Dynamically allocate contact and efc_ arrays on a new memory arena.

- Add private function `mj_arenaAlloc`. This is used internally to allocate memory from the arena.

- Add private function `mj_nefc` to count constraints. This function returns a tight upper bound on `d->nefc`. The number of counted constraints can be slightly bigger than exact `d->nefc` in the case of constraints with empty Jacobian, as when placing a frictional tendon between two world sites.

- Add new `memory` attribute to the `size` XML element for specification of arena memory size. This attribute is mutually exclusive with `nstack` and `njmax` specifications, which are now deprecated (but left around for the time being for legacy compatibility).

- Move `d->stack` to the end of the new arena space. The stack now grows in reverse from the end.

PiperOrigin-RevId: 479341539
Change-Id: Ie019c202e0908577ffc6f833a37920858116f667

src/xml/xml_native_writer.cc

@@ -19,6 +19,8 @@
 #include <cstdio>
 #include <string>
 #include <unordered_set>
+#include <utility>
+#include <vector>
 
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjplugin.h>
@@ -818,6 +820,32 @@ void mjXWriter::Option(XMLElement* root) {
 void mjXWriter::Size(XMLElement* root) {
   XMLElement* section = InsertEnd(root, "size");
 
+  // write memory
+  if (model->memory != -1) {
+    const std::size_t memory = static_cast<std::size_t>(model->memory);
+    const std::vector<std::pair<int, char>> kSuffix = {
+      {60, 'E'}, {50, 'P'}, {40, 'T'}, {30, 'G'}, {20, 'M'}, {10, 'K'},
+    };
+
+    std::ostringstream strm;
+
+    // check for divisibility by each suffixed size
+    for (const auto& [multiplier_bit, suffix_char] : kSuffix) {
+      const std::size_t multiplier = static_cast<std::size_t>(1) << multiplier_bit;
+      if (memory >= multiplier && !(memory & (multiplier - 1))) {
+        strm << (memory >> multiplier_bit) << suffix_char;
+        break;
+      }
+    }
+
+    // doesn't match any suffix, just write the number out as-is
+    if (!strm.tellp()) {
+      strm << memory;
+    }
+
+    WriteAttrTxt(section, "memory", strm.str());
+  }
+
   // write sizes
   WriteAttrInt(section, "njmax", model->njmax, -1);
   WriteAttrInt(section, "nconmax", model->nconmax, -1);