Permutation is the technical name for re-ordering of values.
The need for permutations occurs a lot during (mainly low-level) processing.
To do permutation, you must provide two inputs: an array of values (that you want to re-order in place) and a permutation array which contains the new index of each element (let’s call it `perm`

).
The diagram below shows the input array before and after the re-ordering.

permute: AFTER[ i ] = BEFORE[ perm[i] ] i = 0 .. N-1 inverse: AFTER[ perm[i] ] = BEFORE[ i ] i = 0 .. N-1

The functions here are a re-implementation of the GNU Scientific Library’s `gsl_permute`

function.
The reason we did not use that function was that it uses system-specific types (like `long`

and `int`

) which can have different widths on different systems, hence are not easily convertible to Gnuastro’s fixed width types (see Numeric data types).
There is also a separate function for each type, heavily using macros to allow a `base`

function to work on all the types.
Thus it is hard to read/understand.
Hence, Gnuastro contains a re-write of their steps in a new type-agnostic method which is a single function that can work on any type.

As described in GSL’s source code and manual, this implementation comes from Donald Knuth’s *Art of computer programming* book, in the "Sorting and Searching" chapter of Volume 3 (3rd ed).
Exercise 10 of Section 5.2 defines the problem and in the answers, Knuth describes the solution.
So if you are interested, please have a look there for more.

We are in contact with the GSL developers and in the future^{268} we will submit these implementations to GSL.
If they are finally incorporated there, we will delete this section in future versions.

- Function:

`void`

**gal_permutation_check**`(size_t`

¶`*permutation`

, size_t`size`

) Print how

`permutation`

will re-order an array that has`size`

elements for each element in one one line.

- Function:

`void`

**gal_permutation_apply**`(gal_data_t`

¶`*input`

, size_t`*permutation`

) Apply

`permutation`

on the`input`

dataset (can have any type), see above for the definition of permutation.

- Function:

`void`

**gal_permutation_apply_inverse**`(gal_data_t`

¶`*input`

, size_t`*permutation`

) Apply the inverse of

`permutation`

on the`input`

dataset (can have any type), see above for the definition of permutation.

- Function:

`void`

**gal_permutation_transpose_2d**`(gal_data_t`

¶`*input`

) Transpose an input 2D matrix into a new dataset. If the input is not a square, this function will change the

`input->array`

element to a newly allocated array (the old one will be freed internally). Therefore, in case you have already stored`input->array`

for other usage*before*this function, and the input is not a square, be sure to update the previously stored pointer if the input is not a square.

Gnuastro’s Task 14497. If this task is still “postponed” when you are reading this and you are interested to help, your contributions would be very welcome. Both Gnuastro and GSL developers are very busy, hence both would appreciate your help.

GNU Astronomy Utilities 0.22 manual, February 2024.