Making a fast unstable sorting algorithm stable 1
Arne Maus · 2006
This paper demonstrates how an unstable in place sorting algorithm, the ALR algorithm, can be made stable by temporary changing the sorting keys during the recursion. At ‘the bottom of the recursion ’ all subsequences with equal valued element are then individually sorted with a stable sorting subalgorithm (insertion sort or radix). Later, on backtrack the original keys are restored. This results in a stable sorting of the whole input. Unstable ALR is much faster than Quicksort (which is also unstable). In this paper it is demonstrated that StableALR, which is some 10-30 % slower than the original unstable ALR, is still in most cases 20-60 % faster than Quicksort. It is also shown to be faster than Flashsort, a new unstable in place, bucket type sorting algorithm. This is demonstrated for five different distributions of integers in a array of length from 50 to 97 million elements. The StableALR sorting algorithm can be extended to sort floating point numbers and strings and make effective use of a multi core CPU.