Prevent overflow errors on stream device pointers

The stream device pointers now use wrapping semantics in order to
prevent overflow errors.

1 files changed, 19 insertions, 16 deletions

diff --git a/src/devices/scratch.rs b/src/devices/scratch.rs
index eea011b..950f87a 100644
--- a/src/devices/scratch.rs
+++ b/src/devices/scratch.rs
@@ -1,3 +1,10 @@
+macro_rules! then_inc {
+    ($v:expr) => {{
+        $v = $v.wrapping_add(1);
+        $v as usize
+    }};
+}
+
 pub struct ScratchDevice {
     memory: Vec<u8>,
     pub max_capacity: u32,
@@ -17,38 +24,34 @@ impl ScratchDevice {
     }
 
     pub fn read_head_1(&mut self) -> u8 {
-        let value = self.read_byte(self.pointer_1);
-        self.pointer_1 += 1;
-        return value;
+        let i = then_inc!(self.pointer_1);
+        self.read_byte(i)
     }
 
     pub fn read_head_2(&mut self) -> u8 {
-        let value = self.read_byte(self.pointer_2);
-        self.pointer_2 += 1;
-        return value;
+        let i = then_inc!(self.pointer_2);
+        self.read_byte(i)
     }
 
     pub fn write_head_1(&mut self, value: u8) {
-        self.write_byte(self.pointer_1, value);
-        self.pointer_1 += 1;
+        let i = then_inc!(self.pointer_1);
+        self.write_byte(i, value);
     }
 
     pub fn write_head_2(&mut self, value: u8) {
-        self.write_byte(self.pointer_2, value);
-        self.pointer_2 += 1;
+        let i = then_inc!(self.pointer_2);
+        self.write_byte(i, value);
     }
 
-    fn read_byte(&self, pointer: u32) -> u8 {
-        let pointer = pointer as usize;
-        match self.memory.get(pointer) {
+    fn read_byte(&self, pointer: usize) -> u8 {
+        match self.memory.get(pointer as usize) {
             Some(value) => *value,
             None => 0,
         }
     }
 
-    fn write_byte(&mut self, pointer: u32, value: u8) {
-        if pointer < self.max_capacity {
-            let pointer = pointer as usize;
+    fn write_byte(&mut self, pointer: usize, value: u8) {
+        if pointer < self.max_capacity as usize {
             if pointer >= self.memory.len() {
                 self.memory.resize(pointer + 1, 0);
             }