!>
! @license Apache-2.0
!
! Copyright (c) 2020 The Stdlib Authors.
!
! Licensed under the Apache License, Version 2.0 (the "License");
! you may not use this file except in compliance with the License.
! You may obtain a copy of the License at
!
!    http://www.apache.org/licenses/LICENSE-2.0
!
! Unless required by applicable law or agreed to in writing, software
! distributed under the License is distributed on an "AS IS" BASIS,
! WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
! See the License for the specific language governing permissions and
! limitations under the License.
!<

!> Interchanges two double-precision floating-point vectors.
!
! ## Notes
!
! * Modified version of reference BLAS level1 routine (version 3.7.0). Updated to "free form" Fortran 95.
!
! ## Authors
!
! * Univ. of Tennessee
! * Univ. of California Berkeley
! * Univ. of Colorado Denver
! * NAG Ltd.
!
! ## History
!
! * Jack Dongarra, linpack, 3/11/78.
!
!   - modified 12/3/93, array(1) declarations changed to array(*)
!
! ## License
!
! From <http://netlib.org/blas/faq.html>:
!
! > The reference BLAS is a freely-available software package. It is available from netlib via anonymous ftp and the World Wide Web. Thus, it can be included in commercial software packages (and has been). We only ask that proper credit be given to the authors.
! >
! > Like all software, it is copyrighted. It is not trademarked, but we do ask the following:
! >
! > * If you modify the source for these routines we ask that you change the name of the routine and comment the changes made to the original.
! >
! > * We will gladly answer any questions regarding the software. If a modification is done, however, it is the responsibility of the person who modified the routine to provide support.
!
! @param {integer} N - number of indexed elements
! @param {Array<double>} dx - first input array
! @param {integer} strideX - `dx` stride length
! @param {Array<double>} dy - second input array
! @param {integer} strideY - `dy` stride length
!<
subroutine dswap( N, dx, strideX, dy, strideY )
  implicit none
  ! ..
  ! Scalar arguments:
  integer :: strideX, strideY, N
  ! ..
  ! Array arguments:
  double precision :: dx(*), dy(*)
  ! ..
  ! Local scalars:
  double precision :: tmp
  integer :: mp1, ix, iy, i, m
  ! ..
  ! Intrinsic functions:
  intrinsic mod
  ! ..
  if ( N <= 0 ) then
    return
  end if
  ! ..
  ! If both strides are equal to `1`, use unrolled loops...
  if ( strideX == 1 .AND. strideY == 1 ) then
    m = mod( N, 3 )
    ! ..
    ! If we have a remainder, do a clean-up loop...
    if ( m /= 0 ) then
      do i = 1, m
        tmp = dx( i )
        dx( i ) = dy( i )
        dy( i ) = tmp
      end do
      if ( N < 3 ) then
        return
      end if
    end if
    mp1 = m + 1
    do i = mp1, N, 3
      tmp = dx( i )
      dx( i ) = dy( i )
      dy( i ) = tmp

      tmp = dx( i+1 )
      dx( i+1 ) = dy( i+1 )
      dy( i+1 ) = tmp

      tmp = dx( i+2 )
      dx( i+2 ) = dy( i+2 )
      dy( i+2 ) = tmp
    end do
  else
    if ( strideX < 0 ) then
      ix = ((1-N)*strideX) + 1
    else
      ix = 1
    end if
    if ( strideY < 0 ) then
      iy = ((1-N)*strideY) + 1
    else
      iy = 1
    end if
    do i = 1, N
      tmp = dx( ix )
      dx( ix ) = dy( iy )
      dy( iy ) = tmp
      ix = ix + strideX
      iy = iy + strideY
    end do
  end if
  return
end subroutine dswap