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A general description is here.
The radix sort algorithm and implementation from data.table (forder) replaces the existing but limited radix (counting) sort and adds a new method for ‘order()’. Contributed by Matthew Dowle and Arun Srinivasan, the new algorithm supports logical, integer (even with large values), real, and character vectors. It outperforms all other methods, but there are some caveats (see ‘?sort’).
The ‘order()’ function gains a ‘method’ argument for choosing between ‘"shell"’ and ‘"radix"’.
New function ‘grouping’ returns a permutation that stably rearranges data so that identical values are adjacent. The return value includes extra partitioning information on the groups. The implementation came included with the new radix sort.