C*DECK XERPRN
      SUBROUTINE XERPRN (PREFIX, NPREF, MESSG, NWRAP)
C***BEGIN PROLOGUE  XERPRN
C***SUBSIDIARY
C***PURPOSE  This routine is called by XERMSG to print error messages
C***LIBRARY   SLATEC
C***CATEGORY  R3C
C***TYPE      ALL
C***KEYWORDS  ERROR MESSAGES, PRINTING, XERROR
C***AUTHOR  FONG, KIRBY, (NMFECC AT LLNL)
C***DESCRIPTION
C
C This routine sends one or more lines to each of the (up to five)
C logical units to which error messages are to be sent.  This routine
C is called several times by XERMSG, sometimes with a single line to
C print and sometimes with a (potentially very long) message that may
C wrap around into multiple lines.
C
C PREFIX  Input argument of type CHARACTER.  This argument contains
C         characters to be put at the beginning of each line before
C         the body of the message.  No more than 16 characters of
C         PREFIX will be used.
C
C NPREF   Input argument of type INTEGER.  This argument is the number
C         of characters to use from PREFIX.  If it is negative, the
C         intrinsic function LEN is used to determine its length.  If
C         it is zero, PREFIX is not used.  If it exceeds 16 or if
C         LEN(PREFIX) exceeds 16, only the first 16 characters will be
C         used.  If NPREF is positive and the length of PREFIX is less
C         than NPREF, a copy of PREFIX extended with blanks to length
C         NPREF will be used.
C
C MESSG   Input argument of type CHARACTER.  This is the text of a
C         message to be printed.  If it is a long message, it will be
C         broken into pieces for printing on multiple lines.  Each line
C         will start with the appropriate prefix and be followed by a
C         piece of the message.  NWRAP is the number of characters per
C         piece; that is, after each NWRAP characters, we break and
C         start a new line.  In addition the characters '$$' embedded
C         in MESSG are a sentinel for a new line.  The counting of
C         characters up to NWRAP starts over for each new line.  The
C         value of NWRAP typically used by XERMSG is 72 since many
C         older error messages in the SLATEC Library are laid out to
C         rely on wrap-around every 72 characters.
C
C NWRAP   Input argument of type INTEGER.  This gives the maximum size
C         piece into which to break MESSG for printing on multiple
C         lines.  An embedded '$$' ends a line, and the count restarts
C         at the following character.  If a line break does not occur
C         on a blank (it would split a word) that word is moved to the
C         next line.  Values of NWRAP less than 16 will be treated as
C         16.  Values of NWRAP greater than 132 will be treated as 132.
C         The actual line length will be NPREF + NWRAP after NPREF has
C         been adjusted to fall between 0 and 16 and NWRAP has been
C         adjusted to fall between 16 and 132.
C
C***REFERENCES  (NONE)
C***ROUTINES CALLED  I1MACH, XGETUA
C***REVISION HISTORY  (YYMMDD)
C   880621  DATE WRITTEN
C   880708  REVISED AFTER THE SLATEC CML SUBCOMMITTEE MEETING OF
C           JUNE 29 AND 30 TO CHANGE THE NAME TO XERPRN AND TO REWORK
C           THE HANDLING OF THE NEW LINE SENTINEL TO BEHAVE LIKE THE
C           SLASH CHARACTER IN FORMAT STATEMENTS.
C   890706  REVISED WITH THE HELP OF FRED FRITSCH AND REG CLEMMENS TO
C           STREAMLINE THE CODING AND FIX A BUG THAT CAUSED EXTRA BLANK
C           LINES TO BE PRINTED.
C   890721  REVISED TO ADD A NEW FEATURE.  A NEGATIVE VALUE OF NPREF
C           CAUSES LEN(PREFIX) TO BE USED AS THE LENGTH.
C   891013  REVISED TO CORRECT ERROR IN CALCULATING PREFIX LENGTH.
C   891214  Prologue converted to Version 4.0 format.  (WRB)
C   900510  Added code to break messages between words.  (RWC)
C***END PROLOGUE  XERPRN
      CHARACTER*(*) PREFIX, MESSG
      INTEGER NPREF, NWRAP
      CHARACTER*148 CBUFF
      INTEGER IU(5), NUNIT
      CHARACTER*2 NEWLIN
      PARAMETER (NEWLIN = '$$')
C***FIRST EXECUTABLE STATEMENT  XERPRN
      CALL XGETUA(IU,NUNIT)
C
C       A ZERO VALUE FOR A LOGICAL UNIT NUMBER MEANS TO USE THE STANDARD
C       ERROR MESSAGE UNIT INSTEAD.  I1MACH(4) RETRIEVES THE STANDARD
C       ERROR MESSAGE UNIT.
C
      N = I1MACH(4)
      DO 10 I=1,NUNIT
         IF (IU(I) .EQ. 0) IU(I) = N
   10 CONTINUE
C
C       LPREF IS THE LENGTH OF THE PREFIX.  THE PREFIX IS PLACED AT THE
C       BEGINNING OF CBUFF, THE CHARACTER BUFFER, AND KEPT THERE DURING
C       THE REST OF THIS ROUTINE.
C
      IF ( NPREF .LT. 0 ) THEN
         LPREF = LEN(PREFIX)
      ELSE
         LPREF = NPREF
      ENDIF
      LPREF = MIN(16, LPREF)
      IF (LPREF .NE. 0) CBUFF(1:LPREF) = PREFIX
C
C       LWRAP IS THE MAXIMUM NUMBER OF CHARACTERS WE WANT TO TAKE AT ONE
C       TIME FROM MESSG TO PRINT ON ONE LINE.
C
      LWRAP = MAX(16, MIN(132, NWRAP))
C
C       SET LENMSG TO THE LENGTH OF MESSG, IGNORE ANY TRAILING BLANKS.
C
      LENMSG = LEN(MESSG)
      N = LENMSG
      DO 20 I=1,N
         IF (MESSG(LENMSG:LENMSG) .NE. ' ') GO TO 30
         LENMSG = LENMSG - 1
   20 CONTINUE
   30 CONTINUE
C
C       IF THE MESSAGE IS ALL BLANKS, THEN PRINT ONE BLANK LINE.
C
      IF (LENMSG .EQ. 0) THEN
         CBUFF(LPREF+1:LPREF+1) = ' '
         DO 40 I=1,NUNIT
            WRITE(IU(I), '(A)') CBUFF(1:LPREF+1)
   40    CONTINUE
         RETURN
      ENDIF
C
C       SET NEXTC TO THE POSITION IN MESSG WHERE THE NEXT SUBSTRING
C       STARTS.  FROM THIS POSITION WE SCAN FOR THE NEW LINE SENTINEL.
C       WHEN NEXTC EXCEEDS LENMSG, THERE IS NO MORE TO PRINT.
C       WE LOOP BACK TO LABEL 50 UNTIL ALL PIECES HAVE BEEN PRINTED.
C
C       WE LOOK FOR THE NEXT OCCURRENCE OF THE NEW LINE SENTINEL.  THE
C       INDEX INTRINSIC FUNCTION RETURNS ZERO IF THERE IS NO OCCURRENCE
C       OR IF THE LENGTH OF THE FIRST ARGUMENT IS LESS THAN THE LENGTH
C       OF THE SECOND ARGUMENT.
C
C       THERE ARE SEVERAL CASES WHICH SHOULD BE CHECKED FOR IN THE
C       FOLLOWING ORDER.  WE ARE ATTEMPTING TO SET LPIECE TO THE NUMBER
C       OF CHARACTERS THAT SHOULD BE TAKEN FROM MESSG STARTING AT
C       POSITION NEXTC.
C
C       LPIECE .EQ. 0   THE NEW LINE SENTINEL DOES NOT OCCUR IN THE
C                       REMAINDER OF THE CHARACTER STRING.  LPIECE
C                       SHOULD BE SET TO LWRAP OR LENMSG+1-NEXTC,
C                       WHICHEVER IS LESS.
C
C       LPIECE .EQ. 1   THE NEW LINE SENTINEL STARTS AT MESSG(NEXTC:
C                       NEXTC).  LPIECE IS EFFECTIVELY ZERO, AND WE
C                       PRINT NOTHING TO AVOID PRODUCING UNNECESSARY
C                       BLANK LINES.  THIS TAKES CARE OF THE SITUATION
C                       WHERE THE LIBRARY ROUTINE HAS A MESSAGE OF
C                       EXACTLY 72 CHARACTERS FOLLOWED BY A NEW LINE
C                       SENTINEL FOLLOWED BY MORE CHARACTERS.  NEXTC
C                       SHOULD BE INCREMENTED BY 2.
C
C       LPIECE .GT. LWRAP+1  REDUCE LPIECE TO LWRAP.
C
C       ELSE            THIS LAST CASE MEANS 2 .LE. LPIECE .LE. LWRAP+1
C                       RESET LPIECE = LPIECE-1.  NOTE THAT THIS
C                       PROPERLY HANDLES THE END CASE WHERE LPIECE .EQ.
C                       LWRAP+1.  THAT IS, THE SENTINEL FALLS EXACTLY
C                       AT THE END OF A LINE.
C
      NEXTC = 1
   50 LPIECE = INDEX(MESSG(NEXTC:LENMSG), NEWLIN)
      IF (LPIECE .EQ. 0) THEN
C
C       THERE WAS NO NEW LINE SENTINEL FOUND.
C
         IDELTA = 0
         LPIECE = MIN(LWRAP, LENMSG+1-NEXTC)
         IF (LPIECE .LT. LENMSG+1-NEXTC) THEN
            DO 52 I=LPIECE+1,2,-1
               IF (MESSG(NEXTC+I-1:NEXTC+I-1) .EQ. ' ') THEN
                  LPIECE = I-1
                  IDELTA = 1
                  GOTO 54
               ENDIF
   52       CONTINUE
         ENDIF
   54    CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
         NEXTC = NEXTC + LPIECE + IDELTA
      ELSEIF (LPIECE .EQ. 1) THEN
C
C       WE HAVE A NEW LINE SENTINEL AT MESSG(NEXTC:NEXTC+1).
C       DON'T PRINT A BLANK LINE.
C
         NEXTC = NEXTC + 2
         GO TO 50
      ELSEIF (LPIECE .GT. LWRAP+1) THEN
C
C       LPIECE SHOULD BE SET DOWN TO LWRAP.
C
         IDELTA = 0
         LPIECE = LWRAP
         DO 56 I=LPIECE+1,2,-1
            IF (MESSG(NEXTC+I-1:NEXTC+I-1) .EQ. ' ') THEN
               LPIECE = I-1
               IDELTA = 1
               GOTO 58
            ENDIF
   56    CONTINUE
   58    CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
         NEXTC = NEXTC + LPIECE + IDELTA
      ELSE
C
C       IF WE ARRIVE HERE, IT MEANS 2 .LE. LPIECE .LE. LWRAP+1.
C       WE SHOULD DECREMENT LPIECE BY ONE.
C
         LPIECE = LPIECE - 1
         CBUFF(LPREF+1:LPREF+LPIECE) = MESSG(NEXTC:NEXTC+LPIECE-1)
         NEXTC  = NEXTC + LPIECE + 2
      ENDIF
C
C       PRINT
C
      DO 60 I=1,NUNIT
         WRITE(IU(I), '(A)') CBUFF(1:LPREF+LPIECE)
   60 CONTINUE
C
      IF (NEXTC .LE. LENMSG) GO TO 50
      RETURN
      END
