unprotbas: fixline() commentary.

1 files changed, 9 insertions, 2 deletions

diff --git a/unprotbas.c b/unprotbas.c
index c59a33c..c458e9f 100644
--- a/unprotbas.c
+++ b/unprotbas.c
@@ -34,6 +34,9 @@
 	them anyway. */
 #define MAXVARS 256
 
+/* tokenized colon (statement separator) */
+#define TOK_COLON 0x14
+
 /* entire file gets read into memory (for now) */
 unsigned char data[BUFSIZE];
 
@@ -153,13 +156,17 @@ void set_header_vars(void) {
 	2        09      line length (or, offset to next line) [!]
 	3        06      offset to next statement *from the start of the line*
 	4        28      token for "?"
-	5        14      token for : (end of statement)
+	5        14      token for : (end of statement), we call it TOK_COLON
 	6        09      offset to next statement [!]
 	7        15      token for END
 	8        16      token for end-of-line [*]
 	9        ??      (line number of next statement)
 
 	Note the values marked with [!] are equal.
+	The line length at offset 2 is what gets zeroed out by the
+	protection. To fix it, we follow the next-statement offsets. If
+	there's not a colon before the offset, replace the byte at
+	offset 2 with that statement's offset.
 
 	[*] end-of-line is $16 *except* for REM and DATA, which are
 	    terminated with $9B instead.
@@ -168,7 +175,7 @@ int fixline(int linepos) {
 	/* +3 here to skip the line number + line length */
 	int offset = data[linepos + 3];
 
-	while(data[linepos + offset - 1] == 0x14)
+	while(data[linepos + offset - 1] == TOK_COLON)
 		offset = data[linepos + offset];
 
 	data[linepos + 2] = offset;