SLATEC Routines --- PCHSW ---

*DECK PCHSW
      SUBROUTINE PCHSW (DFMAX, IEXTRM, D1, D2, H, SLOPE, IERR)
C***BEGIN PROLOGUE  PCHSW
C***SUBSIDIARY
C***PURPOSE  Limits excursion from data for PCHCS
C***LIBRARY   SLATEC (PCHIP)
C***TYPE      SINGLE PRECISION (PCHSW-S, DPCHSW-D)
C***AUTHOR  Fritsch, F. N., (LLNL)
C***DESCRIPTION
C
C         PCHSW:  PCHCS Switch Excursion Limiter.
C
C     Called by  PCHCS  to adjust D1 and D2 if necessary to insure that
C     the extremum on this interval is not further than DFMAX from the
C     extreme data value.
C
C ----------------------------------------------------------------------
C
C  Calling sequence:
C
C        INTEGER  IEXTRM, IERR
C        REAL  DFMAX, D1, D2, H, SLOPE
C
C        CALL  PCHSW (DFMAX, IEXTRM, D1, D2, H, SLOPE, IERR)
C
C   Parameters:
C
C     DFMAX -- (input) maximum allowed difference between F(IEXTRM) and
C           the cubic determined by derivative values D1,D2.  (assumes
C           DFMAX.GT.0.)
C
C     IEXTRM -- (input) index of the extreme data value.  (assumes
C           IEXTRM = 1 or 2 .  Any value .NE.1 is treated as 2.)
C
C     D1,D2 -- (input) derivative values at the ends of the interval.
C           (Assumes D1*D2 .LE. 0.)
C          (output) may be modified if necessary to meet the restriction
C           imposed by DFMAX.
C
C     H -- (input) interval length.  (Assumes  H.GT.0.)
C
C     SLOPE -- (input) data slope on the interval.
C
C     IERR -- (output) error flag.  should be zero.
C           If IERR=-1, assumption on D1 and D2 is not satisfied.
C           If IERR=-2, quadratic equation locating extremum has
C                       negative discriminant (should never occur).
C
C    -------
C    WARNING:  This routine does no validity-checking of arguments.
C    -------
C
C  Fortran intrinsics used:  ABS, SIGN, SQRT.
C
C***SEE ALSO  PCHCS
C***ROUTINES CALLED  R1MACH, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   820218  DATE WRITTEN
C   820805  Converted to SLATEC library version.
C   870707  Replaced DATA statement for SMALL with a use of R1MACH.
C   890411  1. Added SAVE statements (Vers. 3.2).
C           2. Added REAL R1MACH for consistency with D.P. version.
C   890411  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   900328  Added TYPE section.  (WRB)
C   910408  Updated AUTHOR and DATE WRITTEN sections in prologue.  (WRB)
C   920526  Eliminated possible divide by zero problem.  (FNF)
C   930503  Improved purpose.  (FNF)
C***END PROLOGUE  PCHSW