remove mutability from mapper read funcs

src/cartridge/cnrom.rs

@@ -15,7 +15,7 @@ impl Cnrom {
 }
 
 impl Mapper for Cnrom {
-    fn read(&mut self, address: usize) -> u8 {
+    fn read(&self, address: usize) -> u8 {
         let pl = self.cart.prg_rom.len();
         let addr = address % 0x4000;
         match address {

src/cartridge/mmc1.rs

@@ -111,7 +111,7 @@ impl Mmc1 {
 }
 
 impl Mapper for Mmc1 {
-    fn read(&mut self, address: usize) -> u8 {
+    fn read(&self, address: usize) -> u8 {
         match address {
             0x0000..=0x1FFF => {
                 if self.cart.chr_rom_size == 0 {

src/cartridge/mmc3.rs

@@ -67,7 +67,7 @@ impl Mmc3 {
 }
 
 impl Mapper for Mmc3 {
-    fn read(&mut self, address: usize) -> u8 {
+    fn read(&self, address: usize) -> u8 {
         let val = match address {
             0x0000..=0x1FFF => { // reading from CHR-ROM
                 let offset_1k = address % 0x400;

src/cartridge/mod.rs

@@ -15,7 +15,7 @@ use std::rc::Rc;
 use std::io::Read;
 
 pub trait Mapper {
-    fn read(&mut self, address: usize) -> u8;
+    fn read(&self, address: usize) -> u8;
     fn write(&mut self, address: usize, value: u8);
     fn get_mirroring(&mut self) -> Mirror;
     fn load_battery_backed_ram(&mut self);

src/cartridge/nrom.rs

@@ -15,7 +15,7 @@ impl Nrom {
 }
 
 impl Mapper for Nrom {
-    fn read(&mut self, address: usize) -> u8 {
+    fn read(&self, address: usize) -> u8 {
         let addr = address % 0x4000;
         match address {
             0x0000..=0x1FFF => {

src/cartridge/uxrom.rs

@@ -17,7 +17,7 @@ impl Uxrom {
 }
 
 impl Mapper for Uxrom {
-    fn read(&mut self, address: usize) -> u8 {
+    fn read(&self, address: usize) -> u8 {
         match address {
             0x0000..=0x1FFF => {
                 if self.cart.chr_rom_size > 0 {

src/cpu/mod.rs

@@ -189,7 +189,7 @@ impl Cpu {
             0x4016          => self.read_controller(),
             0x4000..=0x4017 => 0, // can't read from these APU registers
             0x4018..=0x401F => 0, // APU and I/O functionality that is normally disabled. See CPU Test Mode.
-            0x4020..=0xFFFF => self.mapper.borrow_mut().read(address),
+            0x4020..=0xFFFF => self.mapper.borrow().read(address),
             _ => panic!("invalid read from 0x{:02x}", address),
         };
         val

src/ppu/memory.rs

@@ -4,7 +4,7 @@ impl super::Ppu {
 
     pub fn read(&mut self, address: usize) -> u8 {
         match address {
-            0x0000..=0x1FFF => self.mapper.borrow_mut().read(address),
+            0x0000..=0x1FFF => self.mapper.borrow().read(address),
             0x2000..=0x3EFF => self.read_nametable(address),
             0x3F00..=0x3FFF => {
                 let a = address % 0x0020;