[ui] use GNU wget's algorithms for rate/ETA display

src/ui.h

@@ -47,10 +47,30 @@
 enum update_progress_type {
 	UPT_PERCENT = 0,
 	UPT_SPEED,
-	UPT_TIME,
+	UPT_ETA,
 	UPT_MAX
 };
 
+// Size of the download speed history ring.
+#define SPEED_HISTORY_SIZE 20
+
+// The minimum time length of a history sample. By default, each sample is at least 150ms long,
+// which means that, over the course of 20 samples, "current" download speed spans at least 3s
+// into the past.
+#define SPEED_SAMPLE_MIN 150
+
+// The time after which the download starts to be considered "stalled", i.e. the current
+// bandwidth is not printed and the recent download speeds are scratched.
+#define STALL_START_TIME 5000
+
+// Time between screen refreshes will not be shorter than this.
+// NB: In Rufus' case, "screen" means the text overlaid on the progress bar.
+#define SCREEN_REFRESH_INTERVAL 200
+
+// Don't refresh the ETA too often to avoid jerkiness in predictions.
+// This allows ETA to change approximately once per second.
+#define ETA_REFRESH_INTERVAL 990
+
 extern HWND hMultiToolbar, hSaveToolbar, hHashToolbar, hAdvancedDeviceToolbar, hAdvancedFormatToolbar;
 extern HFONT hInfoFont;
 extern UINT_PTR UM_LANGUAGE_MENU_MAX;