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SUBROUTINE SSWAP( N, SX, INCX, SY, INCY )

c          Interchange s.p vectors  X  and  Y, as follows:

c     For I = 0 to N-1, interchange  SX(LX+I*INCX) and SY(LY+I*INCY),
c     where LX = 1          if INCX .GE. 0,
c              = (-INCX)*N  if INCX .LT. 0
c     and LY is defined analogously using INCY.

c  INPUT--
c        N  Number of elements in input vectors X and Y
c       SX  Sing-prec array containing vector X
c     INCX  Spacing of elements of vector X in SX
c       SY  Sing-prec array containing vector Y
c     INCY  Spacing of elements of vector Y in SY

c OUTPUT--
c       SX  Input vector SY (unchanged if N .LE. 0)
c       SY  Input vector SX (unchanged IF N .LE. 0)
c --------------------------------------------------------------

c     .. Scalar Arguments ..

INTEGER   INCX, INCY, N
c     ..
c     .. Array Arguments ..

REAL      SX( * ), SY( * )
c     ..
c     .. Local Scalars ..

INTEGER   I, IX, IY, M
REAL      STEMP1, STEMP2, STEMP3
c     ..
c     .. Intrinsic Functions ..

INTRINSIC MOD
c     ..

IF( N.LE.0 ) RETURN

IF( INCX.EQ.INCY .AND. INCX.GT.1 ) THEN

DO 10 I = 1, 1 + ( N-1 )*INCX, INCX
STEMP1 = SX( I )
SX( I ) = SY( I )
SY( I ) = STEMP1
10    CONTINUE

ELSE IF( INCX.EQ.INCY .AND. INCX.EQ.1 ) THEN

c                                        ** equal, unit increments
M  = MOD( N, 3 )

IF( M.NE.0 ) THEN
c                            ** clean-up loop so remaining vector length
c                            ** is a multiple of 3.
DO 20 I = 1, M
STEMP1 = SX( I )
SX( I ) = SY( I )
SY( I ) = STEMP1
20       CONTINUE

END IF
c                              ** unroll loop for speed
DO 30 I = M + 1, N, 3
STEMP1 = SX( I )
STEMP2 = SX( I + 1 )
STEMP3 = SX( I + 2 )
SX( I ) = SY( I )
SX( I + 1 ) = SY( I + 1 )
SX( I + 2 ) = SY( I + 2 )
SY( I ) = STEMP1
SY( I + 1 ) = STEMP2
SY( I + 2 ) = STEMP3
30    CONTINUE

ELSE
c               ** nonequal or nonpositive increments.
IX = 1
IY = 1

IF( INCX.LT.0 ) IX = 1 + ( N - 1 )*( -INCX )
IF( INCY.LT.0 ) IY = 1 + ( N - 1 )*( -INCY )

DO 40 I = 1, N
STEMP1 = SX( IX )
SX( IX ) = SY( IY )
SY( IY ) = STEMP1
IX   = IX + INCX
IY   = IY + INCY
40    CONTINUE

END IF

END
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