fix: prevent integer underflows in Land::derelativize

src/state.rs

@@ -16,8 +16,28 @@ impl Land {
         (x % self.width, y % self.height)
     }
 
-    pub fn derelativize(&self, tree: &Tree, relative_x: u16, relative_y: u16) -> (u16, u16) {
-        ((tree.position.0 + relative_x) % self.width, (tree.position.1 + relative_y) % self.height)
+    pub fn derelativize(&self, position: (u16, u16), relative_x: i16, relative_y: i16) -> (u16, u16) {
+        // Note: we don't need to worry about integer overflows directly
+        // because land itself cannot exceed integer limits in size and
+        // so that check's implicitly handled with the modulo operator.
+        // Underflows are a bit trickier to catch, though
+        let absolute_x = if relative_x < 0 && position.0 - relative_x.unsigned_abs() > position.0 {
+            (self.width - (relative_x.unsigned_abs() - position.0)) % self.width
+        } else if relative_x < 0 {
+            (position.0 - relative_x.unsigned_abs()) % self.height
+        } else {
+            (position.0 + relative_x.unsigned_abs()) % self.height
+        };
+
+        let absolute_y = if relative_y < 0 && position.1 - relative_y.unsigned_abs() > position.1 {
+            (self.height - (relative_y.unsigned_abs() - position.1)) % self.height
+        } else if relative_x < 0 {
+            (position.1 - relative_y.unsigned_abs()) % self.height
+        } else {
+            (position.1 + relative_y.unsigned_abs()) % self.height
+        };
+
+        return (absolute_x, absolute_y);
     }
 
     pub fn sow(&mut self, x: u16, y: u16, value: u16) {