compute actual sample rate for profile export (#26)

In cases where due to either an excessive number of goroutines, or excessive
load on the system, we are not able to sample at the given rate, we can compute
the actual rate we were able to sample and use that when exporting the profile
so that timing information in the profile is more accurate and sensible.

With this code in place, a function which is on the stack for the entirety of a
27 second profile, will always display 27s when viewed, whereas previously it
could display less time.

fgprof.go

@@ -6,6 +6,7 @@ package fgprof
 import (
 	"fmt"
 	"io"
+	"math"
 	"runtime"
 	"sort"
 	"strings"
@@ -37,16 +38,19 @@ func Start(w io.Writer, format Format) func() error {
 	const hz = 99
 	ticker := time.NewTicker(time.Second / hz)
 	stopCh := make(chan struct{})
-
 	prof := &profiler{}
 	profile := newWallclockProfile()
 
+	var sampleCount int64
+
 	go func() {
 		defer ticker.Stop()
 
 		for {
 			select {
 			case <-ticker.C:
+				sampleCount++
+
 				stacks := prof.GoroutineProfile()
 				profile.Add(stacks)
 			case <-stopCh:
@@ -59,7 +63,14 @@ func Start(w io.Writer, format Format) func() error {
 		stopCh <- struct{}{}
 		endTime := time.Now()
 		profile.Ignore(prof.SelfFrames()...)
-		return profile.Export(w, format, hz, startTime, endTime)
+
+		// Compute actual sample rate in case, due to performance issues, we
+		// were not actually able to sample at the given hz. Converting
+		// everything to float avoids integers being rounded in the wrong
+		// direction and improves the correctness of times in profiles.
+		duration := endTime.Sub(startTime)
+		actualHz := float64(sampleCount) / (float64(duration) / 1e9)
+		return profile.Export(w, format, int(math.Round(actualHz)), startTime, endTime)
 	}
 }