forked from lijiext/lammps
69 lines
1.6 KiB
Fortran
69 lines
1.6 KiB
Fortran
DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
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* .. Scalar Arguments ..
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INTEGER INCX,INCY,N
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION DX(*),DY(*)
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* ..
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*
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* Purpose
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* =======
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*
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* DDOT forms the dot product of two vectors.
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* uses unrolled loops for increments equal to one.
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*
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* Further Details
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* ===============
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*
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* jack dongarra, linpack, 3/11/78.
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* modified 12/3/93, array(1) declarations changed to array(*)
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*
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* =====================================================================
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*
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* .. Local Scalars ..
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DOUBLE PRECISION DTEMP
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INTEGER I,IX,IY,M,MP1
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC MOD
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* ..
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DDOT = 0.0d0
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DTEMP = 0.0d0
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IF (N.LE.0) RETURN
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IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
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*
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* code for unequal increments or equal increments
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* not equal to 1
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*
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IX = 1
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IY = 1
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IF (INCX.LT.0) IX = (-N+1)*INCX + 1
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IF (INCY.LT.0) IY = (-N+1)*INCY + 1
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DO 10 I = 1,N
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DTEMP = DTEMP + DX(IX)*DY(IY)
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IX = IX + INCX
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IY = IY + INCY
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10 CONTINUE
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DDOT = DTEMP
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RETURN
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*
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* code for both increments equal to 1
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*
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*
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* clean-up loop
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*
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20 M = MOD(N,5)
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IF (M.EQ.0) GO TO 40
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DO 30 I = 1,M
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DTEMP = DTEMP + DX(I)*DY(I)
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30 CONTINUE
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IF (N.LT.5) GO TO 60
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40 MP1 = M + 1
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DO 50 I = MP1,N,5
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DTEMP = DTEMP + DX(I)*DY(I) + DX(I+1)*DY(I+1) +
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+ DX(I+2)*DY(I+2) + DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
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50 CONTINUE
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60 DDOT = DTEMP
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RETURN
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END
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