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*DECK CHFEV
SUBROUTINE CHFEV (X1, X2, F1, F2, D1, D2, NE, XE, FE, NEXT, IERR)
C***BEGIN PROLOGUE CHFEV
C***PURPOSE Evaluate a cubic polynomial given in Hermite form at an
C array of points. While designed for use by PCHFE, it may
C be useful directly as an evaluator for a piecewise cubic
C Hermite function in applications, such as graphing, where
C the interval is known in advance.
C***LIBRARY SLATEC (PCHIP)
C***CATEGORY E3
C***TYPE SINGLE PRECISION (CHFEV-S, DCHFEV-D)
C***KEYWORDS CUBIC HERMITE EVALUATION, CUBIC POLYNOMIAL EVALUATION,
C PCHIP
C***AUTHOR Fritsch, F. N., (LLNL)
C Lawrence Livermore National Laboratory
C P.O. Box 808 (L-316)
C Livermore, CA 94550
C FTS 532-4275, (510) 422-4275
C***DESCRIPTION
C
C CHFEV: Cubic Hermite Function EValuator
C
C Evaluates the cubic polynomial determined by function values
C F1,F2 and derivatives D1,D2 on interval (X1,X2) at the points
C XE(J), J=1(1)NE.
C
C ----------------------------------------------------------------------
C
C Calling sequence:
C
C INTEGER NE, NEXT(2), IERR
C REAL X1, X2, F1, F2, D1, D2, XE(NE), FE(NE)
C
C CALL CHFEV (X1,X2, F1,F2, D1,D2, NE, XE, FE, NEXT, IERR)
C
C Parameters:
C
C X1,X2 -- (input) endpoints of interval of definition of cubic.
C (Error return if X1.EQ.X2 .)
C
C F1,F2 -- (input) values of function at X1 and X2, respectively.
C
C D1,D2 -- (input) values of derivative at X1 and X2, respectively.
C
C NE -- (input) number of evaluation points. (Error return if
C NE.LT.1 .)
C
C XE -- (input) real array of points at which the function is to be
C evaluated. If any of the XE are outside the interval
C [X1,X2], a warning error is returned in NEXT.
C
C FE -- (output) real array of values of the cubic function defined
C by X1,X2, F1,F2, D1,D2 at the points XE.
C
C NEXT -- (output) integer array indicating number of extrapolation
C points:
C NEXT(1) = number of evaluation points to left of interval.
C NEXT(2) = number of evaluation points to right of interval.
C
C IERR -- (output) error flag.
C Normal return:
C IERR = 0 (no errors).
C "Recoverable" errors:
C IERR = -1 if NE.LT.1 .
C IERR = -2 if X1.EQ.X2 .
C (The FE-array has not been changed in either case.)
C
C***REFERENCES (NONE)
C***ROUTINES CALLED XERMSG
C***REVISION HISTORY (YYMMDD)
C 811019 DATE WRITTEN
C 820803 Minor cosmetic changes for release 1.
C 890411 Added SAVE statements (Vers. 3.2).
C 890531 Changed all specific intrinsics to generic. (WRB)
C 890703 Corrected category record. (WRB)
C 890703 REVISION DATE from Version 3.2
C 891214 Prologue converted to Version 4.0 format. (BAB)
C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
C***END PROLOGUE CHFEV
C Programming notes:
C
C To produce a double precision version, simply:
C a. Change CHFEV to DCHFEV wherever it occurs,
C b. Change the real declaration to double precision, and
C c. Change the constant ZERO to double precision.
C
C DECLARE ARGUMENTS.
C
INTEGER NE, NEXT(2), IERR
REAL X1, X2, F1, F2, D1, D2, XE(*), FE(*)
C
C DECLARE LOCAL VARIABLES.
C
INTEGER I
REAL C2, C3, DEL1, DEL2, DELTA, H, X, XMI, XMA, ZERO
SAVE ZERO
DATA ZERO /0./
C
C VALIDITY-CHECK ARGUMENTS.
C
C***FIRST EXECUTABLE STATEMENT CHFEV
IF (NE .LT. 1) GO TO 5001
H = X2 - X1
IF (H .EQ. ZERO) GO TO 5002
C
C INITIALIZE.
C
IERR = 0
NEXT(1) = 0
NEXT(2) = 0
XMI = MIN(ZERO, H)
XMA = MAX(ZERO, H)
C
C COMPUTE CUBIC COEFFICIENTS (EXPANDED ABOUT X1).
C
DELTA = (F2 - F1)/H
DEL1 = (D1 - DELTA)/H
DEL2 = (D2 - DELTA)/H
C (DELTA IS NO LONGER NEEDED.)
C2 = -(DEL1+DEL1 + DEL2)
C3 = (DEL1 + DEL2)/H
C (H, DEL1 AND DEL2 ARE NO LONGER NEEDED.)
C
C EVALUATION LOOP.
C
DO 500 I = 1, NE
X = XE(I) - X1
FE(I) = F1 + X*(D1 + X*(C2 + X*C3))
C COUNT EXTRAPOLATION POINTS.
IF ( X.LT.XMI ) NEXT(1) = NEXT(1) + 1
IF ( X.GT.XMA ) NEXT(2) = NEXT(2) + 1
C (NOTE REDUNDANCY--IF EITHER CONDITION IS TRUE, OTHER IS FALSE.)
500 CONTINUE
C
C NORMAL RETURN.
C
RETURN
C
C ERROR RETURNS.
C
5001 CONTINUE
C NE.LT.1 RETURN.
IERR = -1
CALL XERMSG ('SLATEC', 'CHFEV',
+ 'NUMBER OF EVALUATION POINTS LESS THAN ONE', IERR, 1)
RETURN
C
5002 CONTINUE
C X1.EQ.X2 RETURN.
IERR = -2
CALL XERMSG ('SLATEC', 'CHFEV', 'INTERVAL ENDPOINTS EQUAL', IERR,
+ 1)
RETURN
C------------- LAST LINE OF CHFEV FOLLOWS ------------------------------
END
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