REM CHANGE

TYPE SECTOR
   BYTES AS STRING * 512
END TYPE

DIM RECORD AS SECTOR

REM initialize
RECORD.BYTES = STRING$(512," ")
C$ = CHR$(32)
L = 1

REM open files
INPUT "input filename:  ";F$
INPUT "output filename: ";O$
INPUT "log filename: ";G$
OPEN F$ FOR RANDOM AS #1 LEN = LEN(RECORD)
OPEN O$ FOR RANDOM AS #2 LEN = LEN(RECORD)
OPEN G$ FOR OUTPUT AS #3

INPUT "want to make changes (Y/N)";E$
E = 0
IF E$ = "Y" THEN
   E = 1
ELSEIF E$ = "y" THEN
   E = 1
END IF

OPEN "echo." FOR OUTPUT AS #4

REM initialize sector counter:
S = 0

DO WHILE NOT EOF(1)

REM increment sector counter
    S = S + 1

REM read next sector
    GET #1, S, RECORD

REM log this input byte
    IF L = 1 THEN
       P$ = MID$(RECORD.BYTES,1,1)
       P  = ASC(P$)
       PRINT #3, "byte ";P$;" is ";P
    END IF

REM look thru byte pairs
    FOR I = 2 TO 512

       J = 514 - I
       K = J - 1

REM    pull byte "J":
       M$ = MID$(RECORD.BYTES,J,1)
REM
REM    pull byte "K":
       N$ = MID$(RECORD.BYTES,K,1)

REM    convert to numeric equivalents
       M  = ASC(M$)
       N  = ASC(N$)

REM    log this input byte
       IF L = 1 THEN
          P$ = MID$(RECORD.BYTES,I,1)
          P  = ASC(P$)
          PRINT #3, "byte ";P$;" is ";P
       END IF

       IF E = 1 THEN
       IF J = 512 THEN
          IF M = 13 THEN
             IF N = 13 THEN
                MID$(RECORD.BYTES,K,1) = C$
             ELSE
                MID$(RECORD.BYTES,J,1) = C$
             END IF
          END IF
       ELSE
          IF M = 13 THEN
             IF N = 13 THEN
                MID$(RECORD.BYTES,K,1) = C$
             END IF
          END IF
       END IF
       END IF

    NEXT I

REM write this record:
    PUT #2, S, RECORD
    PRINT     "wrote sector ";S
    PRINT #4, "wrote sector ";S

REM get next record:
LOOP


CLOSE
PRINT "number of sectors written: ";S
PRINT "all done!"
STOP
END