xo-alloc2: work on fomo Arena

include/xo/alloc2/padding.hpp

@@ -0,0 +1,57 @@
+/** @file padding.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include <memory>
+#include <cstdint>
+
+namespace xo {
+    namespace mm {
+
+        struct padding {
+            /** word size for alignment**/
+            static constexpr std::size_t c_alloc_alignment = sizeof(std::uintptr_t);
+
+            /** how much to add to @p z to get a multiple of
+             *  @ref c_alloc_alignment
+             **/
+            static inline std::size_t alloc_padding(std::size_t z,
+                                                    std::size_t align = c_alloc_alignment)
+                {
+
+                    /* round up to multiple of c_bpw, but map 0 -> 0
+                     * (table assuming c_bpw==8)
+                     *
+                     *   z%c_bpw   dz
+                     *   ------------
+                     *         0    0
+                     *         1    7
+                     *         2    6
+                     *        ..   ..
+                     *         7    1
+                     */
+                    std::size_t dz = (align - (z % align)) % align;
+
+                    z += dz;
+
+                    return dz;
+                }
+
+            /** @p z rounded up to an exact multiple
+             *  of @ref c_alloc_alignment
+             **/
+            static inline
+            std::size_t with_padding(std::size_t z,
+                                     std::size_t align)
+                {
+                    return z + alloc_padding(z, align);
+                }
+        };
+
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end padding.hpp */