added tests and all pass with this implementation

src/cartridge.rs

@@ -1,6 +1,7 @@
-
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum Mirroring { Horizontal, Vertical, FourScreen }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Cartridge {
     prg_rom: Vec<u8>,
     chr_rom: Vec<u8>,
@@ -14,6 +15,10 @@ pub struct Cartridge {
 impl Cartridge {
     pub fn from_file(path: &str) -> Result<Cartridge, String> {
         let data: Vec<u8> = std::fs::read(path).map_err(|e| format!("couldn't read {path}: {e}"))?;
+        Self::parse_data(data)
+    }
+
+    pub fn parse_data(data: Vec<u8>) -> Result<Cartridge,String> {
         if data.len() < 16 {
             return Err("file is too short to be an iNES ROM".to_string())
         }
@@ -83,3 +88,73 @@ impl Cartridge {
         self.chr_rom[address as usize] = value
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn load(name: &str) -> Cartridge {
+        let path = format!("{}/roms/{}", env!("CARGO_MANIFEST_DIR"), name);
+        Cartridge::from_file(&path).expect("test ROM should load")
+    }
+
+    // Header parsing requires real test ROMs or actual game backups
+    #[test]
+    fn smb_header() {
+        let cart = load("backup/Super Mario Bros. (Japan, USA).nes");
+        assert_eq!(cart.prg_rom.len(), 32 * 1024);
+        assert_eq!(cart.chr_rom.len(), 8 * 1024);
+        assert_eq!(cart.mapper, 0);
+        assert_eq!(cart.mirroring, Mirroring::Vertical);
+    }
+
+    #[test]
+    fn nestest_header() {
+        let cart = load("test/cpu/nestest/nestest.nes");
+        assert_eq!(cart.prg_rom.len(), 16 * 1024);
+        assert_eq!(cart.mirroring, Mirroring::Horizontal);
+    }
+
+    // PRG ROM mappings
+    #[test]
+    fn nestest_prg_mirrors() {
+        let cart = load("test/cpu/nestest/nestest.nes"); // 16KiB - Mirrored
+        assert_eq!(cart.read_prg(0x8000), cart.read_prg(0xC000));
+    }
+
+    #[test]
+    fn smb_prg_no_mirror() {
+        let cart = load("backup/Super Mario Bros. (Japan, USA).nes");
+        assert_eq!(cart.read_prg(0x8000), cart.prg_rom[0]);
+        assert_eq!(cart.read_prg(0xC000), cart.prg_rom[0x4000]);
+    }
+
+    // Errors via parse_data
+    #[test]
+    fn rejects_bad_magic() {
+        assert!(Cartridge::parse_data(vec![0x00; 16]).is_err());
+    }
+
+    #[test]
+    fn rejects_short_file() {
+        assert!(Cartridge::parse_data(vec![b'N', b'E', b'S', 0x1A]).is_err());
+    }
+
+    #[test]
+    fn rejects_mmc1() {
+        let mut hdr = vec![0u8; 16];
+        hdr[..4].copy_from_slice(b"NES\x1a");
+        hdr[4] = 0x10; // 16 PRG banks
+        hdr[6] = 0x10; // mapper low nibble = 1
+        let err = Cartridge::parse_data(hdr).unwrap_err();
+        assert!(err.contains("mapper 1"), "got: {err}");
+    }
+
+    // CHR-RAM roundtrip (coredump has 0 CHR banks we make CHR-RAM)
+    #[test]
+    fn chr_ram_roundtrip() {
+        let mut cart = load("test/cpu/coredump-v1.3.nes");
+        cart.write_chr(0x1234, 0xAB);
+        assert_eq!(cart.read_chr(0x1234), 0xAB);
+    }
+}