# For developers

Writing code that supports OffsetArrays is generally fairly straightforward. The majority of cases can be handled with these tips:

• replace many uses of size with axes
• replace 1:length(A) with eachindex(A), or if you need an integer index with LinearIndices(A)
• replace explicit allocations like Array{Int}(undef, size(B)) with similar(Array{Int}, axes(B))

More information can be found in Julia's developer documentation. The most subtle issues tend to arise around the axes, and further detail specific to OffsetArrays.jl follows below.

## Internals

How does OffsetArrays work? The fundamental principle is very simple: an OffsetArray is just a wrapper around a "parent" array, together with an index offset:

julia> oa = OffsetArray([1 2; 3 4], 0:1, 5:6)
2×2 OffsetArray(::Matrix{Int64}, 0:1, 5:6) with eltype Int64 with indices 0:1×5:6:
1  2
3  4

julia> parent(oa)
2×2 Matrix{Int64}:
1  2
3  4

julia> oa.offsets
(-1, 4)

So parent(oa) is the original array we constructed it with, and oa.offsets is a tuple, each entry encoding the index-shift to be applied along the corresponding axis. When you index oa[i,j], it "translates" the i,j indexes back to the parent array's indexes and then returns the value in the parent.

## The axes of OffsetArrays

The internal of offset computing is achieved by IdOffsetRange type:

julia> ax = axes(oa, 2)
OffsetArrays.IdOffsetRange(values=5:6, indices=5:6)

This has a similar design to Base.IdentityUnitRange that ax[x] == x always holds.

julia> ax
5
julia> ax
ERROR: BoundsError: attempt to access 2-element Base.OneTo{Int64} at index [-3]
[...]

This property makes sure that they tend to be their own axes:

julia> axes(ax)
(OffsetArrays.IdOffsetRange(values=5:6, indices=5:6),)

julia> axes(ax[ax])
(OffsetArrays.IdOffsetRange(values=5:6, indices=5:6),)

This example of indexing is idempotent. This is a useful characteristic for ensuring the "fundamental axiom" of generalized indexing, that a[ax][i] == a[ax[i]]:

julia> oa2 = OffsetArray([5, 10, 15, 20], 0:3)
4-element OffsetArray(::Vector{Int64}, 0:3) with eltype Int64 with indices 0:3:
5
10
15
20

julia> ax2 = axes(oa2, 1)
OffsetArrays.IdOffsetRange(values=0:3, indices=0:3)

julia> oa2
15

julia> oa2[ax2]
15

julia> oa2[ax2]
15

IdOffsetRanges apply the offset both to the values and the indices of the range, and otherwise preserve the parent range.

Warning

There are circumstances where constructing a specific type of IdOffsetRange cannot be supported without changing the axes of the range (see OffsetArrays.IdOffsetRange.) In the future, this package will distinguish between construction and conversion:

• construction (aka, coercion) will always succeed, even if it has to change the axes of the result (Examples: RangeType(rng), typeof(rng1)(rng2))
• conversion will succeed only if it can preserve both the values and the axes (Examples: convert(RangeType, rng), oftype(rng1, rng2))

While these behave equivalently now (conversion currently performs coercion), developers are encouraged to "future-proof" their code by choosing the behavior appropriate for each usage.

## Wrapping other offset array types

An OffsetArray may wrap any subtype of AbstractArray, including ones that do not use 1-based indexing. Such arrays however need to satisfy the fundamental axiom of idempotent indexing for things to work correctly. In other words, an axis of an offset array needs to have the same values as its own axis. This property is built into OffsetArrays if the parent uses 1-based indexing, but it's up to the user to ensure the correctness in case a type is to be wrapped that uses offset indices.

We demonstrate this through an example by creating a custom 0-based range type that we wrap in an OffsetArray:

julia> struct ZeroBasedRange{T,A<:AbstractRange{T}} <: AbstractRange{T}
a :: A
function ZeroBasedRange(a::AbstractRange{T}) where {T}
@assert !Base.has_offset_axes(a)
new{T, typeof(a)}(a)
end
end;

julia> Base.parent(A::ZeroBasedRange) = A.a;

julia> Base.first(A::ZeroBasedRange) = first(A.a);

julia> Base.length(A::ZeroBasedRange) = length(A.a);

julia> Base.last(A::ZeroBasedRange) = last(A.a);

julia> Base.size(A::ZeroBasedRange) = size(A.a);

julia> Base.axes(A::ZeroBasedRange) = map(x -> 0:x-1, size(A.a));

julia> Base.getindex(A::ZeroBasedRange, i::Int) = A.a[i + 1];

julia> Base.step(A::ZeroBasedRange) = step(A.a);

julia> function Base.show(io::IO, A::ZeroBasedRange)
show(io, A.a)
print(io, " with indices \$(axes(A,1))")
end;

This definition of a ZeroBasedRange appears to have the correct indices, for example:

julia> z = ZeroBasedRange(1:4)
1:4 with indices 0:3

julia> z
1

julia> z
4

However this does not use idempotent indexing, as the axis of a ZeroBasedRange is not its own axis.

julia> axes(z, 1)
0:3

julia> axes(axes(z, 1), 1)
Base.OneTo(4)

This will lead to complications in certain functions –- for example LinearIndices –- that tend to implictly assume idempotent indexing. In this case the LinearIndices of z will not match its axis.

julia> LinearIndices(z)
4-element LinearIndices{1, Tuple{UnitRange{Int64}}}:
1
2
3
4

Wrapping such a type in an OffsetArray might lead to unexpected bugs.

julia> zo = OffsetArray(z, 1);

julia> axes(zo, 1)
OffsetArrays.IdOffsetRange(values=1:4, indices=2:5)

julia> Array(zo)
ERROR: BoundsError: attempt to access 4-element UnitRange{Int64} at index 
[...]

The Array conversion errors despite zo having 1-based indices. The function axes(zo, 1) hints at the underlying problem –- the values and the indices of the axis are different. We may check that the axis of zo is not its own axis:

julia> axes(zo, 1)
OffsetArrays.IdOffsetRange(values=1:4, indices=2:5)

julia> axes(axes(zo, 1), 1)
OffsetArrays.IdOffsetRange(values=2:5, indices=2:5)

In this case the bug may be fixed by defining the axes of a ZeroBasedRange to be idempotent, for example using the OffsetArrays.IdentityUnitRange wrapper:

julia> Base.axes(A::ZeroBasedRange) = map(x -> OffsetArrays.IdentityUnitRange(0:x-1), size(A.a))

julia> axes(zo, 1)
OffsetArrays.IdOffsetRange(values=1:4, indices=1:4)

With this new definition, the values and indices of the axis are identical, which makes indexing idempotent. The conversion to an Array works as expected now:

julia> Array(zo)
4-element Vector{Int64}:
1
2
3
4

## Caveats

Because IdOffsetRange behaves quite differently to the normal UnitRange type, there are some cases that you should be aware of, especially when you are working with multi-dimensional arrays.

One such cases is getindex:

julia> Ao = zeros(-3:3, -3:3); Ao[:] .= 1:49;

julia> Ao[-3:0, :] |> axes # the first dimension does not preserve offsets
(OffsetArrays.IdOffsetRange(values=1:4, indices=1:4), OffsetArrays.IdOffsetRange(values=-3:3, indices=-3:3))

julia> Ao[-3:0, -3:3] |> axes # neither dimensions preserve offsets
(Base.OneTo(4), Base.OneTo(7))

julia> Ao[axes(Ao)...] |> axes # offsets are preserved
(OffsetArrays.IdOffsetRange(values=-3:3, indices=-3:3), OffsetArrays.IdOffsetRange(values=-3:3, indices=-3:3))

julia> Ao[:] |> axes # This is linear indexing
(Base.OneTo(49),)

Note that if you pass a UnitRange, the offsets in corresponding dimension will not be preserved. This might look weird at first, but since it follows the a[ax][i] == a[ax[i]] rule, it is not a bug.

julia> I = -3:0; # UnitRange always starts at index 1

julia> Ao[I, 0] == Ao[I, 0]
true

julia> ax = axes(Ao, 1) # ax starts at index -3
OffsetArrays.IdOffsetRange(values=-3:3, indices=-3:3)

julia> Ao[ax, 0] == Ao[ax, 0]
true

## Using custom axis types

While a wide variety of AbstractUnitRanges provided by Base may be used as indices to construct an OffsetArray, at times it might be convenient to define custom types. The OffsetArray constructor accepts any type that may be converted to an AbstractUnitRange. This proceeds through a two-step process. Let's assume that the constructor called is OffsetArray(A, indstup), where indstup is a Tuple of indices.

1. At the first step, the constructor calls to_indices(A, axes(A), indstup) to lower indstup to a Tuple of AbstractUnitRanges. This step converts –- among other things –- Colons to axis ranges. Custom types may extend Base.to_indices(A, axes(A), indstup) with the desired conversion of indstup to Tuple{Vararg{AbstractUnitRange{Int}}} if this is feasible.

2. At the second step, the result obtained from the previous step treated again to convert it to a Tuple of AbstractUnitRanges to handle cases where the first step doesn't achieve this. An additional customization option may be specified at this stage: a type may be converted either to a single AbstractUnitRange{Int}, or to a Tuple of them. A type might specify which of these two behaviours is desired by extending OffsetArrays.AxisConversionStyle. An example of a type that is acted upon at this stage is CartesianIndices, which is converted to a Tuple of AbstractUnitRanges.

For example, here are a couple of custom type that facilitate zero-based indexing:

julia> struct ZeroBasedIndexing end

julia> Base.to_indices(A, inds, ::Tuple{ZeroBasedIndexing}) = map(x -> 0:length(x)-1, inds)

julia> a = zeros(3, 3);

julia> oa = OffsetArray(a, ZeroBasedIndexing());

julia> axes(oa)
(OffsetArrays.IdOffsetRange(values=0:2, indices=0:2), OffsetArrays.IdOffsetRange(values=0:2, indices=0:2))

In this example we had to define the action of to_indices as the type ZeroBasedIndexing did not have a familiar hierarchy. Things are even simpler if we subtype AbstractUnitRange, in which case we need to define first and length for the custom range to be able to use it as an axis:

julia> struct ZeroTo <: AbstractUnitRange{Int}
n :: Int
ZeroTo(n) = new(n < 0 ? -1 : n)
end

julia> Base.first(::ZeroTo) = 0

julia> Base.length(r::ZeroTo) = r.n + 1

julia> oa = OffsetArray(zeros(2,2), ZeroTo(1), ZeroTo(1));

julia> axes(oa)
(OffsetArrays.IdOffsetRange(values=0:1, indices=0:1), OffsetArrays.IdOffsetRange(values=0:1, indices=0:1))

Note that zero-based indexing may also be achieved using the pre-defined type OffsetArrays.Origin.