10 'PATCHER - file patching program - PROGRAMMERS JOURNAL Vol 1, No 6, Pg. 21
20 'Copyright 1983 - Data Base Decisions, Atlanta, GA
30 'This program is used to patch other programs or files. It requires
40 'a data file containing the patches. The first three items in the
50 'patch file are the name of the file to be patched, a check sum, and
60 'comments. For each byte to be patched, there is one record containing
70 'the offset of the byte to be patched, the old value of the byte,
80 'and the new (patch) value.
90 'Patches are generated using program  GENPATCH.BAS
100 'Note: If the offset is greater than 32,767, BASIC 2.00 must be used.
110 CLS
120 DEFINT A-Z
130 CLEAR
140 ON ERROR GOTO 500
150 CLOSE
160 PRINT : INPUT "Name of file containing patches";PAT$
170 IF PAT$="" THEN  END
180 OPEN "i",#1,PAT$
190 INPUT #1,FIL$,CKSUM!,COMMENT$
200 OPEN "i",#2,FIL$    'is it there
210 PRINT "Patching: " FIL$
220 PRINT "Comments: " COMMENT$
230 CLOSE 2
240 OPEN "r",#2,FIL$,1  'reopen as random file
250 FIELD 2,1 AS R$
260 FILE.LEN = LOF(2)
270 IF EOF(1) THEN 450
280 INPUT# 1,BYTE!,OLDVAL,NEWVAL        'get patch
290 NEWSUM!=(NEWSUM!+BYTE!+OLDVAL!+NEWVAL!)
300 PRINT BYTE!,OLDVAL,NEWVAL, "Checksum " NEWSUM!
310 IF NEWSUM! > 32767 THEN NEWSUM!=NEWSUM!-32767: GOTO 310
320 IF BYTE! > FILE.LEN THEN PRINT "Byte " BYTE! " is beyond end of file": GOTO
330 GET 2,BYTE!
340 R=ASC(R$)
350 IF R <> OLDVAL THEN PRINT "Old value for byte " BYTE! " is " R " not " OLDV
360 LSET R$=CHR$(NEWVAL)
370 PUT 2,BYTE!
380 APPLIED=APPLIED+1
390 GOTO 270
400 REM *** invalid condition ***
410 BEEP:INPUT "Continue (y/n)";ANS$
420 IF ANS$="Y" OR ANS$="y" THEN 390
430 IF ANS$="N" OR ANS$="n" THEN 450
440 GOTO 400
450 REM *** wrap it up ***
460 IF CKSUM!=NEWSUM! THEN PRINT "Checksums match" ELSE PRINT "Checksums do not
470 PRINT "Patches applied: " APPLIED
480 CLOSE
490 END
500 REM *** error handler ***
510 UNABLE$="Unable to "
520 IF ERL=180 OR ERL=280 THEN PRINT UNABLE$ "read " PAT$
530 IF ERL=200 OR ERL=240 OR ERL=330 THEN PRINT UNABLE$ "read " FIL$
540 IF ERL=370 THEN PRINT UNABLE$ "write " FIL$
550 RESUME 120
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