PetscTimSort#
Sorts an array in place in increasing order using Tim Peters https://bugs.python.org/file4451/timsort.txt adaptive sorting algorithm.
Synopsis#
#include "petscsys.h"
PetscErrorCode PetscTimSort(PetscInt n, void *arr, size_t size, int (*cmp)(const void *, const void *, void *), void *ctx)
Not Collective, No Fortran Support
Input Parameters#
n - number of values
arr - array to be sorted
size - size in bytes of the datatype held in arr
cmp - function pointer to comparison function
ctx - optional context to be passed to comparison function, NULL if not needed
Output Parameter#
arr - sorted array
Notes#
Timsort makes the assumption that input data is already likely partially ordered, or that it contains contiguous sections (termed ‘runs’) where the data is locally ordered (but not necessarily globally ordered). It therefore aims to select slices of the array in such a way that resulting mergesorts operate on near perfectly length-balanced arrays. To do so it repeatedly triggers attempts throughout to merge adjacent runs.
Should one run continuously “win” a comparison the algorithm begins the “gallop” phase. It will aggressively search the “winner” for the location of the “losers” next entry (and vice versa) to copy all preceding elements into place in bulk. However if the data is truly unordered (as is the case with random data) the immense gains possible from these searches are expected not to repay their costs. While adjacent arrays are almost all nearly the same size, they likely all contain similar data.
Example Usage#
The comparison function must follow the qsort()
comparison function paradigm, returning the sign of the difference
between its arguments. If left < right : return -1, if left == right : return 0, if left > right : return 1. The user
may also
change or reverse the order of the sort by flipping the above. Note that stability of the sort is only guaranteed if
the comparison function forms a valid trigraph. For example when sorting an array of type “my_type” in increasing
order
int increasing_comparison_function(const void *left, const void *right, void *ctx) {
my_type l = *(my_type *) left, r = *(my_type *) right;
return (l < r) ? -1 : (l > r);
}
Note the context is unused here but you may use it to pass and subsequently access whatever information required inside the comparison function. The context pointer will unaltered except for any changes made inside the comparison function. Then pass the function
PetscTimSort(n, arr, sizeof(arr[0]), increasing_comparison_function, ctx)
Fortran Notes#
To use this from Fortran you must write a comparison subroutine with 4 arguments which accepts left, right, ctx and returns result. For example
subroutine CompareIntegers(left,right,ctx,result)
implicit none
PetscInt,intent(in) :: left, right
type(UserCtx) :: ctx
integer,intent(out) :: result
if (left < right) then
result = -1
else if (left == right) then
result = 0
else
result = 1
end if
return
end subroutine CompareIntegers
See Also#
PetscTimSortWithArray()
, PetscIntSortSemiOrdered()
, PetscRealSortSemiOrdered()
, PetscMPIIntSortSemiOrdered()
Level#
developer
Location#
Index of all Sys routines
Table of Contents for all manual pages
Index of all manual pages