Clarify box_wrap

src/periodic.h

@@ -28,15 +28,15 @@
 /**
  * @brief Limits the value of x to be between a and b
  *
- * Only wraps once. If x > 2b, the returned value will be larger than b.
- * Similarly for x < -b.
+ * Only wraps once. If x > a + 2(b - a), the returned value will be larger than
+ * b. Similarly for x < a - (b - a), the value will be smaller than a.
  */
-#define box_wrap(x, a, b)                                \
-  ({                                                     \
-    const __typeof__(x) _x = (x);                        \
-    const __typeof__(a) _a = (a);                        \
-    const __typeof__(b) _b = (b);                        \
-    _x < _a ? (_x + _b) : ((_x >= _b) ? (_x - _b) : _x); \
+#define box_wrap(x, a, b)                                              \
+  ({                                                                   \
+    const __typeof__(x) _x = (x);                                      \
+    const __typeof__(a) _a = (a);                                      \
+    const __typeof__(b) _b = (b);                                      \
+    _x < _a ? (_x + (_b - _a)) : ((_x >= _b) ? (_x - (_b - _a)) : _x); \
   })
 
 /**