-*DECK DDOT
- DOUBLE PRECISION FUNCTION DDOT (N, DX, INCX, DY, INCY)
-C***BEGIN PROLOGUE DDOT
-C***PURPOSE Compute the inner product of two vectors.
-C***LIBRARY SLATEC (BLAS)
-C***CATEGORY D1A4
-C***TYPE DOUBLE PRECISION (SDOT-S, DDOT-D, CDOTU-C)
-C***KEYWORDS BLAS, INNER PRODUCT, LINEAR ALGEBRA, VECTOR
-C***AUTHOR Lawson, C. L., (JPL)
-C Hanson, R. J., (SNLA)
-C Kincaid, D. R., (U. of Texas)
-C Krogh, F. T., (JPL)
-C***DESCRIPTION
-C
-C B L A S Subprogram
-C Description of Parameters
-C
-C --Input--
-C N number of elements in input vector(s)
-C DX double precision vector with N elements
-C INCX storage spacing between elements of DX
-C DY double precision vector with N elements
-C INCY storage spacing between elements of DY
-C
-C --Output--
-C DDOT double precision dot product (zero if N .LE. 0)
-C
-C Returns the dot product of double precision DX and DY.
-C DDOT = sum for I = 0 to N-1 of DX(LX+I*INCX) * DY(LY+I*INCY),
-C where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
-C defined in a similar way using INCY.
-C
-C***REFERENCES C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
-C Krogh, Basic linear algebra subprograms for Fortran
-C usage, Algorithm No. 539, Transactions on Mathematical
-C Software 5, 3 (September 1979), pp. 308-323.
-C***ROUTINES CALLED (NONE)
-C***REVISION HISTORY (YYMMDD)
-C 791001 DATE WRITTEN
-C 890831 Modified array declarations. (WRB)
-C 890831 REVISION DATE from Version 3.2
-C 891214 Prologue converted to Version 4.0 format. (BAB)
-C 920310 Corrected definition of LX in DESCRIPTION. (WRB)
-C 920501 Reformatted the REFERENCES section. (WRB)
-C***END PROLOGUE DDOT
- DOUBLE PRECISION DX(*), DY(*)
-C***FIRST EXECUTABLE STATEMENT DDOT
- DDOT = 0.0D0
- DTEMP = 0.0D0
- IF (N .LE. 0) RETURN
- IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
-C
-C Code for unequal or nonpositive increments.
-C
- 5 IX = 1
- IY = 1
- IF (INCX .LT. 0) IX = (-N+1)*INCX + 1
- IF (INCY .LT. 0) IY = (-N+1)*INCY + 1
- DO 10 I = 1,N
- DDOT = DDOT + DX(IX)*DY(IY)
- IX = IX + INCX
- IY = IY + INCY
- 10 CONTINUE
- RETURN
-C
-C Code for both increments equal to 1.
-C
-C Clean-up loop so remaining vector length is a multiple of 5.
-C
- 20 M = MOD(N,5)
- IF (M .EQ. 0) GO TO 40
- DO 30 I = 1,M
- DDOT = DDOT + DX(I)*DY(I)
- 30 CONTINUE
- IF (N .LT. 5) RETURN
- 40 MP1 = M + 1
- DO I = MP1,N,5
- DDOT = DDOT + DX(I)*DY(I) + DX(I+1)*DY(I+1) + DX(I+2)*DY(I+2) +
- 1 DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
- END DO
- RETURN
-C
-C Code for equal, positive, non-unit increments.
-C
- 60 NS = N*INCX
- DO 70 I = 1,NS,INCX
- DDOT = DDOT + DX(I)*DY(I)
- 70 CONTINUE
- RETURN
- END