musikr: revamp fscovers

Make it use a scoring system and properly document it.

musikr/src/main/java/org/oxycblt/musikr/covers/fs/FSCovers.kt

@@ -24,6 +24,7 @@ import android.net.Uri
 import android.os.ParcelFileDescriptor
 import androidx.core.net.toUri
 import java.io.InputStream
+import kotlin.math.max
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import org.oxycblt.musikr.covers.Cover
@@ -36,7 +37,16 @@ import org.oxycblt.musikr.metadata.Metadata
 
 private const val PREFIX = "mcf:"
 
-open class FSCovers(private val context: Context) : Covers<FDCover> {
+/**
+ * A [Covers] implementation that obtains cover art from the filesystem, such as cover.jpg.
+ * Cover.jpg is pretty widely used in music libraries to save space, so it's good to use this.
+ *
+ * This implementation does not search the directory tree given that it cannot access it. Rather, it
+ * assumes the provided id ius one yielded by [MutableFSCovers].
+ *
+ * @param context The [Context] to use to access the filesystem and check for ID validity.
+ */
+class FSCovers(private val context: Context) : Covers<FDCover> {
     override suspend fun obtain(id: String): CoverResult<FDCover> {
         if (!id.startsWith(PREFIX)) {
             return CoverResult.Miss()
@@ -64,16 +74,36 @@ open class FSCovers(private val context: Context) : Covers<FDCover> {
     }
 }
 
-class MutableFSCovers(private val context: Context) : FSCovers(context), MutableCovers<FDCover> {
+/**
+ * A [MutableCovers] implementation that obtains cover art from the filesystem, such as cover.jpg.
+ * Cover.jpg is pretty widely used in music libraries to save space, so it's good to use this.
+ *
+ * This implementation will search the parent directory for the best cover art. "Best" being defined
+ * as having cover-art-ish names and having a good format like png/jpg/webp.
+ *
+ * @param context The [Context] to use to access the filesystem and check for ID validity.
+ */
+class MutableFSCovers(private val context: Context) : MutableCovers<FDCover> {
+    private val inner = FSCovers(context)
+
+    override suspend fun obtain(id: String): CoverResult<FDCover> = inner.obtain(id)
+
     override suspend fun create(file: DeviceFile, metadata: Metadata): CoverResult<FDCover> {
         // Since DeviceFiles is a streaming API, we have to wait for the current recursive
         // query to finally finish to be able to have a complete list of siblings to search for.
         val parent = file.parent.await()
-        val coverFile =
-            parent.children.firstNotNullOfOrNull { node ->
-                if (node is DeviceFile && isCoverArtFile(node)) node else null
-            } ?: return CoverResult.Miss()
-        return CoverResult.Hit(FolderCoverImpl(context, coverFile.uri))
+        val bestCover =
+            parent.children
+                .filterIsInstance<DeviceFile>()
+                .map { it to coverArtScore(file) }
+                .maxBy { it.second }
+        if (bestCover.second > 0) {
+            return CoverResult.Hit(FolderCoverImpl(context, bestCover.first.uri))
+        }
+        // No useful cover art was found.
+        // Well, technically we might have found a cover image, but it might be some unrelated
+        // jpeg from poor file organization.
+        return CoverResult.Miss()
     }
 
     override suspend fun cleanup(excluding: Collection<Cover>) {
@@ -81,35 +111,43 @@ class MutableFSCovers(private val context: Context) : FSCovers(context), Mutable
         // that should not be managed by the app
     }
 
-    private fun isCoverArtFile(file: DeviceFile): Boolean {
+    private fun coverArtScore(file: DeviceFile): Int {
         if (!file.mimeType.startsWith("image/", ignoreCase = true)) {
-            return false
+            // Not an image file. You lose!
+            return 0
         }
 
-        val filename = requireNotNull(file.path.name).lowercase()
-        val filenameWithoutExt = filename.substringBeforeLast(".")
-        val extension = filename.substringAfterLast(".", "")
-
-        return coverNames.any { coverName ->
-            filenameWithoutExt.equals(coverName, ignoreCase = true) &&
-                (extension.equals("jpg", ignoreCase = true) ||
-                    extension.equals("jpeg", ignoreCase = true) ||
-                    extension.equals("png", ignoreCase = true))
-        }
+        val filename = requireNotNull(file.path.name)
+        val name = filename.substringBeforeLast('.')
+        val extension = filename.substringAfterLast('.', "")
+        // See if the name contains any of the preferred cover names. This helps weed out
+        // images that are not actually cover art and are just there.
+        var score =
+            preferredCoverNames
+                .withIndex()
+                .filter { name.contains(it.value, ignoreCase = true) }
+                .sumOf { it.index + 1 }
+        // Multiply the score for preferred formats & extensions. Weirder formats are harder for
+        // android to decode, but not the end of the world.
+        score *=
+            max(preferredFormats.indexOfFirst { file.mimeType.equals(it, ignoreCase = true) }, 0)
+        score *=
+            max(preferredExtensions.indexOfFirst { extension.equals(it, ignoreCase = true) }, 0)
+        return score
     }
 
     private companion object {
-        private val coverNames =
+        private val preferredCoverNames = listOf("front", "art", "album", "folder", "cover")
+
+        private val preferredFormats =
             listOf(
-                "cover",
-                "folder",
-                "album",
-                "albumart",
-                "front",
-                "artwork",
-                "art",
-                "folder",
-                "coverart")
+                "image/webp",
+                "image/jpg",
+                "image/jpeg",
+                "image/png",
+            )
+
+        private val preferredExtensions = listOf("webp", "jpg", "jpeg", "png")
     }
 }