addition of specs and of example

doc/specs/stdlib_sorting.md

@@ -46,6 +46,9 @@ data:
 * `ORD_SORT` is intended to sort simple arrays of intrinsic data
   that have significant sections that were partially ordered before
   the sort;
+* `SORT_ADJ` is based on `ORD_SORT`, but in addition to sorting the
+  input array, it returns a related array re-ordered in the
+  same way;
 * `SORT_INDEX` is based on `ORD_SORT`, but in addition to sorting the
   input array, it returns indices that map the original array to its
   sorted version. This enables related arrays to be re-ordered in the
@@ -60,10 +63,10 @@ data:
 The Fortran Standard Library is distributed under the MIT
 License. However components of the library may be based on code with
 additional licensing restrictions. In particular `ORD_SORT`,
-`SORT_INDEX`, and `SORT` are translations of codes with their
+`SORT_ADJ`, `SORT_INDEX`, and `SORT` are translations of codes with their
 own distribution restrictions.
 
-The `ORD_SORT` and `SORT_INDEX` subroutines are essentially
+The `ORD_SORT`, `SORT_ADJ` and `SORT_INDEX` subroutines are essentially
 translations to Fortran 2008 of the `"Rust" sort` of the Rust Language
 distributed as part of
 [`slice.rs`](https://github.com/rust-lang/rust/blob/90eb44a5897c39e3dff9c7e48e3973671dcd9496/src/liballoc/slice.rs).
@@ -140,6 +143,24 @@ argument or allocated internally on the stack.
 Arrays can be also sorted in a decreasing order by providing the argument `reverse
 = .true.`.
 
+#### The `SORT_ADJ` subroutine
+
+The `SORT` and `ORD_SORT` subroutines can sort rank 1 isolated
+arrays of intrinsic types, but do nothing for the coordinated sorting
+of related data, e.g., a related rank 1 array. Therefore the module
+provides a subroutine, `SORT_ADJ`, that re-order such a rank 1 array
+in the same way as the input array based on the `ORD_SORT` algorithm,
+in addition to sorting the input array.
+
+The logic of `SORT_ADJ` parallels that of `ORD_SORT`, with
+additional housekeeping to keep the associated array consistent with
+the sorted positions of the input array. Because of this additional
+housekeeping it has slower runtime performance than `ORD_SORT`.
+`SORT_ADJ` requires the use of two "scratch" arrays, that may be
+provided as optional `work` and `iwork` arguments or allocated
+internally on the stack.
+
+
 #### The `SORT_INDEX` subroutine
 
 The `SORT` and `ORD_SORT` subroutines can sort rank 1 isolated
@@ -198,7 +219,7 @@ factor of six. Still, even when it shows enhanced performance, its
 performance on partially sorted data is typically an order of
 magnitude slower than `ORD_SORT`. Its memory requirements are also
 low, being of order O(Ln(N)), while the memory requirements of
-`ORD_SORT` and `SORT_INDEX` are of order O(N).
+`ORD_SORT`, `SORT_ADJ` and `SORT_INDEX` are of order O(N).
 
 #### The `RADIX_SORT` subroutine
 
@@ -385,6 +406,84 @@ element of `array` is a `NaN`.
 {!example/sorting/example_radix_sort.f90!}
 ```
 
+#### `sort_adj` - sorts an associated array in the same way as the input array, while also sorting the array.
+
+##### Status
+
+Experimental
+
+##### Description
+
+Returns the input `array` sorted in the direction requested while
+retaining order stability, and an associated array whose elements are
+sorted in the same way as the input `array`.
+
+##### Syntax
+
+`call ` [[stdlib_sorting(module):sort_adj(interface)]] `( array, index[, work, iwork, reverse ] )`
+
+##### Class
+
+Generic subroutine.
+
+##### Arguments
+
+`array`: shall be a rank one array of any of the types:
+`integer(int8)`, `integer(int16)`, `integer(int32)`, `integer(int64)`,
+`real(sp)`, `real(dp)`, `real(qp)`, `character(*)`, `type(string_type)`,
+`type(bitset_64)`, or `type(bitset_large)`.
+It is an `intent(inout)` argument. On input it
+will be an array whose sorting indices are to be determined. On return
+it will be the sorted array.
+
+`index`: shall be a rank one `integer` or `real` array of
+the size of `array`. It is an `intent(inout)` argument. On return it
+shall have values that are the indices needed to sort the original
+array in the desired direction.
+
+`work` (optional): shall be a rank one array of any of the same type as
+`array`, and shall have at least `size(array)/2` elements. It is an
+`intent(out)` argument. It is intended to be used as "scratch"
+memory for internal record keeping. If associated with an array in
+static storage, its use can significantly reduce the stack memory
+requirements for the code. Its contents on return are undefined.
+
+`iwork` (optional): shall be a rank one integer array of the same kind
+of the array `index`, and shall have at least `size(array)/2` elements. It
+is an `intent(out)` argument.  It is intended to be used as "scratch"
+memory for internal record keeping. If associated with an array in
+static storage, its use can significantly reduce the stack memory
+requirements for the code. Its contents on return are undefined.
+
+`reverse` (optional): shall be a scalar of type default logical. It
+is an `intent(in)` argument. If present with a value of `.true.` then
+`array` will be sorted in order of non-increasing values in stable
+order. Otherwise `array` will be sorted in order of non-decreasing
+values in stable order.
+
+##### Notes
+
+`SORT_ADJ` implements the hybrid sorting algorithm of `ORD_SORT`,
+keeping the values of `index` consistent with the elements of `array`
+as it is sorted. As a `merge sort` based algorithm, it is a stable
+sorting comparison algorithm. The optional `work` and `iwork` arrays
+replace "scratch" memory that would otherwise be allocated on the
+stack. If `array` is of any kind of `REAL` the order of the elements in
+`index` and `array` on return are undefined if any element of `array`
+is a `NaN`. Sorting of `CHARACTER(*)` and `STRING_TYPE` arrays are
+based on the operator `>`, and not on the function `LGT`.
+
+It should be emphasized that the order of `array` will typically be
+different on return
+
+##### Examples
+
+Sorting a rank one array with `sort_adj`:
+
+```Fortran
+{!example/sorting/example_sort_adj.f90!}
+```
+
 #### `sort_index` - creates an array of sorting indices for an input array, while also sorting the array.
 
 ##### Status

example/sorting/CMakeLists.txt

@@ -1,5 +1,6 @@
 ADD_EXAMPLE(ord_sort)
 ADD_EXAMPLE(sort)
+ADD_EXAMPLE(sort_adj)
 ADD_EXAMPLE(sort_index)
 ADD_EXAMPLE(radix_sort)
 ADD_EXAMPLE(sort_bitset)

example/sorting/example_sort_adj.f90

@@ -0,0 +1,15 @@
+program example_sort_adj
+  use stdlib_sorting, only: sort_adj
+  implicit none
+  integer, allocatable :: array(:)
+  real, allocatable :: adj(:)
+
+  array = [5, 4, 3, 1, 10, 4, 9]
+  allocate(adj, source=real(array))
+
+  call sort_adj(array, adj)
+
+  print *, array   !print [1, 3, 4, 4, 5, 9, 10]
+  print *, adj   !print [1., 3., 4., 4., 5., 9., 10.]
+
+end program example_sort_adj

src/stdlib_sorting.fypp

@@ -295,6 +295,76 @@ module stdlib_sorting
 !!    ! Sort the random data
 !!    call radix_sort( array )
 !!    ...
+!!```
+
+    public sort_adj
+!! Version: experimental
+!!
+!! The generic subroutine implementing the `SORT_ADJ` algorithm to
+!! return an index array whose elements are sorted in the same order
+!! as the input array in the
+!! desired direction. It is primarily intended to be used to sort a
+!! rank 1 `integer` or `real` array based on the values of a component of the array.
+!! Its use has the syntax:
+!!
+!!     call sort_adj( array, index[, work, iwork, reverse ] )
+!!
+!! with the arguments:
+!!
+!! * array: the rank 1 array to be sorted. It is an `intent(inout)`
+!!   argument of any of the types `integer(int8)`, `integer(int16)`,
+!!   `integer(int32)`, `integer(int64)`, `real(real32)`, `real(real64)`,
+!!   `real(real128)`, `character(*)`, `type(string_type)`, 
+!!   `type(bitset_64)`, `type(bitset_large)`. If both the 
+!!   type of `array` is real and at least one of the elements is a `NaN`, 
+!!   then the ordering of the `array` and `index` results is undefined. 
+!!   Otherwise it is defined to be as specified by reverse.
+!!
+!! * index: a rank 1 `integer` or `real` array. It is an `intent(inout)`
+!!   argument of the type `integer(int_index)`. Its size shall be the
+!!   same as `array`. On return, its elements are sorted in the same order
+!!   as the input `array` in the direction specified by `reverse`.
+!!
+!! * work (optional): shall be a rank 1 array of the same type as
+!!   `array`, and shall have at least `size(array)/2` elements. It is an
+!!   `intent(out)` argument to be used as "scratch" memory
+!!   for internal record keeping. If associated with an array in static
+!!   storage, its use can significantly reduce the stack memory requirements
+!!   for the code. Its value on return is undefined.
+!!
+!! * iwork (optional): shall be a rank 1 integer array of the same type as `index`,
+!!   and shall have at least `size(array)/2` elements. It is an
+!!   `intent(out)` argument to be used as "scratch" memory
+!!   for internal record keeping. If associated with an array in static
+!!   storage, its use can significantly reduce the stack memory requirements
+!!   for the code. Its value on return is undefined.
+!!
+!! * `reverse` (optional): shall be a scalar of type default logical. It
+!!   is an `intent(in)` argument. If present with a value of `.true.` then
+!!   `array` will be sorted in order of non-increasing values in stable
+!!   order. Otherwise `array` will be sorted in order of non-decreasing
+!!   values in stable order.
+!!
+!!#### Examples
+!!
+!! Sorting a related rank one array:
+!!
+!!```Fortran
+!!program example_sort_adj
+!!  use stdlib_sorting, only: sort_adj
+!!  implicit none
+!!  integer, allocatable :: array(:)
+!!  real, allocatable :: adj(:)
+!!
+!!  array = [5, 4, 3, 1, 10, 4, 9]
+!!  allocate(adj, source=real(array))
+!!
+!!  call sort_adj(array, adj)
+!!
+!!  print *, array   !print [1, 3, 4, 4, 5, 9, 10]
+!!  print *, adj   !print [1., 3., 4., 4., 5., 9., 10.]
+!!
+!!end program example_sort_adj
 !!```
 
     public sort_index

src/stdlib_sorting_ord_sort.fypp

@@ -117,6 +117,8 @@ contains
         integer :: stat
 
         array_size = size( array, kind=int_index )
+
+! If necessary allocate buffers to serve as scratch memory.
         if ( present(work) ) then
             if ( size(work, kind=int_index) < array_size/2 ) then
                 error stop "${name1}$_${sname}$_ord_sort: work array is too small."

src/stdlib_sorting_sort_adj.fypp

@@ -42,7 +42,7 @@
 !!   TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 !!   SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 !!
-!! The generic subroutine, `SORT_INDEX`, is substantially a translation to
+!! The generic subroutine, `SORT_ADJ`, is substantially a translation to
 !! Fortran 2008 of the `"Rust" sort` sorting routines in
 !! [`slice.rs`](https://github.com/rust-lang/rust/blob/90eb44a5897c39e3dff9c7e48e3973671dcd9496/src/liballoc/slice.rs)
 !! The `rust sort` implementation is distributed with the header:
@@ -95,7 +95,6 @@ contains
 ! estimation of the optimal `run size` as suggested in Tim Peters'
 ! original `listsort.txt`, and the optional `work` and `iwork` arrays to be
 ! used as scratch memory.
-
         ${t1}$, intent(inout)         :: array(0:)
         ${ti}$, intent(inout)         :: index(0:)
         ${t3}$, intent(out), optional :: work(0:)
@@ -104,7 +103,8 @@ contains
 
         ${t2}$, allocatable :: buf(:)
         ${ti}$, allocatable :: ibuf(:)
-        integer(int_index) :: array_size, i, stat
+        integer(int_index) :: array_size, i
+        integer(int_index) :: stat
 
         array_size = size(array, kind=int_index)
 
@@ -136,6 +136,7 @@ contains
                 call merge_sort( array, index, work, ibuf )
             end if
         else
+! Allocate a buffer to use as scratch memory.
             #:if t1[0:4] == "char"
             allocate( ${t3}$ :: buf(0:array_size/2-1), &
                       stat=stat )
@@ -495,3 +496,4 @@ contains
 #:endfor
 
 end submodule stdlib_sorting_sort_adj
+