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PooledElements

This is an in-development package to provide pooled elements. The target use is for columns of DataFrames. The pooled elements provide a mapping to integers. This reduces storage and speeds up times for grouping and sorting. The main types provided are:

• Pool -- A container for elements.
• PooledElement -- A general element that uses a Pool for storage.
• PooledString -- An AbstractString that uses a Pool{AbstractString} for storage.
• PooledStringArray -- An AbstractArray{AbstractString} that uses a vector of integers to reference a pool of strings.

A global string pool is also provided, and many of the PooledString and PooledStringArray methods default to using this global pool.

Many of the features of pools and pooled elements are similar to categorical types from CategoricalData.jl. Pools and pooled elements contain an ID in the type to try to prevent objects from mixing with the wrong pool.

The PooledElement, PooledString, and PooledStringArray all support null values. A null is indicated when the reference to the Pool associated with the element or array is zero. Many of the Nullable methods from Base and NullableArrays are supported (like isnull and nullify!).