Update spec.from_string() and spec.from_file() to match the respective mjModel functions.

PiperOrigin-RevId: 678230403
Change-Id: I7f3d8c5fd44d30b5695eee1f8513cc27f4158d94

python/mujoco/structs.h

@@ -15,7 +15,9 @@
 #ifndef MUJOCO_PYTHON_STRUCTS_H_
 #define MUJOCO_PYTHON_STRUCTS_H_
 
+#include <algorithm>
 #include <array>
+#include <cctype>
 #include <cstddef>
 #include <functional>
 #include <istream>
@@ -39,9 +41,51 @@
 #include <pybind11/pybind11.h>
 #include <pybind11/pytypes.h>
 
+namespace py = ::pybind11;
+
 namespace mujoco::python {
 namespace _impl {
 
+struct VfsAsset {
+  VfsAsset(const char* name, const void* content, std::size_t content_size)
+      : name(name), content(content), content_size(content_size) {}
+  const char* name;
+  const void* content;
+  std::size_t content_size;
+};
+
+// strip path prefix from filename and make lowercase
+inline std::string StripPath(const char* name) {
+  std::string filename(name);
+  size_t start = filename.find_last_of("/\\");
+
+  // get name without path
+  if (start != std::string::npos) {
+    filename = filename.substr(start + 1, filename.size() - start - 1);
+  }
+
+  // make lowercase
+  std::transform(filename.begin(), filename.end(), filename.begin(),
+                 [](unsigned char c) { return std::tolower(c); });
+  return filename;
+}
+
+
+// Converts a dict with py::bytes value to a vector of standard C++ types.
+// This allows us to release the GIL early. Note that the vector consists only
+// of pointers to existing data so no substantial data copies are being made.
+inline std::vector<VfsAsset> ConvertAssetsDict(
+    const std::optional<std::unordered_map<std::string, py::bytes>>& assets) {
+  std::vector<VfsAsset> out;
+  if (assets.has_value()) {
+    for (const auto& [name, content] : *assets) {
+      out.emplace_back(name.c_str(), PYBIND11_BYTES_AS_STRING(content.ptr()),
+                       py::len(content));
+    }
+  }
+  return out;
+}
+
 template <typename T>
 class WrapperBase {
  public: