SimpleCache: Scan sub-directories during initialization

This is the initialization part of mitigating issue #4253. The
remaining work is on the writing side, and is simply a case of
having startFile return File instances that are sharded into
sub-directories. We still need to decide what scheme we want
to use for doing that.

Issue: #4253
PiperOrigin-RevId: 228306327

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -382,30 +382,46 @@ public final class SimpleCache implements Cache {
     }
 
     index.load();
+    loadDirectory(cacheDir, /* isRootDirectory= */ true);
+    index.removeEmpty();
 
-    File[] files = cacheDir.listFiles();
+    try {
+      index.store();
+    } catch (CacheException e) {
+      Log.e(TAG, "Storing index file failed", e);
+    }
+  }
+
+  private void loadDirectory(File directory, boolean isRootDirectory) {
+    File[] files = directory.listFiles();
     if (files == null) {
+      // Not a directory.
+      return;
+    }
+    if (!isRootDirectory && files.length == 0) {
+      // Empty non-root directory.
+      directory.delete();
       return;
     }
     for (File file : files) {
-      if (file.getName().equals(CachedContentIndex.FILE_NAME)) {
-        continue;
-      }
-      SimpleCacheSpan span =
-          file.length() > 0 ? SimpleCacheSpan.createCacheEntry(file, file.length(), index) : null;
-      if (span != null) {
-        addSpan(span);
+      String fileName = file.getName();
+      if (fileName.indexOf('.') == -1) {
+        loadDirectory(file, /* isRootDirectory= */ false);
       } else {
-        file.delete();
+        if (isRootDirectory && CachedContentIndex.FILE_NAME.equals(fileName)) {
+          // Skip the (expected) index file in the root directory.
+          continue;
+        }
+        long fileLength = file.length();
+        SimpleCacheSpan span =
+            fileLength > 0 ? SimpleCacheSpan.createCacheEntry(file, fileLength, index) : null;
+        if (span != null) {
+          addSpan(span);
+        } else {
+          file.delete();
+        }
       }
     }
-
-    index.removeEmpty();
-    try {
-      index.store();
-    } catch (CacheException e) {
-      Log.e(TAG, "Storing index file failed", e);
-    }
   }
 
   /**