Skip calculating IO stats for first sample

Or you get high read/write speeds averaged over process' whole lifetime.

src/iostats.cpp

@@ -1,16 +1,12 @@
 #include "iostats.h"
 #include "string_utils.h"
-#include "timing.hpp"
 #include <fstream>
 
-Clock::time_point lastUpdate = Clock::now(); /* ns */
-
 void getIoStats(void *args) {
     iostats *io = reinterpret_cast<iostats *>(args);
     if (io) {
         Clock::time_point now = Clock::now(); /* ns */
-        Clock::duration timeDiff = now - lastUpdate;
-        float timeDiffSecs = (float)timeDiff.count() / (float)1000000000; // Time diff in seconds
+        std::chrono::duration<float> time_diff = now - io->last_update;
 
         io->prev.read_bytes  = io->curr.read_bytes;
         io->prev.write_bytes = io->curr.write_bytes;
@@ -26,12 +22,14 @@ void getIoStats(void *args) {
             }
         }
 
-        io->diff.read  = (io->curr.read_bytes  - io->prev.read_bytes) / (1024.f * 1024.f);
-        io->diff.write = (io->curr.write_bytes - io->prev.write_bytes) / (1024.f * 1024.f);
+        if (io->last_update.time_since_epoch().count()) {
+            io->diff.read  = (io->curr.read_bytes  - io->prev.read_bytes) / (1024.f * 1024.f);
+            io->diff.write = (io->curr.write_bytes - io->prev.write_bytes) / (1024.f * 1024.f);
 
-        io->per_second.read = io->diff.read / timeDiffSecs;
-        io->per_second.write = io->diff.write / timeDiffSecs;
+            io->per_second.read = io->diff.read / time_diff.count();
+            io->per_second.write = io->diff.write / time_diff.count();
+        }
 
-        lastUpdate = now;
+        io->last_update = now;
     }
 }

src/iostats.h

@@ -3,6 +3,7 @@
 #define MANGOHUD_IOSTATS_H
 
 #include <inttypes.h>
+#include "timing.hpp"
 
 struct iostats {
     struct {
@@ -21,6 +22,7 @@ struct iostats {
       float read;
       float write;
     } per_second;
+    Clock::time_point last_update;
 };
 
 void getIoStats(void *args);