From 6db132ad48a9ecf3193e71cb6ced383be672f0cc Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Mon, 5 Apr 2021 01:43:26 -0500
Subject: [PATCH] feat(cartridge): handle MBC-less games like tetris

TODO: This will panic whenever a write is attempted on the MBC-less
cartridge. This would be fine if games held the implicit rules given by
the hardware, however this is not the case. We will have to modify the
NoMBC implementation to ignore writes to the cartrige (while still
documenting them just in case of bugs)
---
 src/cartridge.rs | 44 +++++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/src/cartridge.rs b/src/cartridge.rs
index 29a63ed..2138348 100644
--- a/src/cartridge.rs
+++ b/src/cartridge.rs
@@ -30,18 +30,20 @@ impl Cartridge {
         let mbc_kind = Self::find_mbc(&memory);
         let ram_byte_count = ram_size.to_byte_count();
 
-        let mbc = match mbc_kind {
-            MbcKind::None => todo!("Handle no MBC Situation"),
-            MbcKind::MBC1 => Mbc1 {
-                ram_size,
-                ram: vec![0; ram_byte_count as usize],
-                bank_count,
-                ..Default::default()
-            },
-            MbcKind::MBC5 => todo!("Implement MBC5"),
-        };
+        match mbc_kind {
+            MbcKind::None => Box::new(NoMbc {}),
+            MbcKind::Mbc1 => {
+                let mbc = Mbc1 {
+                    ram_size,
+                    ram: vec![0; ram_byte_count as usize],
+                    bank_count,
+                    ..Default::default()
+                };
 
-        Box::new(mbc)
+                Box::new(mbc)
+            }
+            MbcKind::Mbc5 => todo!("Implement MBC5"),
+        }
     }
 
     fn find_ram_size(memory: &[u8]) -> RamSize {
@@ -60,8 +62,8 @@ impl Cartridge {
         // TODO: Refactor this to match the other enums in this module
         match id {
             0x00 => MbcKind::None,
-            0x01 => MbcKind::MBC1,
-            0x19 => MbcKind::MBC5,
+            0x01 => MbcKind::Mbc1,
+            0x19 => MbcKind::Mbc5,
             _ => unimplemented!("{} is the id of an unsupported memory bank controller", id),
         }
     }
@@ -211,6 +213,18 @@ impl MemoryBankController for Mbc1 {
         }
     }
 }
+#[derive(Debug, Clone, Copy)]
+struct NoMbc {}
+
+impl MemoryBankController for NoMbc {
+    fn handle_read(&self, addr: u16) -> MbcResult {
+        MbcResult::Address(addr)
+    }
+
+    fn handle_write(&mut self, _addr: u16, _byte: u8) {
+        panic!("A MBC-less cartridge is read only")
+    }
+}
 
 trait MemoryBankController: CloneMbc {
     fn handle_read(&self, addr: u16) -> MbcResult;
@@ -238,8 +252,8 @@ enum MbcResult {
 #[derive(Debug, Clone, Copy)]
 enum MbcKind {
     None,
-    MBC1,
-    MBC5,
+    Mbc1,
+    Mbc5,
 }
 
 impl Default for MbcKind {