n64: fix dd rtc skipping december + validity checks to avoid glitchy …

…rtc (#1290)

- add more details to the dd time.rtc format as comments for readability
- fix dd rtc skipping december entirely (wrong comparison)
  - I feel really dumb for this one.
- add validity checks on every rtc second tick to avoid glitchy rtc time
& date
- This also avoids the user to be able to skip the time & date error
into a glitchy rtc by simply restarting.
- add validity check on load if the time is valid or not, if not valid,
destroy the rtc info to get the user to input new rtc info

ares/n64/dd/dd.hpp

@@ -37,6 +37,7 @@ struct DD : Memory::PI<DD> {
     auto tick(u32 offset) -> void;
     auto tickClock() -> void;
     auto tickSecond() -> void;
+    auto valid() -> bool;
   } rtc;
 
   auto title() const -> string { return information.title; }

ares/n64/dd/rtc.cpp

@@ -4,14 +4,23 @@ auto DD::RTC::load() -> void {
     ram.load(fp);
   }
 
+  //byte 0 to 7 = raw rtc time (last updated, only 6 bytes are used)
   n64 check = 0;
   for(auto n : range(8)) check.byte(n) = ram.read<Byte>(n);
   if(!~check) return;  //new save file
 
+  //check for invalid time info, if invalid, set time info to something invalid and ignore the rest
+  if (!valid()) {
+    for(auto n : range(8)) ram.write<Byte>(n, 0xff);
+    return;
+  }
+
+  //byte 8 to 15 = timestamp of when the last save was made
   n64 timestamp = 0;
   for(auto n : range(8)) timestamp.byte(n) = ram.read<Byte>(8 + n);
   if(!~timestamp) return;  //new save file
 
+  //update based on the amount of time that has passed since the last save
   timestamp = time(0) - timestamp;
   while(timestamp--) tickSecond();
 }
@@ -47,6 +56,8 @@ auto DD::RTC::tickClock() -> void {
 }
 
 auto DD::RTC::tickSecond() -> void {
+  if (!valid()) return;
+
   //second
   tick(5);
   if(ram.read<Byte>(5) < 0x60) return;
@@ -72,9 +83,36 @@ auto DD::RTC::tickSecond() -> void {
 
   //month
   tick(1);
-  if(ram.read<Byte>(1) < 0x12) return;
+  if(ram.read<Byte>(1) <= 0x12) return;
   ram.write<Byte>(1, 1);
 
   //year
   tick(0);
 }
+
+auto DD::RTC::valid() -> bool {
+  //check validity of ram rtc data (if it's BCD valid or not)
+  for(auto n : range(6)) {
+    if ((ram.read<Byte>(n) & 0x0f) >= 0x0a) return false;
+    if (ram.read<Byte>(n) >= 0xa0) return false;
+  }
+
+  //check for valid values of each byte
+  //second
+  if(ram.read<Byte>(5) >= 0x60) return false;
+  //minute
+  if(ram.read<Byte>(4) >= 0x60) return false;
+  //hour
+  if(ram.read<Byte>(3) >= 0x24) return false;
+  //month
+  if(ram.read<Byte>(1) > 0x12) return false;
+  if(ram.read<Byte>(1) < 1) return false;
+  //day
+  if(ram.read<Byte>(2) < 1) return false;
+  u32 daysInMonth[12] = {0x31, 0x28, 0x31, 0x30, 0x31, 0x30, 0x31, 0x31, 0x30, 0x31, 0x30, 0x31};
+  if(ram.read<Byte>(0) && !(BCD::decode(ram.read<Byte>(0)) % 4)) daysInMonth[1]++;
+  if(ram.read<Byte>(2) > daysInMonth[BCD::decode(ram.read<Byte>(1))-1]) return false;
+
+  //everything is valid
+  return true;
+}