Increase the time resolution in rate mode

Times are measured using time(), which has a one second resolution.
Because of this, ttyplot can give vastly underestimated rates if data
points arrive faster than once a second.

Increase the time resolution by using gettimeofday() instead of time().
Also, move the computation of the rates out of the main loop, into their
own function, in order to improve readability.

ttyplot.c

@@ -11,6 +11,7 @@
 #include <unistd.h>
 #include <float.h>
 #include <time.h>
+#include <sys/time.h>
 #include <ncurses.h>
 #include <signal.h>
 #include <errno.h>
@@ -46,14 +47,13 @@
 
 sigset_t sigmsk;
 chtype plotchar, max_errchar, min_errchar;
-time_t t1,t2,td;
+struct timeval now;
+double td;
 struct tm *lt;
 double max=FLT_MIN;
 double softmax=FLT_MIN, hardmax=FLT_MAX, hardmin=0.0;
 char title[256]=".: ttyplot :.", unit[64]={0}, ls[256]={0};
 double values1[1024]={0}, values2[1024]={0};
-double cval1=FLT_MAX, pval1=FLT_MAX;
-double cval2=FLT_MAX, pval2=FLT_MAX;
 double min1=FLT_MAX, max1=FLT_MIN, avg1=0;
 double min2=FLT_MAX, max2=FLT_MIN, avg2=0;
 int width=0, height=0, n=0, r=0, v=0, c=0, rate=0, two=0, plotwidth=0, plotheight=0;
@@ -79,6 +79,35 @@ void version(void) {
     printf("ttyplot %s\n", VERSION_STR);
 }
 
+// Replace *v1 and *v2 (if non-NULL) by their time derivatives.
+//  - v1, v2: addresses of input data and storage for results
+//  - now: current time
+// Return time since previous call.
+double derivative(double *v1, double *v2, const struct timeval *now)
+{
+    static double previous_v1, previous_v2, previous_t = DBL_MAX;
+    const double t = now->tv_sec + 1e-6 * now->tv_usec;
+    const double dt = t - previous_t;
+    previous_t = t;
+    if (v1) {
+        const double dv1 = *v1 - previous_v1;
+        previous_v1 = *v1;
+        if (dt <= 0)
+            *v1 = 0;
+        else
+            *v1 = dv1 / dt;
+    }
+    if (v2) {
+        const double dv2 = *v2 - previous_v2;
+        previous_v2 = *v2;
+        if (dt <= 0)
+            *v2 = 0;
+        else
+            *v2 = dv2 / dt;
+    }
+    return dt;
+}
+
 void getminmax(int pw, double *values, double *min, double *max, double *avg, int v) {
     double tot=0;
     int i=0;
@@ -202,14 +231,14 @@ void paint_plot(void) {
 
     mvaddstr(height-1, width-strlen(verstring)-1, verstring);
 
-    lt=localtime(&t1);
+    lt=localtime(&now.tv_sec);
     asctime_r(lt, ls);
     mvaddstr(height-2, width-strlen(ls), ls);
 
     mvvline(height-2, 5, plotchar|A_NORMAL, 1);
     mvprintw(height-2, 7, "last=%.1f min=%.1f max=%.1f avg=%.1f %s ",  values1[n], min1, max1, avg1, unit);
     if(rate)
-        printw(" interval=%llds", (long long int)td);
+        printw(" interval=%.3gs", td);
 
     if(two) {
         mvaddch(height-1, 5, ' '|A_REVERSE);
@@ -357,7 +386,7 @@ int main(int argc, char *argv[]) {
         err(1, "pledge");
     #endif
 
-    time(&t1);
+    gettimeofday(&now, NULL);
     noecho();
     curs_set(FALSE);
     erase();
@@ -410,39 +439,9 @@ int main(int argc, char *argv[]) {
         if(values2[n] < 0)
             values2[n] = 0;
 
-        if(rate) {
-            t2=t1;
-            time(&t1);
-            td=t1-t2;
-            if(td==0)
-                td=1;
-
-            if(cval1==FLT_MAX)
-                pval1=values1[n];
-            else
-                pval1=cval1;
-            cval1=values1[n];
-
-            values1[n]=(cval1-pval1)/td;
-
-            if(values1[n] < 0) // counter rewind
-                values1[n]=0;
-
-            if(two) {
-                if(cval2==FLT_MAX)
-                    pval2=values2[n];
-                else
-                    pval2=cval2;
-                cval2=values2[n];
-
-                values2[n]=(cval2-pval2)/td;
-
-                if(values2[n] < 0) // counter rewind
-                    values2[n]=0;
-            }
-        } else {
-            time(&t1);
-        }
+        gettimeofday(&now, NULL);
+        if (rate)
+            td=derivative(&values1[n], two ? &values2[n] : NULL, &now);
 
         paint_plot();