A pooled representation of arrays for purposes of compression when there are few unique elements.

Installation: at the Julia REPL, import Pkg; Pkg.add("PooledArrays")

Usage:

Working with PooledArray objects does not differ from working with general AbstractArray objects, with two exceptions:

• If you hold mutable objects in PooledArray it is not allowed to modify them after they are stored in it.
• In multi-threaded context it is not safe to assign values that are not already present in a PooledArray's pool from one thread while either reading or writing to the same array from another thread.

Keeping in mind these two restrictions, as a user, the only thing you need to learn is how to create PooledArray objects. This is accomplished by passing an AbstractArray to the PooledArray constructor:

julia> using PooledArrays
julia> PooledArray(["a" "b"; "c" "d"])
2×2 PooledMatrix{String, UInt32, Matrix{UInt32}}:
 "a"  "b"
 "c"  "d"

PooledArray performs compression by storing an array of reference integers and a mapping from integers to its elements in a dictionary. In this way, if the size of the reference integer is smaller than the size of the actual elements the resulting PooledArray has a smaller memory footprint than the equivalent Array. By default UInt32 is used as a type of reference integers. However, you can specify the reference integer type you want to use by passing it as a second argument to the constructor. This is usually done when you know that you will have only a few unique elements in the PooledArray.

julia> PooledArray(["a", "b", "c", "d"], UInt8)
4-element PooledVector{String, UInt8, Vector{UInt8}}:
 "a"
 "b"
 "c"
 "d"

Alternatively you can pass the compress and signed keyword arguments to the PooledArray constructor to automatically select the reference integer type. When you pass compress=true then the reference integer type is chosen to be the smallest type that is large enough to hold all unique values in array. When you pass signed=true the reference type is signed (by default it is unsigned).

julia> PooledArray(["a", "b", "c", "d"]; compress=true, signed=true)
4-element PooledVector{String, Int8, Vector{Int8}}:
 "a"
 "b"
 "c"
 "d"

Maintenance: PooledArrays is maintained collectively by the JuliaData collaborators. Responsiveness to pull requests and issues can vary, depending on the availability of key collaborators.

• IndirectArrays
• CategoricalArrays