Add missing math.rs

src/math.rs

@@ -0,0 +1,7 @@
+pub fn div_up(v: usize, divisor: usize) -> usize {
+    v / divisor + (v % divisor != 0) as usize
+}
+
+pub fn div_down(v: usize, divisor: usize) -> usize {
+    v / divisor
+}