From dffab8492e33a83ff588cac104e5e8669ac50129 Mon Sep 17 00:00:00 2001
From: Jacob Williams <jacobwilliams@users.noreply.github.com>
Date: Sun, 7 Jan 2024 15:25:00 -0600
Subject: [PATCH] allow for using the HAS_BLAS compiler directive to use an
 external BLAS

---
 README.md                                    | 22 ++++++--
 src/{slsqp_support.f90 => slsqp_support.F90} | 58 ++++++++++++++++++++
 2 files changed, 74 insertions(+), 6 deletions(-)
 rename src/{slsqp_support.f90 => slsqp_support.F90} (80%)

diff --git a/README.md b/README.md
index ced5953..49df2d9 100644
--- a/README.md
+++ b/README.md
@@ -28,10 +28,6 @@ Updates to the original code include:
 * The documentation strings in the code have been converted to [FORD](https://github.com/Fortran-FOSS-Programmers/ford) format, allowing for [nicely formatted documentation](https://jacobwilliams.github.io/slsqp/) to be auto-generated.
 * A couple of bug fixes noted elsewhere have been applied.
 
-### License
-
-  * The original sourcecode and the modifications are released under a [permissive BSD-style license](https://github.com/jacobwilliams/slsqp/blob/master/LICENSE).
-
 ### Building SLSQP
 
 #### **Fortran Package Manager**
@@ -69,14 +65,28 @@ or, to use a specific version:
 slsqp = { git="https://github.com/jacobwilliams/slsqp.git", tag = "1.3.0" }
 ```
 
-### Development
+## Dependencies
 
-  * Development continues on [GitHub](https://github.com/jacobwilliams/slsqp).
+The library requires some [BLAS](https://netlib.org/blas/) routines, which are included. However, the user may also choose to link to an external BLAS library. This can be done by using the `HAS_BLAS` compiler directive. For example:
+
+```
+fpm build --compiler gfortran --flag "-DHAS_BLAS -lblas"
+```
+
+However, note that an external BLAS can only be used if the library is compiled with double precision (`real64`) reals.
 
 ### Documentation
 
   The latest API documentation can be found [here](https://jacobwilliams.github.io/slsqp/). This was generated from the source code using [FORD](https://github.com/Fortran-FOSS-Programmers/ford).
 
+### License
+
+  * The original sourcecode and the modifications are released under a [permissive BSD-style license](https://github.com/jacobwilliams/slsqp/blob/master/LICENSE).
+
+### Development
+
+  * Development continues on [GitHub](https://github.com/jacobwilliams/slsqp).
+
 ### References
 
 * [Original sourcecode at NETLIB](http://www.netlib.org/toms/733)
diff --git a/src/slsqp_support.f90 b/src/slsqp_support.F90
similarity index 80%
rename from src/slsqp_support.f90
rename to src/slsqp_support.F90
index db91435..5f4aadc 100644
--- a/src/slsqp_support.f90
+++ b/src/slsqp_support.F90
@@ -23,6 +23,62 @@ module slsqp_support
 
     public :: daxpy,dcopy,ddot,dnrm2,dscal
 
+#ifdef HAS_BLAS
+
+    ! linking with an external BLAS library.
+    ! Define the interfaces here. Note that this
+    ! will only work if the `wp` is the same kind
+    ! as used in the BLAS library.
+
+    interface
+        pure subroutine daxpy(n,da,dx,incx,dy,incy)
+            import :: wp
+            implicit none
+            integer,intent(in)                  :: n
+            real(wp),intent(in)                 :: da
+            real(wp),dimension(*),intent(in)    :: dx
+            integer,intent(in)                  :: incx
+            real(wp),dimension(*),intent(inout) :: dy
+            integer,intent(in)                  :: incy
+        end subroutine daxpy
+        pure subroutine dcopy(n,dx,incx,dy,incy)
+            import :: wp
+            implicit none
+            integer,intent(in)                :: n
+            real(wp),dimension(*),intent(in)  :: dx
+            integer,intent(in)                :: incx
+            real(wp),dimension(*),intent(out) :: dy
+            integer,intent(in)                :: incy
+        end subroutine dcopy
+        pure real(wp) function ddot(n,dx,incx,dy,incy)
+            import :: wp
+            implicit none
+            integer,intent(in)               :: n
+            real(wp),dimension(*),intent(in) :: dx
+            integer,intent(in)               :: incx
+            real(wp),dimension(*),intent(in) :: dy
+            integer,intent(in)               :: incy
+        end function ddot
+        pure function dnrm2(n,x,incx) result(norm)
+            import :: wp
+            implicit none
+            integer,intent(in)               :: incx
+            integer,intent(in)               :: n
+            real(wp),dimension(*),intent(in) :: x
+            real(wp)                         :: norm
+        end function dnrm2
+        pure subroutine dscal(n,da,dx,incx)
+            import :: wp
+            implicit none
+            integer,intent(in)                  :: n
+            real(wp),intent(in)                 :: da
+            real(wp),dimension(*),intent(inout) :: dx
+            integer,intent(in)                  :: incx
+        end subroutine dscal
+    end interface
+
+#else
+
     contains
 !*******************************************************************************
 
@@ -333,6 +389,8 @@ pure subroutine dscal(n,da,dx,incx)
     end subroutine dscal
 !*******************************************************************************
 
+#endif
+
 !*******************************************************************************
     end module slsqp_support
 !*******************************************************************************