*
         JMP   SUBSTRINGSX
*
*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=*
*                              *
* SUBSTRING ROUTINE LIBRARY    *
*                              *
* AUTHOR:  NATHAN RIGGS        *
* CONTACT: NATHAN.RIGGS@       *
*          OUTLOOK.COM         *
*                              *
* VERSION: 0.1.2               *
* DATE:    30-OCT-2018         *
* ASSEMBLER: MERLIN 8 PRO      *
*                              *
* LICENSE: APACHE 2.0, WHERE   *
*  APPLICABLE. CODE INSPIRED   *
*  BY THE WORKS OF OTHERS MAY  *
*  FALL UNDER A DIFFERENT      *
*  LICENSE (NOTED IN ROUTINE). *
*                              *
* THIS IS A LIBRARY FOR USING  *
* SUBSTRING OPERATIONS. THIS   *
* IS SEPARATE FROM THE MAIN    *
* STRINGS LIBRARY BY VIRTUE OF *
* DEALING WITH FULL MEMORY     *
* ERRORS IN THE ASSEMBLER.     *
* YEEEEE.                      *
*                              *
*------------------------------*
*                              *
* LIST OF ROUTINES             *
*                              *
* SUBPOS : FIND POS OF SUBSTR  *
* SUBCOPY : COPY SUBSTRING     *
* SUBDEL : DELETE SUBSTRING    *
* SUBINS : INSERT SUBSTRING    *
*                              *
*=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-*
*
*``````````````````````````````*
* SUBPOS :: SUBSTRING POSITION *
*-                            -*
* FIND THE POSITION OF A SUB-  *
* STRING WITHIN ANOTHER STRING *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #>STR ; STRING ADDR     *
*  PHA                         *
*  LDA #<STR                   *
*  PHA                         *
*  LDA #>SUB ; SUBSTR ADDR     *
*  PHA                         *
*  LDA #<SUB                   *
*  JSR SUBPOS                  *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* LOW BYTE OF SUBSTRING ADDR   *
* HI BYTE OF SUBSTRING ADDR    *
* LO BYTE OF STRING ADDR       *
* HI BYTE OF STRING ADDRESS    *
* HI BYTE OF RETURN ADDRESS    *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = CLOBBERED; TRASH       *
*   X = CLOBBERED; TRASH       *
*   A = INDEX OF SUBSTRING IF  *
*       FOUND; OTHERWISE, 0    *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*                              *
* THAT SAID, LEVENTHAL WROTE   *
* THAT THE PURPOSE OF THE BOOK *
* WAS TO COPY THE SOURCE CODE, *
* AS REINVENTING THE WHEEL IS  *
* TEDIOUS, AND HE PLACED NO    *
* RESTRICTIONS ON ITS USE.     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUBPOS
*
** SAVE RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   ADDR2
         PLA
         STA   ADDR2+1
         PLA
         STA   ADDR1
         PLA
         STA   ADDR1+1
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
:POS
         LDY   #0
         LDA   (ADDR1),Y  ; GET LENGTH OF STRING
         BEQ   :NOTFND    ; EXIT IF LENGTH = 0
         STA   :SLEN
         LDA   (ADDR2),Y  ; GET SUBSTR LENGTH
         BEQ   :NOTFND    ; EXIT IF SUB LENGTH = 0
         STA   :SUBLEN
*
** IF THE SUBSTR IS LONGER THAN STR, DECLARE THE
** SUBSTR NOT FOUND
*
         LDA   :SUBLEN
         CMP   :SLEN
         BEQ   :LENOK
         BCS   :NOTFND    ; CANNOT FIND SUBSTR IF
                          ; LONGER THAN STR
*
** START, SEARCH, CONTINUE UNTIL
** REMAINING STR SHORTER THAN SUBSTR
*
:LENOK
         LDA   #1
         STA   :SINDEX    ; START LOOKING AT FIRST
                          ; CHARACTER OF STRING
         LDA   :SLEN      ; CONT UNTIL REMAINING STR
                          ; TOO SHORT
         SEC              ; COUNT=STR LEN - SUB LEN+1
         SBC   :SUBLEN
         STA   :SCOUNT
         INC   :SCOUNT
*
** SEARCH FOR SUBSTRING IN STRING
*
:SLP1
         LDA   :SINDEX
         STA   :SIDX      ; START STR AT INDEX
         LDA   #1
         STA   :SUBIDX    ; START SUB IND AT 1
*
** LOOK FOR SUBSTRING BEGINNING AT INDEX
*
:CMPLP
         LDY   :SIDX
         LDA   (ADDR1),Y  ; GET NEXT CHAR FROM STR
         LDY   :SUBIDX
         CMP   (ADDR2),Y  ; COMPARE TO NEXT SUB CHAR
         BNE   :SLP2      ; BR IF SUB NOT HERE
         LDY   :SUBIDX
         CPY   :SUBLEN    ; TEST IF WE ARE DONE
         BEQ   :FOUND     ; BR IF ALL CHARS WERE EQUAL
         INY              ; ELSE INC TO NEXT CHAR
         STY   :SUBIDX
         INC   :SIDX
         JMP   :CMPLP     ; CONTINUE
*
** ARRIVE HERE IF SUBSTRING NOT FOUND
*
:SLP2
         INC   :SINDEX    ; INCREMENT INDEX
         DEC   :SCOUNT    ; DEC COUNT
         BNE   :SLP1      ; BR IF NOT DONE
         BEQ   :NOTFND    ; ELSE EXIT TO NOT FOUND
*
:FOUND
         LDA   :SINDEX    ; FOUND, A = STARTING IDX
         JMP   :EXIT
*
:NOTFND
         LDA   #0         ; SUB NOT FOUND, A=0
         STA   RESULT
:EXIT
         RTS
*
** DATA
*
:SLEN    DS    1
:SUBLEN  DS    1
:SINDEX  DS    1
:SUBIDX  DS    1
:SCOUNT  DS    1
:SIDX    DS    1
*
*``````````````````````````````*
* SUBCOPY :: COPY SUBSTRING    *
*-                            -*
* COPY A SUBSTRING FROM A      *
* STRING TO A DESTINATION      *
* ADDRESS.                     *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #>STR ; SOURCE STRING   *
*  PHA                         *
*  LDA #<STR                   *
*  PHA                         *
*  LDA #IND ; COPY START INDEX *
*  PHA                         *
*  LDA LEN ; LENGTH OF SUBSTR  *
*  PHA                         *
*  LDA #>SUB ; SUBSTRING ADDR  *
*  PHA                         *
*  LDA #<SUB                   *
*  PHA                         *
*  LDA MLEN ; MAX LENGTH       *
*  PHA                         *
*  JSR SUBCOPY                 *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* MAX LENGTH OF DEST STRING    *
* LO BYTE OF DEST STR ADDRESS  *
* HI BYTE OF DEST STRING ADDR  *
* NUMBER OF BYTES TO COPY      *
* STARTING INDEX TO COPY FROM  *
* LO BYTE OF SRC STRING ADDR   *
* HI BYTE OF SOURCE STR ADDR   *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = TRASH                  *
*   X = TRASH                  *
*   A = TRASH                  *
*                              *
* CARRY FLAG WILL BE 0 IF NO   *
* ERRORS; ELSE, CARRY = 1      *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUBCOPY
*
** SAVE RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :MLEN
         PLA
         STA   ADDR2
         PLA
         STA   ADDR2+1
         PLA
         STA   :SCNT
         PLA
         STA   :SINDEX
         PLA
         STA   ADDR1
         PLA
         STA   ADDR1+1
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         LDA   #0
         STA   :S2LEN     ; DESTINATION LENGTH = 0
         STA   :SCERR     ; ASSUME NO ERRORS
*
** CHECK FOR ZERO BYTES TO COPY OR ZERO MAX SUBSTR LENGTH
*
         LDA   :SCNT
         BEQ   :OKEXIT    ; BR IF 0 BYTES TO COPY,
                          ; S2A WILL JUST HAVE ZERO LENGTH
         LDA   :MLEN
         BEQ   :EREXIT    ; ERROR EXIT IF SUBSTR HAS
                          ; ZERO MAX LENGTH
         LDA   :SINDEX
         BEQ   :EREXIT    ; ERROR EXIT IF START IDX = 0
*
** CHECK IF SRC STR REACHES STARTING INDEX
*
         LDY   #0
         LDA   (ADDR1),Y  ;
         STA   :S1LEN     ; GET LENGTH OF SOURCE STRING
         CMP   :SINDEX    ; COMPARE TO STARTING INDEX
         BCC   :EREXIT    ; ERROR EXIT IF INDEX TOO BIG
*
** CHECK THAT WE DO NOT COPY BEYOND THE END OF
** THE SOURCE STRING.
** IF INDEX + COUNT -1 > SLEN THEN
** COUNT = SLEN - SINDEX + 1
*
         LDA   :SINDEX
         CLC
         ADC   :SCNT
         BCS   :RECALC
         TAX              ; BR IF INDEX + COUNT > 255
         DEX
         CPX   :S1LEN
         BCC   :CNT10K    ; BR IF IND + CNT - 1 < S1LEN
         BEQ   :CNT10K    ; OR EQUAL
*
** THE CALLER ASKED FOR TOO MANY CHARS SO
** JUST RETURN EVERYTHING BETWEEN INDEX AND
** END OF STRING. SO CNT = S1LEN - INDEX + 1
*
:RECALC
         LDA   :S1LEN     ; RECALCULATE COUNT
         SEC
         SBC   :SINDEX
         STA   :SCNT
         INC   :SCNT      ; CNT = S1LEN - IND + 1
         LDA   #$0FF
         STA   :SCERR     ; INDICATE TRUNCATION
*
** CHECK IF COUNT IS <= THE MAXIMUM LENGTH
** OF THE DEST STRING. IF NOT, THEN SET COUNT TO
** MAX LENGTH.
** IF COUNT > MAXLEN THEN
**    COUNT = MAXLEN
*
:CNT10K
         LDA   :SCNT
         CMP   :MLEN      ; IF CNT > M SUBSTR LEN ?
         BCC   :CNT20K    ; BR IF CNT < MAXLEN
         BEQ   :CNT20K    ; BR IF CNT = MAXLEN
         LDA   :MLEN
         STA   :SCNT      ; ELSE CNT = MAXLEN
         LDA   #$0FF
         STA   :SCERR     ; INDICATE DEST STR OVERFLOW
*
** EVERYTHING IS SET UP SO MOVE THE
** SUBSTRING TO THE DESTINATION STRING
*
:CNT20K
         LDX   :SCNT      ; REG X WILL BE COUNTER
         BEQ   :EREXIT    ; ERR IF 0
         LDA   #1         ; START WITH 1ST CHAR IN DEST
         STA   :S2LEN     ; RUNNING DEST INDEX
                          ; __SINDEX IS SRC INDEX
:MVLP
         LDY   :SINDEX
         LDA   (ADDR1),Y  ; GET NEXT SRC CHAR
         LDY   :S2LEN
         STA   (ADDR2),Y  ; MOVE NEXT CHAR TO DEST
         INC   :SINDEX    ; INC SRC INDEX
         INC   :S2LEN     ; INC DEST INDEX
         DEX              ; DECREMENT COUNTER
         BNE   :MVLP      ; CONT UNTIL CNT = 0
         DEC   :S2LEN     ; SUBSTR LEN=FINAL DEST IND-1
         LDA   :SCERR     ; CHECK FOR ANY ERRORS
         BNE   :EREXIT    ; BR IF STR TRUNCATED OR OVERFLOW
*
** GOOD EXIT
*
:OKEXIT
         CLC
         BCC   :EXIT
*
** ERROR EXIT
*
:EREXIT
         SEC
*
** STORE LENGTH BYTE IN FRONT OF SUBSTR
*
:EXIT
         LDA   :S2LEN
         LDY   #0
         STA   (ADDR2),Y
         RTS
*
** DATA
*
:S1LEN   DS    1
:S2LEN   DS    1
:MLEN    DS    1
:SCNT    DS    1
:SINDEX  DS    1
:SCERR   DS    1
*
*``````````````````````````````*
* SUBDEL :: DELETE SUBSTRING   *
*-                            -*
* DELETE A SUBSTRING FROM A    *
* STRING.                      *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
* LDA #>STR ; STRING ADDRESS   *
* PHA                          *
* LDA #<STR                    *
* PHA                          *
* LDA IND ; DEL START INDEX    *
* PHA                          *
* LDA LEN ; LENGTH OF DELETE   *
* PHA                          *
* JSR SUBDEL                   *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* NUMBER OF BYTES TO DELETE    *
* STARTING INDEX OF DELETION   *
* LO BYTE OF STRING ADDRESS    *
* HI BYTE OF STRING ADDRESS    *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = TRASH                  *
*   X = TRASH                  *
*   A = TRASH                  *
*                              *
* IF NO ERRORS, CARRY = 0;     *
* ELSE, CARRY = 1              *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUBDEL
*
** SAVE RETURN ADDRES
*
         PLA
         TAY
         PLA
         TAX
*
** GET PARAMTERS
*
         PLA
         STA   :SCNT
         PLA
         STA   :SINDEX
         PLA
         STA   ADDR1
         PLA
         STA   ADDR1+1
*
** RESTORE RETURN ADDRESS
*
         TXA
         PHA
         TYA
         PHA
*
** INITIALIZE ERROR INDICATOR TO 0
*
         LDY   #0
         STY   :SCERR
         LDA   (ADDR1),Y
         STA   :S1LEN     ; GET STRING LENGTH
*
** CHECK FOR A NON-ZERO COUNT AND INDEX
*
         LDA   :SCNT
         BEQ   :OKEXIT
                          ; GOOD EXIT IF NO DELETE
         LDA   :SINDEX
         BEQ   :ERREXIT   ; ERR EXIT IF START = 0
*
** CHECK FOR STARTING INDEX WITHIN THE STRING
*
* EXIT IF IT IS NOT
*
         LDA   :S1LEN
         CMP   :SINDEX
         BCC   :ERREXIT
*
** BE SURE THE NUMBER OF CHARACTERS REQUESTED
** TO BE DELETED ARE PRESENT.
** IF NOT, THEN ONLY DELETE FROM THE INDEX
** TO THE END OF THE STRING.
*
         LDA   :SINDEX
         CLC
         ADC   :SCNT
         BCS   :TRUNC     ;TRUNCATE IF INDEX
                          ; + COUNT > 255
         STA   :SIDX      ; SAVE INDEX + COUNT AS
                          ; THE SOURCE INDEX
         TAX              ; X = INDEX + COUNT
         DEX
         CPX   :S1LEN
         BCC   :CNTOK     ; BR IF IND + CNT - 1
                          ; < __S1LEN
                          ; ELSE JUST TRUNC STRING
         BEQ   :TRUNC     ; TRUNC BUT NO ERROR--
                          ; EXACTLY ENOUGH CHARS
         LDA   #$0FF
         STA   :SCERR     ; INDICATE ERROR - NOT
                          ; ENOUGH CHARS TO DELETE
*
** TRUNCATE THE STRING - NO COMPACTING NECESSARY
*
:TRUNC
         LDX   :SINDEX    ; STRING LENGTH =
                          ;   START INDEX - 1
         DEX
         STX   :S1LEN
         LDA   :SCERR
         BEQ   :OKEXIT
         BNE   :ERREXIT
*
** DELETE THE SUBSTRING BY COMPACTING
** MOVE ALL CHARS ABOVE THE DELETED AREA DOWN
*
:CNTOK
*
** CALCULATE NUMBER OF CHARS TO MOVE
** (SLEN - SIDX + 1)
*
         LDA   :S1LEN     ; GET STR LENGTH
         SEC
         SBC   :SIDX      ; SUBTRACT START INDEX
         TAX
         INX
         BEQ   :OKEXIT    ; ADD 1 TO INCLUDE LAST
                          ; CHAR; BR IF CNT = 0
:MVLP
         LDY   :SIDX
         LDA   (ADDR1),Y  ; GET NEXT CHAR
         LDY   :SINDEX
         STA   (ADDR1),Y  ;MOVE IT DOWN
         INC   :SINDEX
         INC   :SIDX      ; INC DEST, SRC INDEXES
         DEX
         BNE   :MVLP      ; CONT UNTIL CNT = 0
         LDX   :SINDEX
         DEX              ; START LENGTH = FINAL
                          ; DEST INDEX -1
         STX   :S1LEN
*
** GOOD EXIT
*
:OKEXIT
         CLC
         BCC   :EXIT
:ERREXIT
         SEC
*
:EXIT
         LDA   :S1LEN
         LDY   #0
         STA   (ADDR1),Y  ; SET LENGTH OF STRING
         RTS
*
** DATA
*
:S1LEN   DS    1
:SCNT    DS    1
:SINDEX  DS    1
:SIDX    DS    1
:SCERR   DS    1
*
*``````````````````````````````*
* SUBINS :: INSERT SUBSTRING   *
*-                            -*
* INSERT A SUBSTRING INTO      *
* ANOTHER STRING.              *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
* LDA #STR ; STRING TO INS TO  *
* PHA                          *
* LDA #<STR                    *
* PHA                          *
* LDA IND ; INDEX TO START INS *
* PHA                          *
* LDA MLEN ; MAX LENGTH OF STR *
* PHA                          *
* LDA #>SUB ; SUBSTRING TO INS *
* PHA                          *
* LDA #<SUB                    *
* PHA                          *
* JST SUBINS                   *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* LO BYTE OF SUBSTRING ADDRESS *
* HI BYTE OF SUBSTRING ADDRESS *
* MAX LENGTH OF FINAL STRING   *
* STARTING INDEX FOR INSERTION *
* LO BYTE OF SRC STRING ADDR   *
* HI BYTE OF SOURCE STR ADDR   *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = TRASH                  *
*   X = TRASH                  *
*   A = TRASH                  *
*                              *
* CARRY = 0 IF NO ERRORS; ELSE *
* CARRY = 1                    *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
SUBINS
*
** GET RETURN ADDRESS
*
         PLA
         TAY
         PLA
         TAX
*
** GET PARAMETERS
*
         PLA
         STA   ADDR2
         PLA
         STA   ADDR2+1
         PLA
         STA   :MLEN
         PLA
         STA   :SINDEX
         PLA
         STA   ADDR1
         PLA
         STA   ADDR1+1
*
** RESTORE RETURN ADDRESS
*
         TXA
         PHA
         TYA
         PHA
*
*
** ASSUME NO ERRORS
*
         LDA   #0
         STA   :SCERR     ; ASSUME NO ERR WILL BE FOUND
*
** GET SUBSTRING AND STRING LENGTHS
** IF SUB LENGTH = 0 THEN EXIT NO ERROR
*
         LDY   #0
         LDA   (ADDR1),Y
         STA   :S1LEN     ; GET LENGTH OF STRING
         LDA   (ADDR2),Y
         STA   :S2LEN     ; GET LENGTH OF SUB
         BNE   :IDX0
         JMP   :OKEXIT    ; EXIT OF NO INSERT/ERR
*
** IF STARTING INDEX IS 0 THEN ERROR EXIT
*
:IDX0
         LDA   :SINDEX
         BNE   :CHKLEN    ; BR OF INDEX NOT 0
         JMP   :EREXIT    ; ELSE ERROR EXIT
*
** CHECK THAT THE RESULTING STRING AFTER THE
** INSERTION FITS IN THE SOURCE STRING. IF NOT
** THEN TRUNCATE THE SUBSTRING AND SET
** THE TRUNCATION FLAG.
*
:CHKLEN
         LDA   :S2LEN     ; GET SUBSTR LENGTH
         CLC
         ADC   :S1LEN
         BCS   :TRUNC     ;TRUN IF S1+S2 LENGTH > 255
         CMP   :MLEN      ;
         BCC   :IDXLEN    ; BR IF S1+S2 LEN < MAX LENGTH
         BEQ   :IDXLEN    ; BR IF EQUAL
*
** SUBSTRING DOES NOT FIT, SO TRUNCATE IT
*
:TRUNC
         LDA   :MLEN      ; SUBSTR LEN = MLEN - STR LEN
         SEC
         SBC   :S1LEN
         BCC   :EREXIT
         BEQ   :EREXIT    ; ERR IF MLEN < STR LEN OR 0
                          ; (ORIGINAL STRING WAS TOO LONG)
         STA   :S2LEN
         LDA   #$0FF
         STA   :SCERR     ; INDICATE SUBSTR WAS TRUNCATED
*
** CHECK THAT INDEX IS WITHIN STRING. IF NOT, CONCAT
** SUBSTR ONTO THE END OF THE STRING.
*
:IDXLEN
         LDA   :S1LEN
         CMP   :SINDEX    ;
         BCS   :LENOK     ; BR IF INDEX WITHIN STR
         LDX   :S1LEN     ; ELSE CONCAT SUB AT END OF STR
         INX
         STX   :SINDEX    ; START RIGHT AFTER END OF STR
         LDA   #$0FF
         STA   :SCERR     ; INDICATE ERR IN INSERT
         LDA   :S1LEN
         CLC
         ADC   :S2LEN
         STA   :S1LEN     ; ADD LENGTHS TOGETHER
         JMP   :MVESUB    ; PERFORM MOVE, NOTHING ELSE TODO
*
** OPEN UP A SPACE IN THE SOURCE STRING FOR THE
** SUBSTRING BY MOVING THE CHARACTERS FROM THE END
** OF THE SOURCE STRING DOWN TO INDEX, UP BY
** THE SIZE OF THE STRING.
*
:LENOK
*
** CALC NUMBER OF CHARS TO MOVE
**  COUNT = STR LEN - START INDEX + 1
*
         LDA   :S1LEN
         SEC
         SBC   :SINDEX
         TAX
         INX              ; X= NUM OF CHARS TO MOV
*
** SET THE SOURCE INDEX AND CALC DEST INDEX
*
         LDA   :S1LEN
         STA   :SIDX      ; SRC ENDS AT ORIG STR END
         CLC
         ADC   :S2LEN
         STA   :SBIDX     ; DEST ENDS FURTHER BY SUB LEN
         STA   :S1LEN     ; SET NEW LENGTH TO THIS ALSO
*
:OPNLP
         LDY   :SIDX
         LDA   (ADDR1),Y
         LDY   :SBIDX
         STA   (ADDR1),Y  ; MOVE IT UP IN MEM
         DEC   :SIDX
         DEC   :SBIDX     ; DE DEST IDX, COUNTER
         DEX
         BNE   :OPNLP     ; CONT UNTIL COUNTER = 0
*
** MOVE THE SUBSTR INTO THE OPEN AREA
*
:MVESUB
         LDA   #1
         STA   :SIDX
                          ; START AT ONE IN THE SUBSTR
                          ; START AT INDEX IN THE STRING
         LDX   :S2LEN     ; X = NUM OF CHARS TO MOVE
*
:MVELP
         LDY   :SIDX
         LDA   (ADDR2),Y  ; GET NEXT CHAR
         LDY   :SINDEX
         STA   (ADDR1),Y
         INC   :SIDX      ; INC SUBSTR INDEX
         INC   :SINDEX    ; INC STR INDEX
         DEX              ; DEC COUNTER
         BNE   :MVELP     ; CONT UNTIL COUNTER = 0
         LDA   :SCERR     ; GET ERROR FLAG
         BNE   :EREXIT    ; BR IF SUBSTR WAS TRUNCED
*
:OKEXIT
         CLC
         BCC   :EXIT
:EREXIT
         SEC              ; ERROR EXIT
:EXIT
         LDA   :S1LEN
         LDY   #0
         STA   (ADDR1),Y
         RTS
*
** DATA
*
:S1LEN   DS    1
:S2LEN   DS    1
:SUBLEN  DS    1
:MLEN    DS    1
:SINDEX  DS    1
:SIDX    DS    1
:SBIDX   DS    1
:SCERR   DS    1
*
SUBSTRINGSX
*