irc: strip mIRC formatting, drop 004/005 noise, fix a RAM-bank-swap landmine

Against the real irc.sl0p.foo (via a TLS-stripping socat proxy) the
client's screen filled with garbage after connect. Two causes:

1. Color/format codes. The MOTD is a mIRC-color ASCII-art logo. We
   dropped the \x03 control byte (term ignores <32) but left its numeric
   *arguments*, so the logo rendered as digit soup ('09> stream the
   work', '030903 030903 ...'). strip_fmt() now removes \x03 color (+its
   fg[,bg] digits), \x04 hex color, and the \x02/\x0f/\x11/\x16/\x1d/
   \x1e/\x1f toggles, applied to the trailing text of every line (leaving
   \x01 for CTCP). Standalone digits like '3 apples' are untouched.

2. The real bug: handle() defaulted pfx/txt to a read-only "" literal,
   and both bang(pfx) and strip_fmt(txt) write a NUL terminator into it.
   That literal lives in ROM (000-FFF); on MBC5 a write there is a
   RAM-bank-select, silently swapping the  cart-RAM bank - where
   every static lives - out from under the program. One prefix-less or
   empty-trailing server line (real ircds send them: NOTICE AUTH, ERROR,
   registration PING) and all state turns to garbage. Fixed with a
   writable 1-byte 'empty' default (zeroed in main; gbos doesn't clear
   BSS).

Also suppress 004 (MYINFO) and 005 (ISUPPORT) - pure noise on 40 cols,
and InspIRCd splits ISUPPORT across several lines.

Verified end to end against the live server: full InspIRCd MOTD (logo,
LUSERS, links) renders clean and stable; host unit tests cover color/
bold/CTCP-action stripping and prefix-less NOTICE/ERROR/PING.

1 files changed, 42 insertions, 1 deletions

diff --git a/c/irc.c b/c/irc.c
index 817471e..551dca1 100644
--- a/c/irc.c
+++ b/c/irc.c
@@ -30,6 +30,12 @@ static unsigned char namebuf[64];
 static unsigned char dst[4];
 static unsigned char sock;
 static unsigned char quitting;
+/* A WRITABLE empty string for parse defaults. handle() writes terminators
+   into pfx (bang) and txt (strip_fmt); a read-only "" literal lives in ROM
+   ($4000-$5FFF), where a write is an MBC5 *RAM-bank-select* - it would swap
+   the whole $A000 data bank out from under every static (instant garbage).
+   A prefix-less or empty-trailing server line is enough to trigger it. */
+static char empty[1];
 
 /* ---- tiny string helpers (SDCC libc is us) ------------------------------- */
 static unsigned char sceq(const char *a, const char *b) {   /* case-insens. */
@@ -107,9 +113,41 @@ static void bang(char *p) {
 static unsigned char isnum(const char *c) {
     return c[0] >= '0' && c[0] <= '9';
 }
+static unsigned char ishex(char c) {
+    return (c>='0'&&c<='9')||(c>='a'&&c<='f')||(c>='A'&&c<='F');
+}
+
+/* Strip mIRC/IRC formatting in place - crucially their *arguments*, or a
+   color logo (a real MOTD!) leaves its \x03 color numbers on screen as digit
+   soup. \x03 = color (up to 2 digits[,up to 2 digits]); \x04 = hex color
+   (up to 6 hex[,6 hex]); \x02 bold \x0f reset \x11 mono \x16 reverse
+   \x1d italic \x1e strike \x1f underline are lone toggles. Leaves \x01
+   (CTCP) intact for the PRIVMSG path. */
+static void strip_fmt(char *s) {
+    char *w = s, *r = s;
+    unsigned char c, d;
+    while (*r) {
+        c = (unsigned char)*r;
+        if (c == 3) {
+            r++; d = 0;
+            while (*r>='0'&&*r<='9'&&d<2) { r++; d++; }
+            if (*r==','&&r[1]>='0'&&r[1]<='9') { r++; d=0; while(*r>='0'&&*r<='9'&&d<2){r++;d++;} }
+            continue;
+        }
+        if (c == 4) {
+            r++; d = 0;
+            while (ishex(*r)&&d<6) { r++; d++; }
+            if (*r==','&&ishex(r[1])) { r++; d=0; while(ishex(*r)&&d<6){r++;d++;} }
+            continue;
+        }
+        if (c==2||c==15||c==17||c==22||c==29||c==30||c==31) { r++; continue; }
+        *w++ = *r++;
+    }
+    *w = 0;
+}
 
 static void handle(char *l) {
-    char *pfx = (char *)"", *cmd, *arg, *txt = (char *)"";
+    char *pfx = empty, *cmd, *arg, *txt = empty;
     char *p = l;
 
     if (*p == ':') {                       /* :prefix */
@@ -127,6 +165,7 @@ static void handle(char *l) {
         p++;
     }
     /* arg = first param (NUL-joined list), txt = trailing */
+    strip_fmt(txt);                        /* kill color/format codes + args */
 
     if (sceq(cmd, "PING")) {
         slen = 0; sc("PONG :"); sc(txt[0] ? txt : arg); sfin();
@@ -210,6 +249,7 @@ static void handle(char *l) {
             return;
         }
         if (cmd[1]=='3'&&cmd[0]=='3') return;          /* 333 topic-by: skip */
+        if (cmd[0]=='0'&&cmd[1]=='0'&&(cmd[2]=='4'||cmd[2]=='5')) return; /* MYINFO/ISUPPORT noise */
         if (txt[0]) say2("-!- ", txt);                 /* motd & friends */
         return;
     }
@@ -315,6 +355,7 @@ void main(void) {
        parent's RAM bank) - initialize every piece of state explicitly */
     llen = inlen = drawn = slen = quitting = 0;
     target[0] = 0;
+    empty[0] = 0;                          /* gbos doesn't zero BSS */
 
     argp = getargs();                      /* copy args before seg is used */
     for (i = 0; argp[i] && i < 63; i++) namebuf[i] = (unsigned char)argp[i];