decoder_api: move code to _prepare_initial_seek()

.. and add a few code comments.

src/decoder_api.c

@@ -78,6 +78,40 @@ decoder_initialized(struct decoder *decoder,
 					       &af_string));
 }
 
+/**
+ * Checks if we need an "initial seek".  If so, then the initial seek
+ * is prepared, and the function returns true.
+ */
+G_GNUC_PURE
+static bool
+decoder_prepare_initial_seek(struct decoder *decoder)
+{
+	const struct decoder_control *dc = decoder->dc;
+	assert(dc->pipe != NULL);
+
+	if (decoder->initial_seek_running)
+		/* initial seek has already begun - override any other
+		   command */
+		return true;
+
+	if (decoder->initial_seek_pending) {
+		if (dc->command == DECODE_COMMAND_NONE) {
+			/* begin initial seek */
+
+			decoder->initial_seek_pending = false;
+			decoder->initial_seek_running = true;
+			return true;
+		}
+
+		/* skip initial seek when there's another command
+		   (e.g. STOP) */
+
+		decoder->initial_seek_pending = false;
+	}
+
+	return false;
+}
+
 /**
  * Returns the current decoder command.  May return a "virtual"
  * synthesized command, e.g. to seek to the beginning of the CUE
@@ -90,19 +124,9 @@ decoder_get_virtual_command(struct decoder *decoder)
 	const struct decoder_control *dc = decoder->dc;
 	assert(dc->pipe != NULL);
 
-	if (decoder->initial_seek_running)
+	if (decoder_prepare_initial_seek(decoder))
 		return DECODE_COMMAND_SEEK;
 
-	if (decoder->initial_seek_pending) {
-		if (dc->command == DECODE_COMMAND_NONE) {
-			decoder->initial_seek_pending = false;
-			decoder->initial_seek_running = true;
-			return DECODE_COMMAND_SEEK;
-		}
-
-		decoder->initial_seek_pending = false;
-	}
-
 	return dc->command;
 }