!DECK XERPRN
SUBROUTINE XERPRN (PREFIX, NPREF, MESSG, NWRAP)
  IMPLICIT NONE
  !***BEGIN PROLOGUE  XERPRN
  !***SUBSIDIARY
  !***PURPOSE  Print error messages processed by XERMSG.
  !***LIBRARY   SLATEC (XERROR)
  !***CATEGORY  R3C
  !***TYPE      ALL (XERPRN-A)
  !***KEYWORDS  ERROR MESSAGES, PRINTING, XERROR
  !***AUTHOR  Fong, Kirby, (NMFECC at LLNL)
  !***DESCRIPTION
  !
  ! This routine sends one or more lines to each of the (up to five)
  ! logical units to which error messages are to be sent.  This routine
  ! is called several times by XERMSG, sometimes with a single line to
  ! print and sometimes with a (potentially very long) message that may
  ! wrap around into multiple lines.
  !
  ! PREFIX  Input argument of type CHARACTER.  This argument contains
  !     characters to be put at the beginning of each line before
  !     the body of the message.  No more than 16 characters of
  !     PREFIX will be used.
  !
  ! NPREF   Input argument of type INTEGER.  This argument is the number
  !     of characters to use from PREFIX.  If it is negative, the
  !     intrinsic function LEN is used to determine its length.  If
  !         it is zero, PREFIX is not used.  If it exceeds 16 or if
  !     LEN(PREFIX) exceeds 16, only the first 16 characters will be
  !     used.  If NPREF is positive and the length of PREFIX is less
  !     than NPREF, a copy of PREFIX extended with blanks to length
  !     NPREF will be used.
  !
  ! MESSG   Input argument of type CHARACTER.  This is the text of a
  !     message to be printed.  If it is a long message, it will be
  !     broken into pieces for printing on multiple lines.  Each line
  !     will start with the appropriate prefix and be followed by a
  !     piece of the message.  NWRAP is the number of characters per
  !     piece; that is, after each NWRAP characters, we break and
  !     start a new line.  In addition the characters '$$' embedded
  !     in MESSG are a sentinel for a new line.  The counting of
  !     characters up to NWRAP starts over for each new line.  The
  !     value of NWRAP typically used by XERMSG is 72 since many
  !     older error messages in the SLATEC Library are laid out to
  !     rely on wrap-around every 72 characters.
  !
  ! NWRAP   Input argument of type INTEGER.  This gives the maximum size
  !     piece into which to break MESSG for printing on multiple
  !     lines.  An embedded '$$' ends a line, and the count restarts
  !     at the following character.  If a line break does not occur
  !     on a blank (it would split a word) that word is moved to the
  !     next line.  Values of NWRAP less than 16 will be treated as
  !     16.  Values of NWRAP greater than 132 will be treated as 132.
  !     The actual line length will be NPREF + NWRAP after NPREF has
  !     been adjusted to fall between 0 and 16 and NWRAP has been
  !     adjusted to fall between 16 and 132.
  !
  !***REFERENCES  R. E. Jones and D. K. Kahaner, XERROR, the SLATEC
  !             Error-handling Package, SAND82-0800, Sandia
  !             Laboratories, 1982.
  !***ROUTINES CALLED  I1MACH, XGETUA
  !***REVISION HISTORY  (YYMMDD)
  !   880621  DATE WRITTEN
  !   880708  REVISED AFTER THE SLATEC CML SUBCOMMITTEE MEETING OF
  !       JUNE 29 AND 30 TO CHANGE THE NAME TO XERPRN AND TO REWORK
  !       THE HANDLING OF THE NEW LINE SENTINEL TO BEHAVE LIKE THE
  !       SLASH CHARACTER IN FORMAT STATEMENTS.
  !   890706  REVISED WITH THE HELP OF FRED FRITSCH AND REG CLEMENS TO
  !       STREAMLINE THE CODING AND FIX A BUG THAT CAUSED EXTRA BLANK
  !       LINES TO BE PRINTED.
  !   890721  REVISED TO ADD A NEW FEATURE.  A NEGATIVE VALUE OF NPREF
  !       CAUSES LEN(PREFIX) TO BE USED AS THE LENGTH.
  !   891013  REVISED TO CORRECT ERROR IN CALCULATING PREFIX LENGTH.
  !   891214  Prologue converted to Version 4.0 format.  (WRB)
  !   900510  Added code to break messages between words.  (RWC)
  !   920501  Reformatted the REFERENCES section.  (WRB)
  !***END PROLOGUE  XERPRN
  CHARACTER(len=*) :: PREFIX, MESSG
  INTEGER :: NPREF, NWRAP
  CHARACTER(len=148) :: CBUFF
  INTEGER :: IU(5), NUNIT
  CHARACTER(len=2) :: NEWLIN
  PARAMETER (NEWLIN = '$$')
  INTEGER :: N, I1MACH, I, LPREF, LWRAP, LENMSG, NEXTC
  INTEGER :: LPIECE, IDELTA
  !***FIRST EXECUTABLE STATEMENT  XERPRN
  CALL XGETUA(IU,NUNIT)
  !
  !   A ZERO VALUE FOR A LOGICAL UNIT NUMBER MEANS TO USE THE STANDARD
  !   ERROR MESSAGE UNIT INSTEAD.  I1MACH(4) RETRIEVES THE STANDARD
  !   ERROR MESSAGE UNIT.
  !
  N = I1MACH(4)
  DO I=1,NUNIT
     IF (IU(I) .EQ. 0) IU(I) = N
  END DO
  !
  !   LPREF IS THE LENGTH OF THE PREFIX.  THE PREFIX IS PLACED AT THE
  !   BEGINNING OF CBUFF, THE CHARACTER BUFFER, AND KEPT THERE DURING
  !   THE REST OF THIS ROUTINE.
  !
  IF ( NPREF .LT. 0 ) THEN
     LPREF = LEN(PREFIX)
  ELSE
     LPREF = NPREF
  ENDIF
  LPREF = MIN(16, LPREF)
  IF (LPREF .NE. 0) CBUFF(1:LPREF) = PREFIX
  !
  !   LWRAP IS THE MAXIMUM NUMBER OF CHARACTERS WE WANT TO TAKE AT ONE
  !   TIME FROM MESSG TO PRINT ON ONE LINE.
  !
  LWRAP = MAX(16, MIN(132, NWRAP))
  !
  !   SET LENMSG TO THE LENGTH OF MESSG, IGNORE ANY TRAILING BLANKS.
  !
  LENMSG = LEN(MESSG)
  N = LENMSG
  DO I=1,N
     IF (MESSG(LENMSG:LENMSG) .NE. ' ') GO TO 30
     LENMSG = LENMSG - 1
  END DO
   30   CONTINUE
  !
  !   IF THE MESSAGE IS ALL BLANKS, THEN PRINT ONE BLANK LINE.
  !
  IF (LENMSG .EQ. 0) THEN
     CBUFF(LPREF+1:LPREF+1) = ' '
     DO I=1,NUNIT
        WRITE(IU(I), '(A)') CBUFF(1:LPREF+1)
     END DO
     RETURN
  ENDIF
  !
  !   SET NEXTC TO THE POSITION IN MESSG WHERE THE NEXT SUBSTRING
  !   STARTS.  FROM THIS POSITION WE SCAN FOR THE NEW LINE SENTINEL.
  !   WHEN NEXTC EXCEEDS LENMSG, THERE IS NO MORE TO PRINT.
  !   WE LOOP BACK TO LABEL 50 UNTIL ALL PIECES HAVE BEEN PRINTED.
  !
  !   WE LOOK FOR THE NEXT OCCURRENCE OF THE NEW LINE SENTINEL.  THE
  !   INDEX INTRINSIC FUNCTION RETURNS ZERO IF THERE IS NO OCCURRENCE
  !   OR IF THE LENGTH OF THE FIRST ARGUMENT IS LESS THAN THE LENGTH
  !   OF THE SECOND ARGUMENT.
  !
  !   THERE ARE SEVERAL CASES WHICH SHOULD BE CHECKED FOR IN THE
  !   FOLLOWING ORDER.  WE ARE ATTEMPTING TO SET LPIECE TO THE NUMBER
  !   OF CHARACTERS THAT SHOULD BE TAKEN FROM MESSG STARTING AT
  !   POSITION NEXTC.
  !
  !   LPIECE .EQ. 0   THE NEW LINE SENTINEL DOES NOT OCCUR IN THE
  !                   REMAINDER OF THE CHARACTER STRING.  LPIECE
  !                   SHOULD BE SET TO LWRAP OR LENMSG+1-NEXTC,
  !                   WHICHEVER IS LESS.
  !
  !   LPIECE .EQ. 1   THE NEW LINE SENTINEL STARTS AT MESSG(NEXTC:
  !                   NEXTC).  LPIECE IS EFFECTIVELY ZERO, AND WE
  !                   PRINT NOTHING TO AVOID PRODUCING UNNECESSARY
  !                   BLANK LINES.  THIS TAKES CARE OF THE SITUATION
  !                   WHERE THE LIBRARY ROUTINE HAS A MESSAGE OF
  !                   EXACTLY 72 CHARACTERS FOLLOWED BY A NEW LINE
  !                   SENTINEL FOLLOWED BY MORE CHARACTERS.  NEXTC
  !                   SHOULD BE INCREMENTED BY 2.
  !
  !   LPIECE .GT. LWRAP+1  REDUCE LPIECE TO LWRAP.
  !
  !   ELSE            THIS LAST CASE MEANS 2 .LE. LPIECE .LE. LWRAP+1
  !                   RESET LPIECE = LPIECE-1.  NOTE THAT THIS
  !                   PROPERLY HANDLES THE END CASE WHERE LPIECE .EQ.
  !                   LWRAP+1.  THAT IS, THE SENTINEL FALLS EXACTLY
  !                   AT THE END OF A LINE.
  !
  NEXTC = 1
   50   LPIECE = INDEX(MESSG(NEXTC:LENMSG), NEWLIN)
  IF (LPIECE .EQ. 0) THEN
  !
  !   THERE WAS NO NEW LINE SENTINEL FOUND.
  !
     IDELTA = 0
     LPIECE = MIN(LWRAP, LENMSG+1-NEXTC)
     IF (LPIECE .LT. LENMSG+1-NEXTC) THEN
        DO I=LPIECE+1,2,-1
           IF (MESSG(NEXTC+I-1:NEXTC+I-1) .EQ. ' ') THEN
              LPIECE = I-1
              IDELTA = 1
              GOTO 54
           ENDIF
        END DO
     ENDIF
   54   CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
     NEXTC = NEXTC + LPIECE + IDELTA
  ELSEIF (LPIECE .EQ. 1) THEN
  !
  !   WE HAVE A NEW LINE SENTINEL AT MESSG(NEXTC:NEXTC+1).
  !   DON'T PRINT A BLANK LINE.
  !
     NEXTC = NEXTC + 2
     GO TO 50
  ELSEIF (LPIECE .GT. LWRAP+1) THEN
  !
  !   LPIECE SHOULD BE SET DOWN TO LWRAP.
  !
     IDELTA = 0
     LPIECE = LWRAP
     DO I=LPIECE+1,2,-1
        IF (MESSG(NEXTC+I-1:NEXTC+I-1) .EQ. ' ') THEN
           LPIECE = I-1
           IDELTA = 1
           GOTO 58
        ENDIF
     END DO
   58   CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
     NEXTC = NEXTC + LPIECE + IDELTA
  ELSE
  !
  !   IF WE ARRIVE HERE, IT MEANS 2 .LE. LPIECE .LE. LWRAP+1.
  !   WE SHOULD DECREMENT LPIECE BY ONE.
  !
     LPIECE = LPIECE - 1
     CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
     NEXTC  = NEXTC + LPIECE + 2
  ENDIF
  !
  !   PRINT
  !
  DO I=1,NUNIT
     WRITE(IU(I), '(A)') CBUFF(1:LPREF+LPIECE)
  END DO
  !
  IF (NEXTC .LE. LENMSG) GO TO 50
  RETURN
END SUBROUTINE XERPRN