import Base.Slice
import Base._unsafe_getindex!

function mapslices2(f, A::AbstractArray, B::AbstractArray, dims::AbstractVector)
    if isempty(dims)
        return map(f,A,B)
    end

    @assert size(A) == size(B)

    dimsA = [indices(A)...]
    ndimsA = ndims(A)
    alldims = [1:ndimsA;]

    otherdims = setdiff(alldims, dims)

    idx = Any[first(ind) for ind in indices(A)]
    itershape   = tuple(dimsA[otherdims]...)
    for d in dims
        idx[d] = Slice(indices(A, d))
    end

    # Apply the function to the first slice in order to determine the next steps
    Aslice = A[idx...]
    Bslice = B[idx...]
    r1 = f(Aslice, Bslice)
    # In some cases, we can re-use the first slice for a dramatic performance
    # increase. The slice itself must be mutable and the result cannot contain
    # any mutable containers. The following errs on the side of being overly
    # strict (#18570 & #21123).
    safe_for_reuse = isa(Aslice, StridedArray) &&
                     (isa(r1, Number) || (isa(r1, AbstractArray) && eltype(r1) <: Number))

    # determine result size and allocate
    Rsize = copy(dimsA)
    # TODO: maybe support removing dimensions
    if !isa(r1, AbstractArray) || ndims(r1) == 0
        r1 = [r1]
    end
    nextra = max(0, length(dims)-ndims(r1))
    if eltype(Rsize) == Int
        Rsize[dims] = [size(r1)..., ntuple(d->1, nextra)...]
    else
        Rsize[dims] = [indices(r1)..., ntuple(d->OneTo(1), nextra)...]
    end
    R = similar(r1, tuple(Rsize...,))

    ridx = Any[map(first, indices(R))...]
    for d in dims
        ridx[d] = indices(R,d)
    end

    R[ridx...] = r1

    nidx = length(otherdims)
    indexes = Iterators.drop(CartesianRange(itershape), 1)
    inner_mapslices2!(safe_for_reuse, indexes, nidx, idx, otherdims, ridx, Aslice,
        Bslice, A, B, f, R)
end

function inner_mapslices2!(safe_for_reuse, indexes, nidx, idx, otherdims, ridx, Aslice,
        Bslice, A, B, f, R)
    if safe_for_reuse
        # when f returns an array, R[ridx...] = f(Aslice) line copies elements,
        # so we can reuse Aslice
        for I in indexes # skip the first element, we already handled it
            replace_tuples2!(nidx, idx, ridx, otherdims, I)
            _unsafe_getindex!(Aslice, A, idx...)
            _unsafe_getindex!(Bslice, B, idx...)
            R[ridx...] = f(Aslice, Bslice)
        end
    else
        # we can't guarantee safety (#18524), so allocate new storage for each slice
        for I in indexes
            replace_tuples!(nidx, idx, ridx, otherdims, I)
            R[ridx...] = f(A[idx...], B[idx...])
        end
    end

    return R
end

function mapslicesN(f, AN::Array{T,1}, dims::AbstractVector) where T<:AbstractArray
    if isempty(dims)
        return map(f,AN...)
    end

    A = AN[1]

    dimsA = [indices(A)...]
    ndimsA = ndims(A)
    alldims = [1:ndimsA;]

    otherdims = setdiff(alldims, dims)

    idx = Any[first(ind) for ind in indices(A)]
    itershape   = tuple(dimsA[otherdims]...)
    for d in dims
        idx[d] = Slice(indices(A, d))
    end

    # Apply the function to the first slice in order to determine the next steps
    ANslices = [x[idx...] for x in AN]
    r1 = f(ANslices...)
    # In some cases, we can re-use the first slice for a dramatic performance
    # increase. The slice itself must be mutable and the result cannot contain
    # any mutable containers. The following errs on the side of being overly
    # strict (#18570 & #21123).
    safe_for_reuse = isa(ANslices[1], StridedArray) &&
                     (isa(r1, Number) || (isa(r1, AbstractArray) && eltype(r1) <: Number))

    # determine result size and allocate
    Rsize = copy(dimsA)
    # TODO: maybe support removing dimensions
    if !isa(r1, AbstractArray) || ndims(r1) == 0
        r1 = [r1]
    end
    nextra = max(0, length(dims)-ndims(r1))
    if eltype(Rsize) == Int
        Rsize[dims] = [size(r1)..., ntuple(d->1, nextra)...]
    else
        Rsize[dims] = [indices(r1)..., ntuple(d->OneTo(1), nextra)...]
    end
    R = similar(r1, tuple(Rsize...,))

    ridx = Any[map(first, indices(R))...]
    for d in dims
        ridx[d] = indices(R,d)
    end

    R[ridx...] = r1

    nidx = length(otherdims)
    indexes = Iterators.drop(CartesianRange(itershape), 1)
    inner_mapslicesN!(safe_for_reuse, indexes, nidx, idx, otherdims, ridx, ANslices,
        AN, f, R)
end

function inner_mapslicesN!(safe_for_reuse, indexes, nidx, idx, otherdims, ridx, ANslices,
        AN, f, R)
    if safe_for_reuse
        # when f returns an array, R[ridx...] = f(Aslice) line copies elements,
        # so we can reuse Aslice
        for I in indexes # skip the first element, we already handled it
            replace_tuples2!(nidx, idx, ridx, otherdims, I)
            for (ind,slice) in enumerate(ANslices)
                _unsafe_getindex!(slice, AN[ind], idx...)
            end
            R[ridx...] = f(ANslices...)
        end
    else
        # we can't guarantee safety (#18524), so allocate new storage for each slice
        for I in indexes
            replace_tuples!(nidx, idx, ridx, otherdims, I)
            R[ridx...] = f([x[idx...] for x in AN]...)
        end
    end

    return R
end

function replace_tuples!(nidx, idx, ridx, otherdims, I)
    for i in 1:nidx
        idx[otherdims[i]] = ridx[otherdims[i]] = I.I[i]
    end
end