take the checksum from before the closing checkpoint

LogicGameMode::encode reads its return value out of getCheckSum() and
only then writes it:

  v15 = ChecksumEncoder::getCheckSum(a2);
  (...vptr+88)(a2, v15);        // the checkpoint vint
  return v15;

we were reading ours after that write, so the closing checkpoint was
folded into the number we compared. the two could never match, whatever
the simulation did - which is why tick 41 disagreed with six untouched
towers on the field.

write() now hands back the value it wrote, the way encode() does.

crates/logic/src/battle/logic_game_mode.rs

@@ -23,7 +23,7 @@ impl LogicGameMode {
     }
 }
 impl LogicGameMode {
-    pub fn write(&self, writer: &mut ByteStreamWriter, with_commands: bool) -> Result<()> {
+    pub fn write(&self, writer: &mut ByteStreamWriter, with_commands: bool) -> Result<i32> {
         writer.write_vint(self.time.tick);
         writer.write_checksum_checkpoint();
         writer.write_vint(SECTION_BATTLE);
@@ -36,21 +36,21 @@ impl LogicGameMode {
         }
         writer.write_vint(SECTION_TUTORIAL);
         self.tutorial_manager.encode(writer)?;
-        writer.write_checksum_checkpoint();
+        let checksum = writer.write_checksum_checkpoint();
         if with_commands {
             writer.write_vint(0);
         }
-        Ok(())
+        Ok(checksum)
     }
     pub fn calculate_checksum(&self) -> Result<i32> {
         let mut writer = ByteStreamWriter::new();
-        self.write(&mut writer, false)?;
-        Ok(writer.checksum() as i32)
+        self.write(&mut writer, false)
     }
 }
 impl Payload for LogicGameMode {
     fn encode(&self, writer: &mut ByteStreamWriter) -> Result<()> {
-        self.write(writer, true)
+        self.write(writer, true)?;
+        Ok(())
     }
     fn decode(_reader: &mut titan::ByteStreamReader<'_>) -> Result<Self> {
         Err(titan::Error::Unsupported("LogicGameMode is encode only"))

crates/titan/src/io/writer.rs

@@ -33,9 +33,10 @@ impl ByteStreamWriter {
     pub fn checksum(&self) -> u32 {
         self.checksum.value()
     }
-    pub fn write_checksum_checkpoint(&mut self) {
+    pub fn write_checksum_checkpoint(&mut self) -> i32 {
         let running = self.checksum.value() as i32;
         self.write_vint(running);
+        running
     }
     pub fn write_byte(&mut self, value: u8) {
         self.checksum.write_byte(value as i8);