Refactor code structure for improved readability and maintainability

android/app/src/main/kotlin/org/xtimms/psu/schedule/data/Api.kt

@@ -15,7 +15,15 @@ import java.io.File
 import java.io.IOException
 import java.util.concurrent.TimeUnit
 
-class ApiException(message: String, val offline: Boolean = false) : Exception(message)
+/**
+ * An error ready to show. [code] is the server's machine-readable reason (server/errors.js);
+ * [offline] means the data could not be fetched — no network, or the university's catalog
+ * is down — so a saved copy stays on screen and "Повторить" makes sense.
+ */
+class ApiException(message: String, val offline: Boolean = false, val code: String? = null) : Exception(message)
+
+/** Server codes after which trying again later can help. */
+private val SOURCE_DOWN = setOf("source_unavailable", "source_timeout", "busy")
 
 val json = Json {
     ignoreUnknownKeys = true
@@ -44,14 +52,20 @@ class Api(baseUrl: String, cacheDir: File? = null) {
                 client.newCall(Request.Builder().url(url).build()).execute().use { response ->
                     val text = response.body?.string().orEmpty()
                     if (!response.isSuccessful) {
-                        val message = runCatching {
-                            json.parseToJsonElement(text).jsonObject["error"]?.jsonPrimitive?.content
-                        }.getOrNull()
-                        // The server's catch-all 404 means it runs an older version without this endpoint.
-                        if (response.code == 404 && path.startsWith("api/") && message == "Адрес не найден.") throw ApiException(
+                        val body = runCatching { json.parseToJsonElement(text).jsonObject }.getOrNull()
+                        val message = body?.get("error")?.jsonPrimitive?.content
+                        // Servers before API version 5 send no code; their catch-all 404 has this text.
+                        val code = body?.get("code")?.jsonPrimitive?.content
+                            ?: if (response.code == 404 && message == "Адрес не найден.") "unknown_endpoint" else null
+                        if (code == "unknown_endpoint") throw ApiException(
                             "Сервер расписания не знает запрос /$path — на нём старая версия. Обновите сервер.",
+                            code = code,
+                        )
+                        throw ApiException(
+                            message ?: "Сервер ответил ошибкой ${response.code}.",
+                            offline = code in SOURCE_DOWN,
+                            code = code,
                         )
-                        throw ApiException(message ?: "Сервер ответил ошибкой ${response.code}.")
                     }
                     text
                 }
@@ -70,12 +84,12 @@ class Api(baseUrl: String, cacheDir: File? = null) {
                 return call()
             } catch (e: IOException) {
                 android.util.Log.w("Api", "GET $url failed (attempt ${attempt + 1})", e)
-                if (attempt == RETRIES - 1) throw ApiException(OFFLINE, offline = true)
+                if (attempt == RETRIES - 1) throw ApiException(OFFLINE, offline = true, code = "offline")
                 delay(wait)
                 wait *= 3
             }
         }
-        throw ApiException(OFFLINE, offline = true)
+        throw ApiException(OFFLINE, offline = true, code = "offline")
     }
 
     private companion object {