01:       SUBROUTINE DSWAP(N,DX,INCX,DY,INCY)
02: *     .. Scalar Arguments ..
03:       INTEGER INCX,INCY,N
04: *     ..
05: *     .. Array Arguments ..
06:       DOUBLE PRECISION DX(*),DY(*)
07: *     ..
08: *
09: *  Purpose
10: *  =======
11: *
12: *     interchanges two vectors.
13: *     uses unrolled loops for increments equal one.
14: *     jack dongarra, linpack, 3/11/78.
15: *     modified 12/3/93, array(1) declarations changed to array(*)
16: *
17: *
18: *     .. Local Scalars ..
19:       DOUBLE PRECISION DTEMP
20:       INTEGER I,IX,IY,M,MP1
21: *     ..
22: *     .. Intrinsic Functions ..
23:       INTRINSIC MOD
24: *     ..
25:       IF (N.LE.0) RETURN
26:       IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
27: *
28: *       code for unequal increments or equal increments not equal
29: *         to 1
30: *
31:       IX = 1
32:       IY = 1
33:       IF (INCX.LT.0) IX = (-N+1)*INCX + 1
34:       IF (INCY.LT.0) IY = (-N+1)*INCY + 1
35:       DO 10 I = 1,N
36:           DTEMP = DX(IX)
37:           DX(IX) = DY(IY)
38:           DY(IY) = DTEMP
39:           IX = IX + INCX
40:           IY = IY + INCY
41:    10 CONTINUE
42:       RETURN
43: *
44: *       code for both increments equal to 1
45: *
46: *
47: *       clean-up loop
48: *
49:    20 M = MOD(N,3)
50:       IF (M.EQ.0) GO TO 40
51:       DO 30 I = 1,M
52:           DTEMP = DX(I)
53:           DX(I) = DY(I)
54:           DY(I) = DTEMP
55:    30 CONTINUE
56:       IF (N.LT.3) RETURN
57:    40 MP1 = M + 1
58:       DO 50 I = MP1,N,3
59:           DTEMP = DX(I)
60:           DX(I) = DY(I)
61:           DY(I) = DTEMP
62:           DTEMP = DX(I+1)
63:           DX(I+1) = DY(I+1)
64:           DY(I+1) = DTEMP
65:           DTEMP = DX(I+2)
66:           DX(I+2) = DY(I+2)
67:           DY(I+2) = DTEMP
68:    50 CONTINUE
69:       RETURN
70:       END
71: