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SUBROUTINE DROTMG(DD1,DD2,DX1,DY1,DPARAM)
* .. Scalar Arguments ..
DOUBLE PRECISION DD1,DD2,DX1,DY1
* ..
* .. Array Arguments ..
DOUBLE PRECISION DPARAM(5)
* ..
*
* Purpose
* =======
*
* CONSTRUCT THE MODIFIED GIVENS TRANSFORMATION MATRIX H WHICH ZEROS
* THE SECOND COMPONENT OF THE 2-VECTOR (DSQRT(DD1)*DX1,DSQRT(DD2)*
* DY2)**T.
* WITH DPARAM(1)=DFLAG, H HAS ONE OF THE FOLLOWING FORMS..
*
* DFLAG=-1.D0 DFLAG=0.D0 DFLAG=1.D0 DFLAG=-2.D0
*
* (DH11 DH12) (1.D0 DH12) (DH11 1.D0) (1.D0 0.D0)
* H=( ) ( ) ( ) ( )
* (DH21 DH22), (DH21 1.D0), (-1.D0 DH22), (0.D0 1.D0).
* LOCATIONS 2-4 OF DPARAM CONTAIN DH11, DH21, DH12, AND DH22
* RESPECTIVELY. (VALUES OF 1.D0, -1.D0, OR 0.D0 IMPLIED BY THE
* VALUE OF DPARAM(1) ARE NOT STORED IN DPARAM.)
*
* THE VALUES OF GAMSQ AND RGAMSQ SET IN THE DATA STATEMENT MAY BE
* INEXACT. THIS IS OK AS THEY ARE ONLY USED FOR TESTING THE SIZE
* OF DD1 AND DD2. ALL ACTUAL SCALING OF DATA IS DONE USING GAM.
*
*
* Arguments
* =========
*
* DD1 (input/output) DOUBLE PRECISION
*
* DD2 (input/output) DOUBLE PRECISION
*
* DX1 (input/output) DOUBLE PRECISION
*
* DY1 (input) DOUBLE PRECISION
*
* DPARAM (input/output) DOUBLE PRECISION array, dimension 5
* DPARAM(1)=DFLAG
* DPARAM(2)=DH11
* DPARAM(3)=DH21
* DPARAM(4)=DH12
* DPARAM(5)=DH22
*
* =====================================================================
*
* .. Local Scalars ..
DOUBLE PRECISION DFLAG,DH11,DH12,DH21,DH22,DP1,DP2,DQ1,DQ2,DTEMP,
$ DU,GAM,GAMSQ,ONE,RGAMSQ,TWO,ZERO
* ..
* .. Intrinsic Functions ..
INTRINSIC DABS
* ..
* .. Data statements ..
*
DATA ZERO,ONE,TWO/0.D0,1.D0,2.D0/
DATA GAM,GAMSQ,RGAMSQ/4096.D0,16777216.D0,5.9604645D-8/
* ..
IF (DD1.LT.ZERO) THEN
* GO ZERO-H-D-AND-DX1..
DFLAG = -ONE
DH11 = ZERO
DH12 = ZERO
DH21 = ZERO
DH22 = ZERO
*
DD1 = ZERO
DD2 = ZERO
DX1 = ZERO
ELSE
* CASE-DD1-NONNEGATIVE
DP2 = DD2*DY1
IF (DP2.EQ.ZERO) THEN
DFLAG = -TWO
DPARAM(1) = DFLAG
RETURN
END IF
* REGULAR-CASE..
DP1 = DD1*DX1
DQ2 = DP2*DY1
DQ1 = DP1*DX1
*
IF (DABS(DQ1).GT.DABS(DQ2)) THEN
DH21 = -DY1/DX1
DH12 = DP2/DP1
*
DU = ONE - DH12*DH21
*
IF (DU.GT.ZERO) THEN
DFLAG = ZERO
DD1 = DD1/DU
DD2 = DD2/DU
DX1 = DX1*DU
END IF
ELSE
IF (DQ2.LT.ZERO) THEN
* GO ZERO-H-D-AND-DX1..
DFLAG = -ONE
DH11 = ZERO
DH12 = ZERO
DH21 = ZERO
DH22 = ZERO
*
DD1 = ZERO
DD2 = ZERO
DX1 = ZERO
ELSE
DFLAG = ONE
DH11 = DP1/DP2
DH22 = DX1/DY1
DU = ONE + DH11*DH22
DTEMP = DD2/DU
DD2 = DD1/DU
DD1 = DTEMP
DX1 = DY1*DU
END IF
END IF
* PROCEDURE..SCALE-CHECK
IF (DD1.NE.ZERO) THEN
DO WHILE ((DD1.LE.RGAMSQ) .OR. (DD1.GE.GAMSQ))
IF (DFLAG.EQ.ZERO) THEN
DH11 = ONE
DH22 = ONE
DFLAG = -ONE
ELSE
DH21 = -ONE
DH12 = ONE
DFLAG = -ONE
END IF
IF (DD1.LE.RGAMSQ) THEN
DD1 = DD1*GAM**2
DX1 = DX1/GAM
DH11 = DH11/GAM
DH12 = DH12/GAM
ELSE
DD1 = DD1/GAM**2
DX1 = DX1*GAM
DH11 = DH11*GAM
DH12 = DH12*GAM
END IF
ENDDO
END IF
IF (DD2.NE.ZERO) THEN
DO WHILE ( (DABS(DD2).LE.RGAMSQ) .OR. (DABS(DD2).GE.GAMSQ) )
IF (DFLAG.EQ.ZERO) THEN
DH11 = ONE
DH22 = ONE
DFLAG = -ONE
ELSE
DH21 = -ONE
DH12 = ONE
DFLAG = -ONE
END IF
IF (DABS(DD2).LE.RGAMSQ) THEN
DD2 = DD2*GAM**2
DH21 = DH21/GAM
DH22 = DH22/GAM
ELSE
DD2 = DD2/GAM**2
DH21 = DH21*GAM
DH22 = DH22*GAM
END IF
END DO
END IF
END IF
IF (DFLAG.LT.ZERO) THEN
DPARAM(2) = DH11
DPARAM(3) = DH21
DPARAM(4) = DH12
DPARAM(5) = DH22
ELSE IF (DFLAG.EQ.ZERO) THEN
DPARAM(3) = DH21
DPARAM(4) = DH12
ELSE
DPARAM(2) = DH11
DPARAM(5) = DH22
END IF
DPARAM(1) = DFLAG
RETURN
END
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