day6: explain basic

day6/both.bas

@@ -2,11 +2,14 @@ REM Advent of Code 2022: Day 6, part 1 and 2
 REM Written in Applesoft BASIC
 
  5  HOME 
- 10 A = 1000
- 20 R = 0
+ 10 R = 0
  50  PRINT  CHR$ (4),"OPEN INPUT"
  60  PRINT  CHR$ (4),"READ INPUT"
  70  ONERR  GOTO 900
+
+REM Print some fun messages, going for a hacker feel.
+REM The routine at 1000 spins an empty FOR I = 0 TO A loop for a delay.
+
  100  PRINT "INITIALIZING COMMUNICATION SYSTEM";
  105 A = 350
  110  FOR I = 0 TO 6
@@ -22,12 +25,20 @@ REM Written in Applesoft BASIC
  250  PRINT 
  260  PRINT 
  300  PRINT "SCANNING FOR START-OF-PACKET MARKER:"
+
+REM Show bars above and below the four characters being tested for part one.
+
  310  HTAB 1
  320  VTAB 8
  330  PRINT "||||";
  340  HTAB 1
  350  VTAB 10
  360  PRINT "||||";
+
+REM Main loop: Print the current string. If it can be checked for uniqueness,
+REM GOTO 500 (which comes back to 440). At 440, we add the next character to
+REM the string and trim off the end if its too long.
+
  400  HTAB 1
  410  VTAB 9
  420  PRINT S$;
@@ -35,10 +46,18 @@ REM Written in Applesoft BASIC
  440  GET C$
  450 S$ = C$ + S$
  455  IF  LEN (S$) > 40 THEN S$ =  MID$ (S$,1,40)
+
+REM Delay a little for visual effect, count the added character, then repeat.
+
  460 A = 80
  470  GOSUB 1000
  475 R = R + 1
  480  GOTO 400
+
+REM Here, we check for matching characters. It's hard-coded because there are
+REM only a few combinations. A little ASCII art "connects" the matched bytes.
+REM GOTO 700 if all unique, otherwise back to 440 for the next character.
+
  500  HTAB 1
  510  VTAB 7
  530  IF  MID$ (S$,1,1) =  MID$ (S$,2,1) THEN  PRINT "/\  ";: GOTO 600
@@ -50,7 +69,9 @@ REM Written in Applesoft BASIC
  590  PRINT "    ";
  595  GOTO 700
  600  GOTO 440
- 620  VTAB 14
+
+REM Part 1 complete! Indicate that part 2 is beginning...
+
  700  HTAB 1
  710  VTAB 13
  715 A = 1300
@@ -60,39 +81,71 @@ REM Written in Applesoft BASIC
  740  PRINT "ENGAGING FULL-SPEED SEARCH FOR START-OF-MESSAGE MARKER:"
  750  GOSUB 1000
  760  GOTO 2000
+
+REM I keep the exit routine at 900 out of habit.
+
  900  PRINT  CHR$ (4),"CLOSE"
  910  END 
+
+REM The delay routine.
+
  1000  FOR Z = 0 TO A: NEXT Z
  1010  RETURN 
+
+REM Print bars for the 14-character window.
+
  2000  HTAB 1
  2010  VTAB 19
  2020  PRINT "||||||||||||||";
  2030  HTAB 1
  2040  VTAB 21
  2050  PRINT "||||||||||||||";
+
+REM Re-open the input data and reset variables (var M is new).
+
  2060  PRINT  CHR$ (4),"CLOSE"
  2070  PRINT  CHR$ (4),"OPEN INPUT"
  2080  PRINT  CHR$ (4),"READ INPUT"
  2090 S$ = "":R = 0
  2095 M = 0
+
+REM Print the string, then add and count the next character.
+
  2100  HTAB 1
  2110  VTAB 20
  2120  PRINT S$;
  2130  GET C$
  2140 S$ = C$ + S$
  2145 R = R + 1
+
+REM Same checks as part one, except for 2170 which skips M checks.
+REM See 2345 for explanation.
+
  2150  IF  LEN (S$) < 14 GOTO 2100
  2160  IF  LEN (S$) > 40 THEN S$ =  MID$ (S$,1,40)
  2170  IF M > 0 THEN M = M - 1: GOTO 2100
+
+REM Prepare to search for matches.
+
  2190 I = 1
  2200 M = 0
+
+REM Search for a matching pair. If found, store 2nd index in M.
+
  2210  FOR J = I + 1 TO 14
  2220  IF  MID$ (S$,I,1) =  MID$ (S$,J,1) THEN M = J:J = 14
  2230  NEXT J
  2240  IF M <  > 0 GOTO 2300
+
+REM If no matches, move to next index and repeat.
+REM If we've gone through all indices, marker found. GOTO 2400.
+
  2250 I = I + 1
  2260  IF I < 14 GOTO 2200
  2270  GOTO 2400
+
+REM A match was found. Show dashes between the two characters.
+
  2300  HTAB I
  2310  VTAB 18
  2314  PRINT "              ";
@@ -100,8 +153,16 @@ REM Written in Applesoft BASIC
  2320  FOR J = I TO M
  2330  PRINT "-";
  2340  NEXT J
+
+REM M becomes the number of characters to skip/shift in the input string.
+REM E.g. if indices 1 and 3 matched, they will continue to be in the 14-char
+REM window for the next 11 iterations. Skip those.
+
  2345 M = 14 - M
  2350  GOTO 2100
+
+REM Yay, we did it!
+
  2400  HTAB 1
  2410  VTAB 23
  2420  PRINT "MESSAGE FOUND AT INDEX ";R"."