Update README.md

README.md

@@ -2,3 +2,75 @@
 
 [![Build Status](https://github.com/jpivarski/AwkwardArray.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/jpivarski/AwkwardArray.jl/actions/workflows/CI.yml?query=branch%3Amain)
 [![Coverage](https://codecov.io/gh/jpivarski/AwkwardArray.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/jpivarski/AwkwardArray.jl)
+
+Julia-based Awkward Arrays are only layouts, no `ak.Array` wrapper. `PrimitiveArray` (renamed from `NumpyArray`) and `ListOffsetArray` implemented so far.
+
+The buffers that the layouts wrap can be any `AbstractArray`, and we are careful to count between `firstindex` and `lastindex`, rather than assume `1` and `length`.
+
+Index values in buffers are 0-indexed so that they can be zero-copy shared with Python.
+
+Since Julia has penalty-free imperative iteration, layouts and LayoutBuilders are unified in the same interface.
+
+Example:
+
+```julia
+julia> using AwkwardArray
+[ Info: Precompiling AwkwardArray [7d259134-7f60-4bf1-aa00-7452e11bde56]
+
+julia> layout = AwkwardArray.ListOffsetArray{AwkwardArray.Index64, AwkwardArray.PrimitiveArray{Float64}}()
+0-element AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Float64, Vector{Float64}}}
+
+julia> sublayout = layout.content
+0-element AwkwardArray.PrimitiveArray{Float64, Vector{Float64}}
+
+julia> AwkwardArray.push!(sublayout, 1.1);
+
+julia> AwkwardArray.push!(sublayout, 2.2);
+
+julia> AwkwardArray.push!(sublayout, 3.3);
+
+julia> AwkwardArray.end_list!(layout);
+
+julia> AwkwardArray.end_list!(layout);
+
+julia> AwkwardArray.push!(sublayout, 4.4);
+
+julia> AwkwardArray.push!(sublayout, 5.5);
+
+julia> AwkwardArray.end_list!(layout);
+
+julia> length(layout)
+3
+
+julia> layout[1]
+3-element AwkwardArray.PrimitiveArray{Float64, Vector{Float64}}:
+ 1.1
+ 2.2
+ 3.3
+
+julia> layout[2]
+0-element AwkwardArray.PrimitiveArray{Float64, Vector{Float64}}
+
+julia> layout[3]
+2-element AwkwardArray.PrimitiveArray{Float64, Vector{Float64}}:
+ 4.4
+ 5.5
+
+julia> layout[1][3]
+3.3
+
+julia> layout == AwkwardArray.ListOffsetArray(
+           [0, 3, 3, 5],
+           AwkwardArray.PrimitiveArray([1.1, 2.2, 3.3, 4.4, 5.5]),
+       )
+true
+
+julia> for x in layout
+           print("[ ")
+           for y in x
+               print(y, " ")
+           end
+           print("] ")
+       end
+[ 1.1 2.2 3.3 ] [ ] [ 4.4 5.5 ] 
+```