Filesystem: Add a cache for finding file info by disc offset

FileMonitor calls this every time a read happens, and there's no code that
only calls this a small number of times, so having a cache is worthwhile.

Source/Core/DiscIO/FileSystemGCWii.cpp

@@ -6,6 +6,7 @@
 #include <cinttypes>
 #include <cstddef>
 #include <cstring>
+#include <map>
 #include <memory>
 #include <optional>
 #include <string>
@@ -277,26 +278,27 @@ std::unique_ptr<FileInfo> FileSystemGCWii::FindFileInfo(u64 disc_offset) const
   if (!IsValid())
     return nullptr;
 
-  return FindFileInfo(disc_offset, m_root);
+  // Build a cache (unless there already is one)
+  if (m_offset_file_info_cache.empty())
+  {
+    u32 fst_entries = m_root.GetSize();
+    for (u32 i = 0; i < fst_entries; i++)
+    {
+      FileInfoGCWii file_info(m_root, i);
+      if (!file_info.IsDirectory())
+        m_offset_file_info_cache.emplace(file_info.GetOffset() + file_info.GetSize(), i);
+    }
   }
 
-std::unique_ptr<FileInfo> FileSystemGCWii::FindFileInfo(u64 disc_offset,
+  // Get the first file that ends after disc_offset
-                                                        const FileInfo& file_info) const
+  const auto it = m_offset_file_info_cache.upper_bound(disc_offset);
-{
+  if (it == m_offset_file_info_cache.end())
-  for (const FileInfo& child : file_info)
+    return nullptr;
-  {
+  std::unique_ptr<FileInfo> result(std::make_unique<FileInfoGCWii>(m_root, it->second));
-    if (child.IsDirectory())
+
-    {
+  // If the file's start isn't after disc_offset, success
-      std::unique_ptr<FileInfo> result = FindFileInfo(disc_offset, child);
+  if (result->GetOffset() <= disc_offset)
-      if (result)
     return result;
-    }
-    else if ((file_info.GetOffset() <= disc_offset) &&
-             ((file_info.GetOffset() + file_info.GetSize()) > disc_offset))
-    {
-      return file_info.clone();
-    }
-  }
 
   return nullptr;
 }

Source/Core/DiscIO/FileSystemGCWii.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <cstddef>
+#include <map>
 #include <memory>
 #include <optional>
 #include <string>
@@ -101,9 +102,10 @@ private:
   u32 m_offset_shift;
   std::vector<u8> m_file_system_table;
   FileInfoGCWii m_root;
+  // Maps the end offset of files to FST indexes
+  mutable std::map<u64, u32> m_offset_file_info_cache;
 
   std::unique_ptr<FileInfo> FindFileInfo(const std::string& path, const FileInfo& file_info) const;
-  std::unique_ptr<FileInfo> FindFileInfo(u64 disc_offset, const FileInfo& file_info) const;
 };
 
 }  // namespace