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SUBROUTINE PCHSW (DFMAX, IEXTRM, D1, D2, H, SLOPE, IERR)
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C***BEGIN PROLOGUE PCHSW
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C***PURPOSE Limits excursion from data for PCHCS
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C***LIBRARY SLATEC (PCHIP)
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C***TYPE SINGLE PRECISION (PCHSW-S, DPCHSW-D)
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C***AUTHOR Fritsch, F. N., (LLNL)
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C PCHSW: PCHCS Switch Excursion Limiter.
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C Called by PCHCS to adjust D1 and D2 if necessary to insure that
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C the extremum on this interval is not further than DFMAX from the
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C ----------------------------------------------------------------------
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C INTEGER IEXTRM, IERR
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C REAL DFMAX, D1, D2, H, SLOPE
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C CALL PCHSW (DFMAX, IEXTRM, D1, D2, H, SLOPE, IERR)
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C DFMAX -- (input) maximum allowed difference between F(IEXTRM) and
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C the cubic determined by derivative values D1,D2. (assumes
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C IEXTRM -- (input) index of the extreme data value. (assumes
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C IEXTRM = 1 or 2 . Any value .NE.1 is treated as 2.)
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C D1,D2 -- (input) derivative values at the ends of the interval.
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C (Assumes D1*D2 .LE. 0.)
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C (output) may be modified if necessary to meet the restriction
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C H -- (input) interval length. (Assumes H.GT.0.)
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C SLOPE -- (input) data slope on the interval.
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C IERR -- (output) error flag. should be zero.
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C If IERR=-1, assumption on D1 and D2 is not satisfied.
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C If IERR=-2, quadratic equation locating extremum has
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C negative discriminant (should never occur).
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C WARNING: This routine does no validity-checking of arguments.
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C Fortran intrinsics used: ABS, SIGN, SQRT.
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C***ROUTINES CALLED R1MACH, XERMSG
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C***REVISION HISTORY (YYMMDD)
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C 820805 Converted to SLATEC library version.
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C 870707 Replaced DATA statement for SMALL with a use of R1MACH.
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C 890411 1. Added SAVE statements (Vers. 3.2).
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C 2. Added REAL R1MACH for consistency with D.P. version.
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C 890411 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
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C 900328 Added TYPE section. (WRB)
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C 910408 Updated AUTHOR and DATE WRITTEN sections in prologue. (WRB)
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C 920526 Eliminated possible divide by zero problem. (FNF)
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C 930503 Improved purpose. (FNF)
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C***END PROLOGUE PCHSW
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REAL DFMAX, D1, D2, H, SLOPE
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C DECLARE LOCAL VARIABLES.
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REAL CP, FACT, HPHI, LAMBDA, NU, ONE, PHI, RADCAL, RHO, SIGMA,
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* SMALL, THAT, THIRD, THREE, TWO, ZERO
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SAVE ZERO, ONE, TWO, THREE, FACT
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DATA ZERO /0./, ONE /1./, TWO /2./, THREE /3./, FACT /100./
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C THIRD SHOULD BE SLIGHTLY LESS THAN 1/3.
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C NOTATION AND GENERAL REMARKS.
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C RHO IS THE RATIO OF THE DATA SLOPE TO THE DERIVATIVE BEING TESTED.
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C LAMBDA IS THE RATIO OF D2 TO D1.
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C THAT = T-HAT(RHO) IS THE NORMALIZED LOCATION OF THE EXTREMUM.
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C PHI IS THE NORMALIZED VALUE OF P(X)-F1 AT X = XHAT = X-HAT(RHO),
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C WHERE THAT = (XHAT - X1)/H .
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C THAT IS, P(XHAT)-F1 = D*H*PHI, WHERE D=D1 OR D2.
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C SIMILARLY, P(XHAT)-F2 = D*H*(PHI-RHO) .
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C SMALL SHOULD BE A FEW ORDERS OF MAGNITUDE GREATER THAN MACHEPS.
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C***FIRST EXECUTABLE STATEMENT PCHSW
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SMALL = FACT*R1MACH(4)
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C DO MAIN CALCULATION.
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IF (D1 .EQ. ZERO) THEN
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C SPECIAL CASE -- D1.EQ.ZERO .
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C IF D2 IS ALSO ZERO, THIS ROUTINE SHOULD NOT HAVE BEEN CALLED.
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IF (D2 .EQ. ZERO) GO TO 5001
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C EXTREMUM IS OUTSIDE INTERVAL WHEN RHO .GE. 1/3 .
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IF (RHO .GE. THIRD) GO TO 5000
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THAT = (TWO*(THREE*RHO-ONE)) / (THREE*(TWO*RHO-ONE))
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PHI = THAT**2 * ((THREE*RHO-ONE)/THREE)
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C CONVERT TO DISTANCE FROM F2 IF IEXTRM.NE.1 .
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IF (IEXTRM .NE. 1) PHI = PHI - RHO
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C TEST FOR EXCEEDING LIMIT, AND ADJUST ACCORDINGLY.
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IF (HPHI*ABS(D2) .GT. DFMAX) THEN
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C AT THIS POINT, HPHI.GT.0, SO DIVIDE IS OK.
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D2 = SIGN (DFMAX/HPHI, D2)
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IF (D2 .EQ. ZERO) THEN
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C SPECIAL CASE -- D2.EQ.ZERO .
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C EXTREMUM IS OUTSIDE INTERVAL WHEN RHO .GE. 1/3 .
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IF (RHO .GE. THIRD) GO TO 5000
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THAT = ONE / (THREE*NU)
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IF (LAMBDA .LE. ZERO) GO TO 5001
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C NORMAL CASE -- D1 AND D2 BOTH NONZERO, OPPOSITE SIGNS.
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NU = ONE - LAMBDA - TWO*RHO
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IF (ABS(NU) .GT. SMALL) THEN
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RADCAL = (NU - (TWO*RHO+ONE))*NU + SIGMA**2
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IF (RADCAL .LT. ZERO) GO TO 5002
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THAT = (CP - SQRT(RADCAL)) / (THREE*NU)
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THAT = ONE/(TWO*SIGMA)
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PHI = THAT*((NU*THAT - CP)*THAT + ONE)
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C CONVERT TO DISTANCE FROM F2 IF IEXTRM.NE.1 .
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IF (IEXTRM .NE. 1) PHI = PHI - RHO
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C TEST FOR EXCEEDING LIMIT, AND ADJUST ACCORDINGLY.
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IF (HPHI*ABS(D1) .GT. DFMAX) THEN
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C AT THIS POINT, HPHI.GT.0, SO DIVIDE IS OK.
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D1 = SIGN (DFMAX/HPHI, D1)
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C D1 AND D2 BOTH ZERO, OR BOTH NONZERO AND SAME SIGN.
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CALL XERMSG ('SLATEC', 'PCHSW', 'D1 AND/OR D2 INVALID', IERR, 1)
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C NEGATIVE VALUE OF RADICAL (SHOULD NEVER OCCUR).
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CALL XERMSG ('SLATEC', 'PCHSW', 'NEGATIVE RADICAL', IERR, 1)
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C------------- LAST LINE OF PCHSW FOLLOWS ------------------------------