perf: Migrate epub caching system from Json to Protocol Buffers

app/build.gradle

@@ -117,7 +117,8 @@ dependencies {
     // Android 12+ splash API.
     implementation 'androidx.core:core-splashscreen:1.0.1'
     // KotlinX Serialization library.
-    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3"
+    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.7.3"
+    implementation "org.jetbrains.kotlinx:kotlinx-serialization-protobuf:1.7.3"
     // OkHttp library.
     implementation "com.squareup.okhttp3:okhttp:4.12.0"
     implementation "com.squareup.okhttp3:logging-interceptor:4.12.0"

app/src/main/java/com/starry/myne/epub/EpubParser.kt

@@ -21,6 +21,7 @@ import android.content.Context
 import android.graphics.Bitmap
 import android.graphics.BitmapFactory
 import android.util.Log
+import com.starry.myne.epub.cache.EpubCache
 import com.starry.myne.epub.models.EpubBook
 import com.starry.myne.epub.models.EpubChapter
 import com.starry.myne.epub.models.EpubImage

app/src/main/java/com/starry/myne/epub/BitmapSerializer.kt → app/src/main/java/com/starry/myne/epub/cache/BitmapSerializer.kt

@@ -14,36 +14,38 @@
  * limitations under the License.
  */
 
-package com.starry.myne.epub
+package com.starry.myne.epub.cache
 
 import android.graphics.Bitmap
 import android.graphics.BitmapFactory
 import kotlinx.serialization.KSerializer
-import kotlinx.serialization.descriptors.PrimitiveKind
-import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
+import kotlinx.serialization.builtins.ByteArraySerializer
 import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.descriptors.buildClassSerialDescriptor
+import kotlinx.serialization.descriptors.element
 import kotlinx.serialization.encoding.Decoder
 import kotlinx.serialization.encoding.Encoder
 import java.io.ByteArrayOutputStream
 
+/**
+ * A [KSerializer] for [Bitmap] objects.
+ * It serializes the bitmap to a byte array and deserializes it back to a bitmap.
+ */
 object BitmapSerializer : KSerializer<Bitmap> {
-    override val descriptor: SerialDescriptor =
-        PrimitiveSerialDescriptor("Bitmap", PrimitiveKind.STRING)
+    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Bitmap") {
+        element<ByteArray>("bytes")
+    }
 
     override fun serialize(encoder: Encoder, value: Bitmap) {
         val stream = ByteArrayOutputStream()
         value.compress(Bitmap.CompressFormat.PNG, 100, stream)
         val byteArray = stream.toByteArray()
-        encoder.encodeString(
-            android.util.Base64.encodeToString(
-                byteArray, android.util.Base64.DEFAULT
-            )
-        )
+        encoder.encodeSerializableValue(ByteArraySerializer(), byteArray)
     }
 
     override fun deserialize(decoder: Decoder): Bitmap {
-        val byteArray =
-            android.util.Base64.decode(decoder.decodeString(), android.util.Base64.DEFAULT)
+        val byteArray = decoder.decodeSerializableValue(ByteArraySerializer())
         return BitmapFactory.decodeByteArray(byteArray, 0, byteArray.size)
     }
 }
+

app/src/main/java/com/starry/myne/epub/EpubCache.kt → app/src/main/java/com/starry/myne/epub/cache/EpubCache.kt

@@ -14,19 +14,21 @@
  * limitations under the License.
  */
 
-package com.starry.myne.epub
+package com.starry.myne.epub.cache
 
 import android.content.Context
 import android.util.Log
 import com.starry.myne.epub.models.EpubBook
-import kotlinx.serialization.encodeToString
-import kotlinx.serialization.json.Json
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.protobuf.ProtoBuf
+import kotlinx.serialization.protobuf.schema.ProtoBufSchemaGenerator
 import java.io.File
 
 /**
- * A cache for storing epub books.
+ * A cache storage based on Protocol Buffers for storing [EpubBook] objects.
+ * The cache is stored in the app's cache directory.
  *
- * @param context The context.
+ * @param context The context of the application.
  */
 class EpubCache(private val context: Context) {
 
@@ -36,14 +38,17 @@ class EpubCache(private val context: Context) {
         private const val CACHE_VERSION_FILE = "cache_version"
 
         // Increment this if the cache format changes.
-        private const val EPUB_CACHE_VERSION = 1
+        private const val EPUB_CACHE_VERSION = 2
     }
 
     init {
         create()
         checkCacheVersion()
     }
 
+    @OptIn(ExperimentalSerializationApi::class)
+    private val protobuf = ProtoBuf { encodeDefaults = true }
+
     private fun getPath(): String {
         return context.cacheDir.absolutePath + File.separator + EPUB_CACHE
     }
@@ -52,6 +57,7 @@ class EpubCache(private val context: Context) {
         return getPath() + File.separator + CACHE_VERSION_FILE
     }
 
+    // Checks if the cache version matches the current version.
     private fun checkCacheVersion() {
         Log.d(TAG, "Checking cache version")
         val versionFile = File(getVersionFilePath())
@@ -69,6 +75,7 @@ class EpubCache(private val context: Context) {
         }
     }
 
+    // Saves the current cache version.
     private fun saveCacheVersion() {
         Log.d(TAG, "Saving cache version")
         val versionFile = File(getVersionFilePath())
@@ -91,35 +98,45 @@ class EpubCache(private val context: Context) {
         }
     }
 
+    // Used for debugging purposes.
+    @Suppress("unused")
+    @OptIn(ExperimentalSerializationApi::class)
+    private fun printSchema() {
+        val protoSchema =
+            ProtoBufSchemaGenerator.generateSchemaText(EpubBook.serializer().descriptor)
+        Log.d(TAG, "Proto schema: $protoSchema")
+    }
+
     /**
-     * Adds a book to the cache.
+     * Inserts a book into the cache.
      *
-     * @param book The book to add.
+     * @param book The book to insert.
      * @param filepath The path to the book file.
      */
+    @OptIn(ExperimentalSerializationApi::class)
     fun put(book: EpubBook, filepath: String) {
         Log.d(TAG, "Inserting book into cache: ${book.title}")
         val fileName = File(filepath).nameWithoutExtension
-        val bookFile = File(getPath(), "$fileName.json")
-        val jsonString = Json.encodeToString(book)
-        bookFile.writeText(jsonString)
+        val bookFile = File(getPath(), "$fileName.protobuf")
+        val protoBytes = protobuf.encodeToByteArray(EpubBook.serializer(), book)
+        bookFile.writeBytes(protoBytes)
     }
 
     /**
      * Gets a book from the cache.
-     * If the book is not cached, null is returned.
      *
      * @param filepath The path to the book file.
      * @return The book if it is cached, null otherwise.
      */
+    @OptIn(ExperimentalSerializationApi::class)
     fun get(filepath: String): EpubBook? {
         Log.d(TAG, "Getting book from cache: $filepath")
         val fileName = File(filepath).nameWithoutExtension
-        val bookFile = File(getPath(), "$fileName.json")
+        val bookFile = File(getPath(), "$fileName.protobuf")
         return if (bookFile.exists()) {
             Log.d(TAG, "Book found in cache: $filepath")
-            val jsonString = bookFile.readText()
-            Json.decodeFromString(jsonString)
+            val protoBytes = bookFile.readBytes()
+            protobuf.decodeFromByteArray(EpubBook.serializer(), protoBytes)
         } else {
             Log.d(TAG, "Book not found in cache: $filepath")
             null
@@ -134,7 +151,7 @@ class EpubCache(private val context: Context) {
     fun remove(filepath: String): Boolean {
         Log.d(TAG, "Removing book from cache: $filepath")
         val fileName = File(filepath).nameWithoutExtension
-        val bookFile = File(getPath(), "$fileName.json")
+        val bookFile = File(getPath(), "$fileName.protobuf")
         return if (bookFile.exists()) {
             bookFile.delete()
         } else {
@@ -150,7 +167,7 @@ class EpubCache(private val context: Context) {
      */
     fun isCached(filepath: String): Boolean {
         val fileName = File(filepath).nameWithoutExtension
-        val bookFile = File(getPath(), "$fileName.json")
+        val bookFile = File(getPath(), "$fileName.protobuf")
         return bookFile.exists()
     }
 }

app/src/main/java/com/starry/myne/epub/cache/epub_cache.proto

@@ -0,0 +1,36 @@
+// Generated schema file for 'com.starry.myne.epub.models.EpubBook'
+// For reference only, not actually used to generate protobuf code
+
+syntax = "proto2";
+
+// serial name 'com.starry.myne.epub.models.EpubBook'
+message EpubBook {
+  required string fileName = 1;
+  required string title = 2;
+  required string author = 3;
+  required string language = 4;
+  optional Bitmap coverImage = 5;
+  // WARNING: a default value decoded when value is missing
+  repeated EpubChapter chapters = 6;
+  // WARNING: a default value decoded when value is missing
+  repeated EpubImage images = 7;
+}
+
+// serial name 'Bitmap?'
+message Bitmap {
+  required bytes bytes = 1;
+}
+
+// serial name 'com.starry.myne.epub.models.EpubChapter'
+message EpubChapter {
+  required string chapterId = 1;
+  required string absPath = 2;
+  required string title = 3;
+  required string body = 4;
+}
+
+// serial name 'com.starry.myne.epub.models.EpubImage'
+message EpubImage {
+  required string absPath = 1;
+  required bytes image = 2;
+}

app/src/main/java/com/starry/myne/epub/models/EpubBook.kt

@@ -18,8 +18,10 @@
 package com.starry.myne.epub.models
 
 import android.graphics.Bitmap
-import com.starry.myne.epub.BitmapSerializer
+import com.starry.myne.epub.cache.BitmapSerializer
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.protobuf.ProtoNumber
 
 /**
  * Represents an epub book.
@@ -33,13 +35,12 @@ import kotlinx.serialization.Serializable
  * @param images The list of images in the book.
  */
 @Serializable
-data class EpubBook(
-    val fileName: String,
-    val title: String,
-    val author: String,
-    val language: String,
-    @Serializable(with = BitmapSerializer::class)
-    val coverImage: Bitmap?,
-    val chapters: List<EpubChapter>,
-    val images: List<EpubImage>
+data class EpubBook @OptIn(ExperimentalSerializationApi::class) constructor(
+    @ProtoNumber(1) val fileName: String,
+    @ProtoNumber(2) val title: String,
+    @ProtoNumber(3) val author: String,
+    @ProtoNumber(4) val language: String,
+    @ProtoNumber(5) @Serializable(with = BitmapSerializer::class) val coverImage: Bitmap?,
+    @ProtoNumber(6) val chapters: List<EpubChapter> = emptyList(),
+    @ProtoNumber(7) val images: List<EpubImage> = emptyList()
 )

app/src/main/java/com/starry/myne/epub/models/EpubChapter.kt

@@ -17,7 +17,9 @@
 
 package com.starry.myne.epub.models
 
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.protobuf.ProtoNumber
 
 /**
  * Represents a chapter in an epub book.
@@ -27,9 +29,9 @@ import kotlinx.serialization.Serializable
  * @param body The body of the chapter.
  */
 @Serializable
-data class EpubChapter(
-    val chapterId: String,
-    val absPath: String,
-    val title: String,
-    val body: String
+data class EpubChapter @OptIn(ExperimentalSerializationApi::class) constructor(
+    @ProtoNumber(1) val chapterId: String,
+    @ProtoNumber(2) val absPath: String,
+    @ProtoNumber(3) val title: String,
+    @ProtoNumber(4) val body: String
 )

app/src/main/java/com/starry/myne/epub/models/EpubImage.kt

@@ -17,7 +17,9 @@
 
 package com.starry.myne.epub.models
 
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.protobuf.ProtoNumber
 
 /**
  * Represents an image in an epub book.
@@ -26,7 +28,10 @@ import kotlinx.serialization.Serializable
  * @param image The image data.
  */
 @Serializable
-data class EpubImage(val absPath: String, val image: ByteArray) {
+data class EpubImage @OptIn(ExperimentalSerializationApi::class) constructor(
+    @ProtoNumber(1) val absPath: String,
+    @ProtoNumber(2) val image: ByteArray
+) {
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
         if (javaClass != other?.javaClass) return false