Make source ID generation function reusable to extensions (#9836)

* Make source ID generation function reusable to extensions.

* Add parameters and return documentation.

source-api/src/commonMain/kotlin/eu/kanade/tachiyomi/source/online/HttpSource.kt

@@ -43,15 +43,16 @@ abstract class HttpSource : CatalogueSource {
     open val versionId = 1
 
     /**
-     * Id of the source. By default it uses a generated id using the first 16 characters (64 bits)
-     * of the MD5 of the string: sourcename/language/versionId
-     * Note the generated id sets the sign bit to 0.
+     * ID of the source. By default it uses a generated id using the first 16 characters (64 bits)
+     * of the MD5 of the string `"${name.lowercase()}/$lang/$versionId"`.
+     *
+     * The ID is generated by the [generateId] function, which can be reused if needed
+     * to generate outdated IDs for cases where the source name or language needs to
+     * be changed but migrations can be avoided.
+     *
+     * Note: the generated ID sets the sign bit to `0`.
      */
-    override val id by lazy {
-        val key = "${name.lowercase()}/$lang/$versionId"
-        val bytes = MessageDigest.getInstance("MD5").digest(key.toByteArray())
-        (0..7).map { bytes[it].toLong() and 0xff shl 8 * (7 - it) }.reduce(Long::or) and Long.MAX_VALUE
-    }
+    override val id by lazy { generateId(name, lang, versionId) }
 
     /**
      * Headers used for requests.
@@ -64,6 +65,28 @@ abstract class HttpSource : CatalogueSource {
     open val client: OkHttpClient
         get() = network.client
 
+    /**
+     * Generates a unique ID for the source based on the provided [name], [lang] and
+     * [versionId]. It will use the first 16 characters (64 bits) of the MD5 of the string
+     * `"${name.lowercase()}/$lang/$versionId"`.
+     *
+     * Note: the generated ID sets the sign bit to `0`.
+     *
+     * Can be used to generate outdated IDs, such as when the source name or language
+     * needs to be changed but migrations can be avoided.
+     *
+     * @since extensions-lib 1.5
+     * @param name [String] the name of the source
+     * @param lang [String] the language of the source
+     * @param versionId [Int] the version ID of the source
+     * @return a unique ID for the source
+     */
+    protected fun generateId(name: String, lang: String, versionId: Int): Long {
+        val key = "${name.lowercase()}/$lang/$versionId"
+        val bytes = MessageDigest.getInstance("MD5").digest(key.toByteArray())
+        return (0..7).map { bytes[it].toLong() and 0xff shl 8 * (7 - it) }.reduce(Long::or) and Long.MAX_VALUE
+    }
+
     /**
      * Headers builder for requests. Implementations can override this method for custom headers.
      */