tuple{Int,Int}
    @test_throws MethodError size(z) # by convention, the zip of a finite and
                         # an infinite iterator has only `length`
    @test_throws MethodError axes(z)
    @test length(z) == 2
end

let z = zip([i*j for i in 1:3, j in -1:2:1], 1:6)
    @test collect(z) == [(-1, 1)
                         (-2, 2)
                         (-3, 3)
                         (1, 4)
                         (2, 5)
                         (3, 6) ]
    @test eltype(z) == Tuple{Int,Int}
    @test_throws DimensionMismatch size(z)
    @test_throws DimensionMismatch axes(z)
    @test length(z) == 6
end

let z = zip([i*j for i in 1:3, j in -1:2:1], [i*j for i in 1:3, j in -1:2:1])
    @test collect(z) == [(-1, -1) (1, 1)
                        (-2, -2) (2, 2)
                        (-3, -3) (3, 3)]
    @test eltype(z) == Tuple{Int,Int}
    @test size(z) == (3, 2)
    @test axes(z) == (Base.OneTo(3), Base.OneTo(2))
    @test length(z) == 6
end

let z = zip(1:2, 3:4, 5:6)
    @test size(z) == (2,)
    @test collect(z) == [(1,3,5), (2,4,6)]
    @test eltype(z) == Tuple{Int,Int,Int}
end

@test eltype(Iterators.filter(isodd, 1:5)) == Int

# typed `collect`
@test collect(Float64, Iterators.filter(isodd, [1,2,3,4]))[1] === 1.0

# check direct EachLine constructor
let b = IOBuffer("foo\n")
    @test collect(Base.EachLine(b)) == ["foo"]
    seek(b, 0)
    @test collect(Base.EachLine(b, keep=true)) == ["foo\n"]
    seek(b, 0)
    @test collect(Base.EachLine(b, ondone=()->0)) == ["foo"]
    seek(b, 0)
    @test collect(Base.EachLine(b, keep=true, ondone=()->0)) == ["foo\n"]
end

# enumerate (issue #6284)
let b = IOBuffer("1\n2\n3\n"), a = []
    for (i,x) in enumerate(eachline(b))
        push!(a, (i,x))
    end
    @test a == [(1,"1"),(2,"2"),(3,"3")]
end

# zip eachline (issue #7369)
let zeb = IOBuffer("1\n2\n3\n4\n5\n"),
    letters = ['a', 'b', 'c', 'd', 'e'],
    res     = []
    for (number, letter) in zip(eachline(zeb), letters)
        push!(res, (parse(Int,strip(number)), letter))
    end
    @test res == [(1, 'a'), (2, 'b'), (3, 'c'), (4, 'd'), (5, 'e')]
end

@test length(zip(cycle(1:3), 1:7)) == 7
@test length(zip(cycle(1:3), 1:7, cycle(1:3))) == 7
@test length(zip(1:3,product(1:7,cycle(1:3)))) == 3
@test length(zip(1:3,product(1:7,cycle(1:3)),8)) == 1

# map
# ----
@testset "Iterators.map" begin
    @test collect(Iterators.map(string, 1:3)::Base.Generator) == map(string, 1:3)
    @test collect(Iterators.map(tuple, 1:3, 4:6)::Base.Generator) == map(tuple, 1:3, 4:6)
end

# rest
# ----
let s = "hello"
    _, st = iterate(s)
    c = collect(rest(s, st))
    @test c == ['e','l','l','o']
    @test c isa Vector{Char}
    @test rest(s, st) == rest(rest(s,4),st)
end

@test_throws MethodError collect(rest(countfrom(1), 5))

# countfrom
# ---------
let i = 0, k = 1, l = 0
    for j = countfrom(0, 2)
        @test j == i*2
        i += 1
        i <= 10 || break
    end
    for j = countfrom()
        @test j == k
        k += 1
        k <= 10 || break
    end
    # test that `start` promotes to `typeof(start+step)`
    for j = countfrom(Int[0, 0], Float64[1.0, 2.0])
        @test j isa Vector{Float64}
        @test j == l*[1, 2]
        l += 1
        l <= 10 || break
    end
    # test with `start` and `step` having different types
    @test collect(take(countfrom(Date(2020,12,25), Day(1)), 12)) == range(Date(2020,12,25), step=Day(1), length=12)
end

# take
# ----
let t = take(0:2:8, 10), i = 0
    @test length(collect(t)) == 5 == length(t)

    for j = t
        @test j == i*2
        i += 1
    end
    @test i == 5
end

let i = 0
    for j = take(0:2:100, 10)
        @test j == i*2
        i += 1
    end
    @test i == 10
end

@test isempty(take(0:2:8, 0))
@test_throws ArgumentError take(0:2:8, -1)
@test length(take(1:3,typemax(Int))) == 3
@test length(take(countfrom(1),3)) == 3
@test length(take(1:6,3)) == 3

# drop
# ----
let i = 0
    for j = drop(0:2:10, 2)
        @test j == (i+2)*2
        i += 1
    end
    @test i == 4
end

@test isempty(drop(0:2:10, 100))
@test isempty(collect(drop(0:2:10, 100)))
@test_throws ArgumentError drop(0:2:8, -1)
@test length(drop(1:3,typemax(Int))) == 0
@test Base.IteratorSize(drop(countfrom(1),3)) == Base.IsInfinite()
@test_throws MethodError length(drop(countfrom(1), 3))

# double take
# and take/drop canonicalization
# -----------
for xs in Any["abc", [1, 2, 3]]
    @test take(take(xs, 2), 3) === take(xs, 2)
    @test take(take(xs, 4), 2) === take(xs, 2)
    @test drop(drop(xs, 1), 1) === drop(xs, 2)
    @test take(drop(xs, 1), 1) === drop(take(xs, 2), 1)
    @test take(drop(xs, 3), 0) === drop(take(xs, 2), 3)
    @test isempty(drop(drop(xs, 2), 2))
    @test drop(take(drop(xs, 1), 2), 1) === take(drop(xs, 2), 1)
    @test take(drop(take(xs, 3), 1), 1) === take(drop(xs, 1), 1)
end

# takewhile
# --------
@testset begin
    @test collect(takewhile(<(4),1:10)) == [1,2,3]
    @test collect(takewhile(<(4),Iterators.countfrom(1))) == [1,2,3]
    @test collect(takewhile(<(4),5:10)) == []
    @test collect(takewhile(Returns(true),5:10)) == 5:10
    @test collect(takewhile(isodd,[1,1,2,3])) == [1,1]
    @test collect(takewhile(<(2), takewhile(<(3), [1,1,2,3]))) == [1,1]
    @test Base.IteratorEltype(typeof(takewhile(<(4),Iterators.map(identity, 1:10)))) isa Base.EltypeUnknown
end

# dropwhile
# --------
@testset begin
    @test collect(dropwhile(<(4), 1:10)) == 4:10
    @test collect(dropwhile(<(4), 1:10)) isa Vector{Int}
    @test isempty(dropwhile(<(4), []))
    @test collect(dropwhile(Returns(false),1:3)) == 1:3
    @test isempty(dropwhile(Returns(true), 1:3))
    @test collect(dropwhile(isodd,[1,1,2,3])) == [2,3]
    @test collect(dropwhile(iseven,dropwhile(isodd,[1,1,2,3]))) == [3]
    @test Base.IteratorEltype(typeof(dropwhile(<(4),Iterators.map(identity, 1:10)))) isa Base.EltypeUnknown
end

# cycle
# -----
let i = 0
    for j = cycle(0:3)
        @test j == i % 4
        i += 1
        i <= 10 || break
    end
end

# repeated
# --------
let i = 0
    for j = repeated(1, 10)
        @test j == 1
        i += 1
    end
    @test i == 10
end
let i = 0
    for j = repeated(1)
        @test j == 1
        i += 1
        i <= 10 || break
    end
end
@test eltype(repeated(0))    == Int
@test eltype(repeated(0, 5)) == Int
@test Base.IteratorSize(repeated(0))      == Base.IsInfinite()
@test Base.IteratorSize(repeated(0, 5))   == Base.HasLength()
@test Base.IteratorEltype(repeated(0))    == Base.HasEltype()
@test Base.IteratorEltype(repeated(0, 5)) == Base.HasEltype()
@test Base.IteratorSize(zip(repeated(0), repeated(0))) == Base.IsInfinite()

# product
# -------

# empty?
for itr in [product(1:0),
            product(1:2, 1:0),
            product(1:0, 1:2),
            product(1:0, 1:1, 1:2),
            product(1:1, 1:0, 1:2),
            product(1:1, 1:2 ,1:0)]
    @test isempty(itr)
    @test isempty(collect(itr))
end

# collect a product - first iterators runs faster
@test collect(product(1:2))           == [(i,)      for i=1:2]
@test collect(product(1:2, 3:4))      == [(i, j)    for i=1:2, j=3:4]
@test collect(product(1:2, 3:4, 5:6)) == [(i, j, k) for i=1:2, j=3:4, k=5:6]

# iteration order
let expected = [(1,3,5), (2,3,5), (1,4,5), (2,4,5), (1,3,6), (2,3,6), (1,4,6), (2,4,6)]
    actual = product(1:2, 3:4, 5:6)
    for (exp, act) in zip(expected, actual)
        @test exp == act
    end
end

# collect multidimensional array
let (a, b) = (1:3, [4 6;
                    5 7])
    p = product(a, b)
    @test size(p)    == (3, 2, 2)
    @test length(p)  == 12
    @test ndims(p)   == 3
    @test eltype(p)  == NTuple{2, Int}
    cp = collect(p)
    for i = 1:3
        @test cp[i, :, :] == [(i, 4) (i, 6);
                              (i, 5) (i, 7)]
    end
end

# collect stateful iterator
let itr
    itr = Iterators.Stateful(Iterators.map(identity, 1:5))
    @test collect(itr) == 1:5
    @test collect(itr) == Int[] # Stateful do not preserve shape
    itr = (i+1 for i in Base.Stateful([1, 2, 3]))
    @test collect(itr) == [2, 3, 4]
    @test collect(itr) == Int[] # Stateful do not preserve shape
    itr = (i-1 for i in Base.Stateful(zeros(Int, 0, 0)))
    @test collect(itr) == Int[] # Stateful do not preserve shape
end

# with 1D inputs
let a = 1:2,
    b = 1.0:10.0,
    c = Int32(1):Int32(0)

    # length
    @test length(product(a))       == 2
    @test length(product(a, b))    == 20
    @test length(product(a, b, c)) == 0

    # size
    @test size(product(a))         == (2,)
    @test size(product(a, b))      == (2, 10)
    @test size(product(a, b, c))   == (2, 10, 0)

    # eltype
    @test eltype(product(a))       == Tuple{Int}
    @test eltype(product(a, b))    == Tuple{Int, Float64}
    @test eltype(product(a, b, c)) == Tuple{Int, Float64, Int32}

    # ndims
    @test ndims(product(a))        == 1
    @test ndims(product(a, b))     == 2
    @test ndims(product(a, b, c))  == 3
end

# with multidimensional inputs
let a = randn(4, 4),
    b = randn(3, 3, 3),
    c = randn(2, 2, 2, 2)

    args = Any[(a,),
               (a, a),
               (a, b),
               (a, a, a),
               (a, b, c)]
    sizes = Any[(4, 4),
                (4, 4, 4, 4),
                (4, 4, 3, 3, 3),
                (4, 4, 4, 4, 4, 4),
                (4, 4, 3, 3, 3, 2, 2, 2, 2)]
    for (method, fun) in zip([size, ndims, length], [x->x, length, prod])
        for i in 1:length(args)
            @test method(product(args[i]...)) == method(collect(product(args[i]...))) == fun(sizes[i])
        end
    end
end

# more tests on product with iterators of various type
let iters = (1:2,
             rand(2, 2, 2),
             take(1:4, 2),
             product(1:2, 1:3),
             product(rand(2, 2), rand(1, 1, 1)),
             repeated([1, -1], 2)  # 28497
             )
    for method in [size, length, ndims, eltype]
        for i = 1:length(iters)
            args = (iters[i],)
            @test method(product(args...)) == method(collect(product(args...)))
            for j = 1:length(iters)
                args = iters[i], iters[j]
                @test method(product(args...)) == method(collect(product(args...)))
                for k = 1:length(iters)
                    args = iters[i], iters[j], iters[k]
                    @test method(product(args...)) == method(collect(product(args...)))
                end
            end
        end
    end
end

# product of finite length and infinite length iterators
let a = 1:2,
    b = countfrom(1),
    ab = product(a, b),
    ba = product(b, a),
    abexp = [(1, 1), (2, 1), (1, 2), (2, 2), (1, 3), (2, 3)],
    baexp = [(1, 1), (2, 1), (3, 1), (4, 1), (5, 1), (6, 1)]
    for (expected, actual) in zip([abexp, baexp], [ab, ba])
        for (i, el) in enumerate(actual)
            @test el == expected[i]
            i == length(expected) && break
        end
        @test_throws ArgumentError length(actual)
        @test_throws ArgumentError size(actual)
        @test_throws ArgumentError ndims(actual)
    end

    # size infinite or unknown raises an error
    for itr in Any[countfrom(1), Iterators.filter(Returns(0), 1:10)]
        @test_throws ArgumentError length(product(itr))
        @test_throws ArgumentError   size(product(itr))
        @test_throws ArgumentError  ndims(product(itr))
    end
end

# IteratorSize trait business
let f1 = Iterators.filter(i->i>0, 1:10)
    @test Base.IteratorSize(product(f1))               == Base.SizeUnknown()
    @test Base.IteratorSize(product(1:2, f1))          == Base.SizeUnknown()
    @test Base.IteratorSize(product(f1, 1:2))          == Base.SizeUnknown()
    @test Base.IteratorSize(product(f1, f1))           == Base.SizeUnknown()
    @test Base.IteratorSize(product(f1, countfrom(1))) == Base.IsInfinite()
    @test Base.IteratorSize(product(countfrom(1), f1)) == Base.IsInfinite()
end
@test Base.IteratorSize(product(1:2, countfrom(1)))          == Base.IsInfinite()
@test Base.IteratorSize(product(countfrom(2), countfrom(1))) == Base.IsInfinite()
@test Base.IteratorSize(product(countfrom(1), 1:2))          == Base.IsInfinite()
@test Base.IteratorSize(product(1:2))                        == Base.HasShape{1}()
@test Base.IteratorSize(product(1:2, 1:2))                   == Base.HasShape{2}()
@test Base.IteratorSize(product(take(1:2, 1), take(1:2, 1))) == Base.HasShape{2}()
@test Base.IteratorSize(product(take(1:2, 2)))               == Base.HasShape{1}()
@test Base.IteratorSize(product([1 2; 3 4]))                 == Base.HasShape{2}()

# IteratorEltype trait business
let f1 = Iterators.filter(i->i>0, 1:10)
    @test Base.IteratorEltype(product(f1))               == Base.HasEltype() # FIXME? eltype(f1) is Any
    @test Base.IteratorEltype(product(1:2, f1))          == Base.HasEltype() # FIXME? eltype(f1) is Any
    @test Base.IteratorEltype(product(f1, 1:2))          == Base.HasEltype() # FIXME? eltype(f1) is Any
    @test Base.IteratorEltype(product(f1, f1))           == Base.HasEltype() # FIXME? eltype(f1) is Any
    @test Base.IteratorEltype(product(f1, countfrom(1))) == Base.HasEltype() # FIXME? eltype(f1) is Any
    @test Base.IteratorEltype(product(countfrom(1), f1)) == Base.HasEltype() # FIXME? eltype(f1) is Any
end
@test Base.IteratorEltype(product(1:2, countfrom(1)))          == Base.HasEltype()
@test Base.IteratorEltype(product(countfrom(1), 1:2))          == Base.HasEltype()
@test Base.IteratorEltype(product(1:2))                        == Base.HasEltype()
@test Base.IteratorEltype(product(1:2, 1:2))                   == Base.HasEltype()
@test Base.IteratorEltype(product(take(1:2, 1), take(1:2, 1))) == Base.HasEltype()
@test Base.IteratorEltype(product(take(1:2, 2)))               == Base.HasEltype()
@test Base.IteratorEltype(product([1 2; 3 4]))                 == Base.HasEltype()

@test collect(product(1:2,3:4)) == [(1,3) (1,4); (2,3) (2,4)]
@test isempty(collect(product(1:0,1:2)))
@test length(product(1:2,1:10,4:6)) == 60
@test Base.IteratorSize(product(1:2, countfrom(1))) == Base.IsInfinite()

# intersection
@test intersect(product(1:3, 4:6), product(2:4, 3:5)) == Iterators.ProductIterator((2:3, 4:5))
@test intersect(product(1:3, [4 5 ; 6 7]), product(2:4, [7 6 ; 5 4])).iterators == (2:3, [4, 6, 5, 7])

# flatten
# -------
@test collect(flatten(Any[1:2, 4:5])) == Any[1,2,4,5]
@test collect(flatten(Any[flatten(Any[1:2, 6:5]), flatten(Any[10:7, 10:9])])) == Any[1,2]
@test collect(flatten(Any[flatten(Any[1:2, 4:5]), flatten(Any[6:7, 8:9])])) == Any[1,2,4,5,6,7,8,9]
@test collect(flatten(Any[flatten(Any[1:2, 6:5]), flatten(Any[6:7, 8:9])])) == Any[1,2,6,7,8,9]
@test collect(flatten(Any[2:1])) == Any[]
@test eltype(flatten(UnitRange{Int8}[1:2, 3:4])) == Int8
@test length(flatten(zip(1:3, 4:6))) == 6
@test length(flatten(1:6)) == 6
@test collect(flatten(Any[])) == Any[]
@test collect(flatten(())) == Union{}[]
@test_throws ArgumentError length(flatten(NTuple[(1,), ()])) # #16680
@test_throws ArgumentError length(flatten([[1], [1]]))

@test Base.IteratorEltype(Base.Flatten((i for i=1:2) for j=1:1)) == Base.EltypeUnknown()
# see #29112, #29464, #29548
@test Base.return_types(Base.IteratorEltype, Tuple{Array}) == [Base.HasEltype]

# partition(c, n)
let v = collect(partition([1,2,3,4,5], 1))
    @test all(i->v[i][1] == i, v)
end

let v1 = collect(partition([1,2,3,4,5], 2)),
    v2 = collect(partition(flatten([[1,2],[3,4],5]), 2)) # collecting partition with SizeUnkown
    @test v1[1] == v2[1] == [1,2]
    @test v1[2] == v2[2] == [3,4]
    @test v1[3] == v2[3] == [5]
end

let v = collect(partition([1,2,3,4,5], 2))
    @test v[1] == [1,2]
    @test v[2] == [3,4]
    @test v[3] == [5]
end

let v = collect(partition(enumerate([1,2,3,4,5]), 3))
    @test v[1] == [(1,1),(2,2),(3,3)]
    @test v[2] == [(4,4),(5,5)]
end

for n in [5,6]
    @test collect(partition([1,2,3,4,5], n))[1] == [1,2,3,4,5]
    @test collect(partition(enumerate([1,2,3,4,5]), n))[1] ==
          [(1,1),(2,2),(3,3),(4,4),(5,5)]
end

function iterate_length(iter)
    n=0
    for i in iter
        n += 1
    end
    return n
end
function simd_iterate_length(iter)
    n=0
    @simd for i in iter
        n += 1
    end
    return n
end
function simd_trip_count(iter)
    return sum(Base.SimdLoop.simd_inner_length(iter, i) for i in Base.SimdLoop.simd_outer_range(iter))
end
function iterate_elements(iter)
    vals = Vector{eltype(iter)}(undef, length(iter))
    i = 1
    for v in iter
        @inbounds vals[i] = v
        i += 1
    end
    return vals
end
function simd_iterate_elements(iter)
    vals = Vector{eltype(iter)}(undef, length(iter))
    i = 1
    @simd for v in iter
        @inbounds vals[i] = v
        i += 1
    end
    return vals
end
function index_elements(iter)
    vals = Vector{eltype(iter)}(undef, length(iter))
    i = 1
    for j in eachindex(iter)
        @inbounds vals[i] = iter[j]
        i += 1
    end
    return vals
end

@testset "CartesianPartition optimizations" for dims in ((1,), (64,), (101,),
                                                         (1,1), (8,8), (11, 13),
                                                         (1,1,1), (8, 4, 2), (11, 13, 17)),
                                                part in (1, 7, 8, 11, 63, 64, 65, 142, 143, 144)
    for fun in (i -> 1:i, i -> 1:2:2i, i -> Base.IdentityUnitRange(-i:i))
        iter = CartesianIndices(map(fun, dims))
        P = partition(iter, part)
        for I in P
            @test length(I) == iterate_length(I) == simd_iterate_length(I) == simd_trip_count(I)
            @test collect(I) == iterate_elements(I) == simd_iterate_elements(I) == index_elements(I)
        end
        @test all(Base.splat(==), zip(Iterators.flatten(map(collect, P)), iter))
    end
end
@testset "empty/invalid partitions" begin
    @test_throws ArgumentError partition(1:10, 0)
    @test_throws ArgumentError partition(1:10, -1)
    @test_throws ArgumentError partition(1:0, 0)
    @test_throws ArgumentError partition(1:0, -1)
    @test isempty(partition(1:0, 1))
    @test isempty(partition(CartesianIndices((0,1)), 1))
end
@testset "exact partition eltypes" for a in (Base.OneTo(24), 1:24, 1:1:24, LinRange(1,10,24), .1:.1:2.4, Vector(1:24),
                                             CartesianIndices((4, 6)), Dict((1:24) .=> (1:24)))
    P = partition(a, 2)
    @test eltype(P) === typeof(first(P))
    @test Iterators.IteratorEltype(P) == Iterators.HasEltype()
    if a isa AbstractArray
        P = partition(vec(a), 2)
        @test eltype(P) === typeof(first(P))
        P = partition(reshape(a, 6, 4), 2)
        @test eltype(P) === typeof(first(P))
        P = partition(reshape(a, 2, 3, 4), 2)
        @test eltype(P) === typeof(first(P))
    end
end

@test join(map(x->string(x...), partition("Hello World!", 5)), "|") ==
      "Hello| Worl|d!"

let s = "Monkey 🙈🙊🙊"
    tf = (n)->join(map(x->string(x...), partition(s,n)), "|")
    @test tf(10) == s
    @test tf(9) == "Monkey 🙈🙊|🙊"
    @test tf(8) == "Monkey 🙈|🙊🙊"
    @test tf(7) == "Monkey |🙈🙊🙊"
    @test tf(6) == "Monkey| 🙈🙊🙊"
    @test tf(5) == "Monke|y 🙈🙊🙊"
    @test tf(4) == "Monk|ey 🙈|🙊🙊"
    @test tf(3) == "Mon|key| 🙈🙊|🙊"
    @test tf(2) == "Mo|nk|ey| 🙈|🙊🙊"
    @test tf(1) == "M|o|n|k|e|y| |🙈|🙊|🙊"
end

@test Base.IteratorEltype(partition([1,2,3,4], 2)) == Base.HasEltype()
@test Base.IteratorEltype(partition((2x for x in 1:3), 2)) == Base.EltypeUnknown()

# take and friends with arbitrary integers (#19214)
for T in (UInt8, UInt16, UInt32, UInt64, UInt128, Int8, Int16, Int128, BigInt)
    @test length(take(1:6, T(3))) == 3
    @test length(drop(1:6, T(3))) == 3
    @test length(repeated(1, T(5))) == 5
    @test collect(partition(1:5, T(5)))[1] == 1:5
end

@testset "collect finite iterators issue #12009" begin
    @test eltype(collect(enumerate(Iterators.Filter(x -> x>0, randn(10))))) == Tuple{Int, Float64}
end

@testset "product iterator infinite loop" begin
    @test collect(product(1:1, (1, "2"))) == [(1, 1) (1, "2")]
end

@testset "filter empty iterable #16704" begin
    arr = filter(Returns(true), 1:0)
    @test length(arr) == 0
    @test eltype(arr) == Int
end

@testset "Pairs type" begin
    for A in ([4.0 5.0 6.0],
              [],
              (4.0, 5.0, 6.0),
              (a=4.0, b=5.0, c=6.0),
              (),
              NamedTuple(),
              (a=1.1, b=2.0),
             )
        d = pairs(A)
        @test d === pairs(d)
        @test isempty(d) == isempty(A)
        @test length(d) == length(A)
        @test keys(d) == keys(A)
        @test values(d) == A
        @test Base.IteratorSize(d) == Base.IteratorSize(A)
        @test Base.IteratorEltype(d) == Base.HasEltype()
        @test Base.IteratorSize(pairs([1 2;3 4])) isa Base.HasShape{2}
        @test isempty(d) || haskey(d, first(keys(d)))
        @test collect(v for (k, v) in d) == collect(A)
        if A isa NamedTuple
            K = Symbol
            V = isempty(d) ? Union{} : Float64
            @test isempty(d) || haskey(d, :a)
            @test !haskey(d, :abc)
            @test !haskey(d, 1)
            @test get(A, :key) do; 99; end == 99
        elseif A isa Tuple
            K = Int
            V = isempty(d) ? Union{} : Float64
        else
            K = A isa AbstractVector ? Int : CartesianIndex{2}
            V = isempty(d) ? Any : Float64
            @test get(A, 4, "not found") === "not found"
            if !isempty(A)
                @test get(A, 2, "not found") === 5.0
                @test getindex(d, 3) === 6.0
                @test setindex!(d, 9, 3) === d
                @test A[3] === 9.0
            end
        end
        @test keytype(d) == K
        @test valtype(d) == V
        @test eltype(d) == Pair{K, V}
    end

    let io = IOBuffer()
        Base.showarg(io, pairs([1,2,3]), true)
        @test String(take!(io)) == "pairs(::Vector{$Int})"
        Base.showarg(io, pairs((a=1, b=2)), true)
        @test String(take!(io)) == "pairs(::NamedTuple)"
        Base.showarg(io, pairs(IndexLinear(), zeros(3,3)), true)
        @test String(take!(io)) == "pairs(IndexLinear(), ::Matrix{Float64})"
        Base.showarg(io, pairs(IndexCartesian(), zeros(3)), true)
        @test String(take!(io)) == "pairs(IndexCartesian(), ::Vector{Float64})"
    end
end

@testset "reverse iterators" begin
    squash(x::Number) = x
    squash(A) = reshape(A, length(A))
    Z = Array{Int,0}(undef); Z[] = 17 # zero-dimensional test case
    for itr in (2:10, "∀ϵ>0", 1:0, "", (2,3,5,7,11), [2,3,5,7,11], rand(5,6), Z, 3, true, 'x', 4=>5,
                eachindex("∀ϵ>0"), view(Z), view(rand(5,6),2:4,2:6), (x^2 for x in 1:10),
                Iterators.Filter(isodd, 1:10), flatten((1:10, 50:60)), enumerate("foo"),
                pairs(50:60), zip(1:10,21:30,51:60), product(1:3, 10:12), repeated(3.14159, 5),
                (a=2, b=3, c=5, d=7, e=11))
        arr = reverse(squash(collect(itr)))
        itr = Iterators.reverse(itr)
        @test squash(collect(itr)) == arr
        if !isempty(arr)
            @test first(itr) == first(arr)
            @test last(itr) == last(arr)
        end
    end
    @test collect(take(Iterators.reverse(cycle(1:3)), 7)) == collect(take(cycle(3:-1:1), 7))
    let r = repeated(3.14159)
        @test Iterators.reverse(r) === r
        @test last(r) === 3.14159
    end
    for t in [(1,), (2, 3, 5, 7, 11), (a=1,), (a=2, b=3, c=5, d=7, e=11)]
        @test Iterators.reverse(Iterators.reverse(t)) === t
        @test first(Iterators.reverse(t)) === last(t)
        @test last(Iterators.reverse(t)) === first(t)
        @test collect(Iterators.reverse(t)) == reverse(collect(t))
    end
end

@testset "Iterators.Stateful" begin
    let a = @inferred(Iterators.Stateful("abcdef"))
        @test !isempty(a)
        @test popfirst!(a) == 'a'
        @test collect(Iterators.take(a, 3)) == ['b','c','d']
        @test collect(a) == ['e', 'f']
    end
    let a = @inferred(Iterators.Stateful([1, 1, 1, 2, 3, 4]))
        for x in a; x == 1 || break; end
        @test peek(a) == 3
        @test sum(a) == 7
    end
    @test eltype(Iterators.Stateful("a")) == Char
    # Interaction of zip/Stateful
    let a = Iterators.Stateful("a"), b = ""
    @test isempty(collect(zip(a,b)))
    @test !isempty(a)
    @test isempty(collect(zip(b,a)))
    @test !isempty(a)
    end
    let a = Iterators.Stateful("a"), b = "", c = Iterators.Stateful("c")
        @test isempty(collect(zip(a,b,c)))
        @test !isempty(a)
        @test !isempty(c)
        @test isempty(collect(zip(a,c,b)))
        @test !isempty(a)
        @test !isempty(c)
        @test isempty(collect(zip(b,a,c)))
        @test !isempty(a)
        @test !isempty(c)
        @test isempty(collect(zip(b,c,a)))
        @test !isempty(a)
        @test !isempty(c)
        @test isempty(collect(zip(c,a,b)))
        @test !isempty(a)
        @test !isempty(c)
        @test isempty(collect(zip(c,b,a)))
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(a,b,c))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(a,c,b))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(b,a,c))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(b,c,a))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(c,a,b))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let a = Iterators.Stateful("aa"), b = "b", c = Iterators.Stateful("cc")
        @test length(collect(zip(c,b,a))) == 1
        @test !isempty(a)
        @test !isempty(c)
    end
    let z = zip(Iterators.Stateful("ab"), Iterators.Stateful("b"), Iterators.Stateful("c"))
        v, s = iterate(z)
        @test Base.isdone(z, s)
    end
end

@testset "pair for Svec" begin
    ps = pairs(Core.svec(:a, :b))
    @test ps isa Iterators.Pairs
    @test collect(ps) == [1 => :a, 2 => :b]
end

@testset "inference for large zip #26765" begin
    x = zip(1:2, ["a", "b"], (1.0, 2.0), Base.OneTo(2), Iterators.repeated("a"), 1.0:0.2:2.0,
            (1 for i in 1:2), Iterators.Stateful(["a", "b", "c"]), (1.0 for i in 1:2, j in 1:3))
    @test @inferred(length(x)) == 2
    z = Iterators.filter(x -> x[1] >= 1, x)
    @test @inferred(eltype(z)) <: Tuple{Int,String,Float64,Int,String,Float64,Any,String,Any}
    @test @inferred(first(z)) == (1, "a", 1.0, 1, "a", 1.0, 1, "a", 1.0)
    @test @inferred(first(Iterators.drop(z, 1))) == (2, "b", 2.0, 2, "a", 1.2, 1, "c", 1.0)
end

@testset "Stateful fix #30643" begin
    @test Base.IteratorSize(1:10) isa Base.HasShape
    a = Iterators.Stateful(1:10)
    @test Base.IteratorSize(a) isa Base.HasLength
    @test length(a) == 10
    @test length(collect(a)) == 10
    @test length(a) == 0
    b = Iterators.Stateful(Iterators.take(1:10,3))
    @test Base.IteratorSize(b) isa Base.HasLength
    @test length(b) == 3
    @test length(collect(b)) == 3
    @test length(b) == 0
    c = Iterators.Stateful(Iterators.countfrom(1))
    @test Base.IteratorSize(c) isa Base.IsInfinite
    @test length(Iterators.take(c,3)) == 3
    @test length(collect(Iterators.take(c,3))) == 3
    d = Iterators.Stateful(Iterators.filter(isodd,1:10))
    @test Base.IteratorSize(d) isa Base.SizeUnknown
    @test length(collect(Iterators.take(d,3))) == 3
    @test length(collect(d)) == 2
    @test length(collect(d)) == 0
end

@testset "only" begin
    @test only([3]) === 3
    @test_throws ArgumentError only([])
    @test_throws ArgumentError only([3, 2])

    @test @inferred(only((3,))) === 3
    @test_throws ArgumentError only(())
    @test_throws ArgumentError only((3, 2))

    @test only(Dict(1=>3)) === (1=>3)
    @test_throws ArgumentError only(Dict{Int,Int}())
    @test_throws ArgumentError only(Dict(1=>3, 2=>2))

    @test only(Set([3])) === 3
    @test_throws ArgumentError only(Set(Int[]))
    @test_throws ArgumentError only(Set([3,2]))

    @test @inferred(only((;a=1))) === 1
    @test_throws ArgumentError only(NamedTuple())
    @test_throws ArgumentError only((a=3, b=2.0))

    @test @inferred(only(1)) === 1
    @test @inferred(only('a')) === 'a'
    @test @inferred(only(Ref([1, 2]))) == [1, 2]
    @test_throws ArgumentError only(Pair(10, 20))

    @test only(1 for ii in 1:1) === 1
    @test only(1 for ii in 1:10 if ii < 2) === 1
    @test_throws ArgumentError only(1 for ii in 1:10)
    @test_throws ArgumentError only(1 for ii in 1:10 if ii > 2)
    @test_throws ArgumentError only(1 for ii in 1:10 if ii > 200)
end

@testset "flatten empty tuple" begin
    @test isempty(collect(Iterators.flatten(())))
end

@testset "Iterators.accumulate" begin
    @test collect(Iterators.accumulate(+, [])) == []
    @test collect(Iterators.accumulate(+, [1])) == [1]
    @test collect(Iterators.accumulate(+, [1,2])) == [1,3]
    @test collect(Iterators.accumulate(+, [1,2,3])) == [1,3,6]
    @test collect(Iterators.accumulate(=>, [:a,:b,:c])) == [:a, :a => :b, (:a => :b) => :c]
    @test collect(Iterators.accumulate(+, (x for x in [true])))::Vector{Int} == [1]
    @test collect(Iterators.accumulate(+, (x for x in [true, true, false])))::Vector{Int} == [1, 2, 2]
    @test collect(Iterators.accumulate(+, (x for x in [true]), init=10.0))::Vector{Float64} == [11.0]
    @test length(Iterators.accumulate(+, [10,20,30])) == 3
    @test size(Iterators.accumulate(max, rand(2,3))) == (2,3)
    @test Base.IteratorSize(Iterators.accumulate(max, rand(2,3))) === Base.IteratorSize(rand(2,3))
    @test Base.IteratorEltype(Iterators.accumulate(*, ())) isa Base.EltypeUnknown
end

@testset "Base.accumulate" begin
    @test cumsum(x^2 for x in 1:3) == [1, 5, 14]
    @test cumprod(x + 1 for x in 1:3) == [2, 6, 24]
    @test accumulate(+, (x^2 for x in 1:3); init=100) == [101, 105, 114]
end

@testset "proper patition for non-1-indexed vector" begin
    @test partition(IdentityUnitRange(11:19), 5) |> collect == [11:15,16:19] # IdentityUnitRange
end

@testset "Iterators.peel" begin
    @test Iterators.peel([]) == nothing
    @test Iterators.peel(1:10)[1] == 1
    @test Iterators.peel(1:10)[2] |> collect == 2:10
    @test Iterators.peel(x^2 for x in 2:4)[1] == 4
    @test Iterators.peel(x^2 for x in 2:4)[2] |> collect == [9, 16]
end

@testset "last for iterators" begin
    @test last(Iterators.map(identity, 1:3)) == 3
    @test last(Iterators.filter(iseven, (Iterators.map(identity, 1:3)))) == 2
end

@testset "isempty and isdone for Generators" begin
    itr = eachline(IOBuffer("foo\n"))
    gen = (x for x in itr)
    @test !isempty(gen)
    @test !Base.isdone(gen)
    @test collect(gen) == ["foo"]
end