Refactor download architecture to be protocol-agnostic

app/shared/src/commonMain/kotlin/com/linroid/ketch/app/instance/InstanceManager.kt

@@ -243,7 +243,7 @@ private object DisconnectedApi : KetchApi {
 
   override suspend fun resolve(
     url: String,
-    headers: Map<String, String>,
+    properties: Map<String, String>,
   ): ResolvedSource {
     throw IllegalStateException(
       "No instance connected. Add a remote server first.",

app/shared/src/commonTest/kotlin/com/linroid/ketch/app/FakeKetchApi.kt

@@ -41,7 +41,7 @@ class FakeKetchApi(
 
   override suspend fun resolve(
     url: String,
-    headers: Map<String, String>,
+    properties: Map<String, String>,
   ): ResolvedSource {
     throw UnsupportedOperationException(
       "FakeKetchApi does not support resolve"

library/api/src/commonMain/kotlin/com/linroid/ketch/api/KetchApi.kt

@@ -27,11 +27,13 @@ interface KetchApi {
    * during [download].
    *
    * @param url the URL to resolve
-   * @param headers optional HTTP headers to include in the probe
+   * @param properties source-specific key-value pairs. For HTTP
+   *   sources this contains HTTP headers; other sources may
+   *   interpret them differently or ignore them.
    */
   suspend fun resolve(
     url: String,
-    headers: Map<String, String> = emptyMap(),
+    properties: Map<String, String> = emptyMap(),
   ): ResolvedSource
 
   /**

library/core/src/commonMain/kotlin/com/linroid/ketch/core/Ketch.kt

@@ -181,11 +181,11 @@ class Ketch(
 
   override suspend fun resolve(
     url: String,
-    headers: Map<String, String>,
+    properties: Map<String, String>,
   ): ResolvedSource {
     log.i { "Resolving URL: $url" }
     val source = sourceResolver.resolve(url)
-    return source.resolve(url, headers)
+    return source.resolve(url, properties)
   }
 
   override suspend fun start() {

library/core/src/commonMain/kotlin/com/linroid/ketch/core/engine/DownloadContext.kt

@@ -21,18 +21,19 @@ import kotlinx.coroutines.flow.MutableStateFlow
  *   call this with the number of bytes before writing each chunk.
  *   This replaces direct [SpeedLimiter] access to avoid cross-module
  *   visibility issues with internal types.
- * @property headers HTTP headers or source-specific metadata headers
+ * @property headers request headers from [DownloadRequest.headers].
+ *   Used by HTTP sources for custom headers; other sources may ignore.
  * @property preResolved pre-resolved URL metadata, allowing the
  *   download source to skip its own probe/HEAD request
  * @property maxConnections observable override for the number of
  *   concurrent segment connections. When positive, takes precedence
  *   over [DownloadRequest.connections]. Emitting a new value triggers
- *   live resegmentation in [HttpDownloadSource]. Reduced automatically
- *   on HTTP 429 (Too Many Requests) responses.
+ *   live resegmentation in sources that support it. Reduced
+ *   automatically on HTTP 429 (Too Many Requests) responses.
  * @property pendingResegment target connection count for a pending
  *   resegmentation. Set by the connection-change watcher before
- *   canceling the download batch scope. Read by [HttpDownloadSource]
- *   to distinguish resegment-cancel from external cancel.
+ *   canceling the download batch scope. Read by sources to
+ *   distinguish resegment-cancel from external cancel.
  */
 class DownloadContext(
   val taskId: String,

library/core/src/commonMain/kotlin/com/linroid/ketch/core/engine/DownloadExecution.kt

@@ -140,9 +140,7 @@ internal class DownloadExecution(
     totalBytes = total
 
     val fileName = resolvedUrl.suggestedFileName
-      ?: fileNameResolver.resolve(
-        request, toServerInfo(resolvedUrl),
-      )
+      ?: fileNameResolver.resolve(request, resolvedUrl)
     val outputPath = resolveDestPath(
       destination = request.destination,
       defaultDir = config.defaultDirectory ?: "downloads",
@@ -162,12 +160,10 @@ internal class DownloadExecution(
         outputPath = outputPath,
         state = TaskState.DOWNLOADING,
         totalBytes = total,
-        acceptRanges = resolvedUrl.supportsResume,
-        etag = resolvedUrl.metadata[HttpDownloadSource.META_ETAG],
-        lastModified = resolvedUrl.metadata[
-          HttpDownloadSource.META_LAST_MODIFIED,
-        ],
         sourceType = source.type,
+        sourceResumeState = source.buildResumeState(
+          resolvedUrl, total,
+        ),
         updatedAt = now,
       )
     }
@@ -177,6 +173,7 @@ internal class DownloadExecution(
     val preResolved = if (resolved != null) resolvedUrl else null
     runDownload(
       outputPath, total, source.managesOwnFileIo, preResolved,
+      source = source,
     ) { ctx ->
       source.download(ctx)
     }
@@ -185,7 +182,12 @@ internal class DownloadExecution(
   private suspend fun executeResume(info: ResumeInfo) {
     val taskRecord = info.record
 
-    val sourceType = taskRecord.sourceType ?: HttpDownloadSource.TYPE
+    val sourceType = taskRecord.sourceType
+      ?: throw KetchError.Unknown(
+        IllegalStateException(
+          "No sourceType for taskId=${taskRecord.taskId}",
+        ),
+      )
     val source = sourceResolver.resolveByType(sourceType)
     log.i {
       "Resuming download for taskId=$taskId via " +
@@ -202,14 +204,13 @@ internal class DownloadExecution(
     taskLimiter.delegate = createLimiter(taskRecord.request.speedLimit)
 
     val resumeState = taskRecord.sourceResumeState
-      ?: HttpDownloadSource.buildResumeState(
-        etag = taskRecord.etag,
-        lastModified = taskRecord.lastModified,
-        totalBytes = taskRecord.totalBytes,
+      ?: throw KetchError.CorruptResumeState(
+        "No resume state for taskId=${taskRecord.taskId}",
       )
 
     runDownload(
       outputPath, taskRecord.totalBytes, source.managesOwnFileIo,
+      source = source,
     ) { ctx ->
       context = ctx
       source.resume(ctx, resumeState)
@@ -229,6 +230,7 @@ internal class DownloadExecution(
     total: Long,
     selfManagedIo: Boolean = false,
     preResolved: ResolvedSource? = null,
+    source: DownloadSource? = null,
     downloadBlock: suspend (DownloadContext) -> Unit,
   ) {
     val fa = if (selfManagedIo) {
@@ -248,10 +250,14 @@ internal class DownloadExecution(
           while (true) {
             delay(config.saveIntervalMs)
             val snapshot = handle.mutableSegments.value
-            val downloaded = snapshot.sumOf { it.downloadedBytes }
+            val updatedResume = context?.let {
+              source?.updateResumeState(it)
+            }
             handle.record.update {
               it.copy(
                 segments = snapshot,
+                sourceResumeState = updatedResume
+                  ?: it.sourceResumeState,
                 updatedAt = Clock.System.now(),
               )
             }
@@ -278,11 +284,6 @@ internal class DownloadExecution(
         it.copy(
           state = TaskState.COMPLETED,
           segments = null,
-          sourceResumeState = HttpDownloadSource.buildResumeState(
-            etag = it.etag,
-            lastModified = it.lastModified,
-            totalBytes = it.totalBytes,
-          ),
           updatedAt = Clock.System.now(),
         )
       }
@@ -479,17 +480,6 @@ internal class DownloadExecution(
     }
   }
 
-  private fun toServerInfo(resolved: ResolvedSource): ServerInfo {
-    return ServerInfo(
-      contentLength = resolved.totalBytes,
-      acceptRanges = resolved.supportsResume,
-      etag = resolved.metadata[HttpDownloadSource.META_ETAG],
-      lastModified = resolved.metadata[
-        HttpDownloadSource.META_LAST_MODIFIED,
-      ],
-    )
-  }
-
   private fun resolveDestPath(
     destination: com.linroid.ketch.api.Destination?,
     defaultDir: String,

library/core/src/commonMain/kotlin/com/linroid/ketch/core/engine/DownloadSource.kt

@@ -7,8 +7,7 @@ import com.linroid.ketch.api.ResolvedSource
  *
  * Each source handles a specific protocol or download mechanism.
  * The default implementation is [HttpDownloadSource] for HTTP/HTTPS
- * downloads. Future implementations may include torrent, media
- * extraction, or other protocols.
+ * downloads. Other implementations include FTP, BitTorrent, etc.
  *
  * Sources are registered with [SourceResolver] which routes
  * download requests to the appropriate source based on URL matching.
@@ -32,10 +31,15 @@ interface DownloadSource {
    * Resolves source metadata for the given URL without downloading.
    * This is analogous to an HTTP HEAD request but generalized for
    * any source type.
+   *
+   * @param url the URL to resolve
+   * @param properties source-specific key-value pairs. For HTTP
+   *   sources this contains HTTP headers; other sources may
+   *   interpret them differently or ignore them.
    */
   suspend fun resolve(
     url: String,
-    headers: Map<String, String> = emptyMap(),
+    properties: Map<String, String> = emptyMap(),
   ): ResolvedSource
 
   /**
@@ -56,4 +60,36 @@ interface DownloadSource {
    * [com.linroid.ketch.api.KetchError.Unsupported].
    */
   suspend fun resume(context: DownloadContext, resumeState: SourceResumeState)
+
+  /**
+   * Builds an opaque [SourceResumeState] from resolved metadata.
+   *
+   * Called after [resolve] completes to persist source-specific
+   * state needed for resume validation (e.g., HTTP ETag/Last-Modified,
+   * FTP MDTM, torrent info hash). The returned state is stored in
+   * the task record and passed back to [resume] on restart.
+   *
+   * @param resolved the metadata returned by [resolve]
+   * @param totalBytes total download size in bytes
+   */
+  fun buildResumeState(
+    resolved: ResolvedSource,
+    totalBytes: Long,
+  ): SourceResumeState
+
+  /**
+   * Called periodically during download to let the source update
+   * its resume state. The returned state replaces the current
+   * resume state in the task record.
+   *
+   * Default implementation returns `null` (no update needed).
+   * Sources like BitTorrent override this to persist bitfield
+   * progress incrementally.
+   *
+   * @param context the active download context
+   * @return updated resume state, or `null` to keep the current one
+   */
+  suspend fun updateResumeState(
+    context: DownloadContext,
+  ): SourceResumeState? = null
 }