chore: Arith placeholder.

include/arith.h

@@ -0,0 +1,6 @@
+extern int Arith_max(int x, int y);
+extern int Arith_min(int x, int y);
+extern int Arith_div(int x, int y);
+extern int Arith_mod(int x, int y);
+extern int Arith_ceiling(int x, int y);
+extern int Arith_floor  (int x, int y);

src/arith.c

@@ -0,0 +1,30 @@
+#include "arith.h"
+
+int Arith_max(int x, int y) {
+  return x > y ? x : y;
+}
+
+int Arith_min(int x, int y) {
+  return x > y ? y : x;
+}
+
+int Arith_div(int x, int y) {
+  if (/* division truncates toward 0 */ -13/5 == -2
+  && /* x and y have different signs */ (x < 0) != (y < 0)
+  && x%y != 0)
+    return x/y - 1;
+  else
+    return x/y;
+}
+
+int Arith_mod(int x, int y) {
+  return x - y*Arith_div(x, y);
+}
+
+int Arith_floor(int x, int y) {
+  return Arith_div(x, y);
+}
+
+int Arith_ceiling(int x, int y) {
+  return Arith_div(x, y) + (x%y != 0);
+}