*> \brief \b DGER
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
*  Definition:
*  ===========
*
*       SUBROUTINE DGER(M,N,ALPHA,X,INCX,Y,INCY,A,LDA)
* 
*       .. Scalar Arguments ..
*       REAL(kind=rk) ALPHA
*       INTEGER INCX,INCY,LDA,M,N
*       ..
*       .. Array Arguments ..
*       REAL(kind=rk) A(LDA,*),X(*),Y(*)
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> DGER   performs the rank 1 operation
*>
*>    A := alpha*x*y**T + A,
*>
*> where alpha is a scalar, x is an m element vector, y is an n element
*> vector and A is an m by n matrix.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] M
*> \verbatim
*>          M is INTEGER
*>           On entry, M specifies the number of rows of the matrix A.
*>           M must be at least zero.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>           On entry, N specifies the number of columns of the matrix A.
*>           N must be at least zero.
*> \endverbatim
*>
*> \param[in] ALPHA
*> \verbatim
*>          ALPHA is REAL(kind=rk).
*>           On entry, ALPHA specifies the scalar alpha.
*> \endverbatim
*>
*> \param[in] X
*> \verbatim
*>          X is REAL(kind=rk) array of dimension at least
*>           ( 1 + ( m - 1 )*abs( INCX ) ).
*>           Before entry, the incremented array X must contain the m
*>           element vector x.
*> \endverbatim
*>
*> \param[in] INCX
*> \verbatim
*>          INCX is INTEGER
*>           On entry, INCX specifies the increment for the elements of
*>           X. INCX must not be zero.
*> \endverbatim
*>
*> \param[in] Y
*> \verbatim
*>          Y is REAL(kind=rk) array of dimension at least
*>           ( 1 + ( n - 1 )*abs( INCY ) ).
*>           Before entry, the incremented array Y must contain the n
*>           element vector y.
*> \endverbatim
*>
*> \param[in] INCY
*> \verbatim
*>          INCY is INTEGER
*>           On entry, INCY specifies the increment for the elements of
*>           Y. INCY must not be zero.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*>          A is REAL(kind=rk) array of DIMENSION ( LDA, n ).
*>           Before entry, the leading m by n part of the array A must
*>           contain the matrix of coefficients. On exit, A is
*>           overwritten by the updated matrix.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>           On entry, LDA specifies the first dimension of A as declared
*>           in the calling (sub) program. LDA must be at least
*>           max( 1, m ).
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee 
*> \author Univ. of California Berkeley 
*> \author Univ. of Colorado Denver 
*> \author NAG Ltd. 
*
*> \date November 2011
*
*> \ingroup double_blas_level2
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  Level 2 Blas routine.
*>
*>  -- Written on 22-October-1986.
*>     Jack Dongarra, Argonne National Lab.
*>     Jeremy Du Croz, Nag Central Office.
*>     Sven Hammarling, Nag Central Office.
*>     Richard Hanson, Sandia National Labs.
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE DGER(M,N,ALPHA,X,INCX,Y,INCY,A,LDA)
      use env_module, only: rk
*
*  -- Reference BLAS level2 routine (version 3.4.0) --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2011
*
*     .. Scalar Arguments ..
      REAL(kind=rk) ALPHA
      INTEGER INCX,INCY,LDA,M,N
*     ..
*     .. Array Arguments ..
      REAL(kind=rk) A(LDA,*),X(*),Y(*)
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL(kind=rk) ZERO
      PARAMETER (ZERO=0.0D+0)
*     ..
*     .. Local Scalars ..
      REAL(kind=rk) TEMP
      INTEGER I,INFO,IX,J,JY,KX
*     ..
*     .. External Subroutines ..
      EXTERNAL XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC MAX
*     ..
*
*     Test the input parameters.
*
      INFO = 0
      IF (M.LT.0) THEN
          INFO = 1
      ELSE IF (N.LT.0) THEN
          INFO = 2
      ELSE IF (INCX.EQ.0) THEN
          INFO = 5
      ELSE IF (INCY.EQ.0) THEN
          INFO = 7
      ELSE IF (LDA.LT.MAX(1,M)) THEN
          INFO = 9
      END IF
      IF (INFO.NE.0) THEN
          CALL XERBLA('DGER  ',INFO)
          RETURN
      END IF
*
*     Quick return if possible.
*
      IF ((M.EQ.0) .OR. (N.EQ.0) .OR. (ALPHA.EQ.ZERO)) RETURN
*
*     Start the operations. In this version the elements of A are
*     accessed sequentially with one pass through A.
*
      IF (INCY.GT.0) THEN
          JY = 1
      ELSE
          JY = 1 - (N-1)*INCY
      END IF
      IF (INCX.EQ.1) THEN
          DO 20 J = 1,N
              IF (Y(JY).NE.ZERO) THEN
                  TEMP = ALPHA*Y(JY)
                  DO 10 I = 1,M
                      A(I,J) = A(I,J) + X(I)*TEMP
   10             CONTINUE
              END IF
              JY = JY + INCY
   20     CONTINUE
      ELSE
          IF (INCX.GT.0) THEN
              KX = 1
          ELSE
              KX = 1 - (M-1)*INCX
          END IF
          DO 40 J = 1,N
              IF (Y(JY).NE.ZERO) THEN
                  TEMP = ALPHA*Y(JY)
                  IX = KX
                  DO 30 I = 1,M
                      A(I,J) = A(I,J) + X(IX)*TEMP
                      IX = IX + INCX
   30             CONTINUE
              END IF
              JY = JY + INCY
   40     CONTINUE
      END IF
*
      RETURN
*
*     End of DGER  .
*
      END
