nmon/internal/rollup/rate.go

package rollup

import "math"

// wrap32 is the modulus of a 32-bit octet counter.
const wrap32 = 1 << 32

// counterWindow is the result of measuring one counter over one window.
type counterWindow struct {
	rateBps float64 // average bits per second over the span actually observed
	inside  int     // samples inside the window; fewer than expected means a gap
}

// measureCounter computes the average rate of a counter over the window
// (startMs, endMs] from its raw samples, which must be sorted by time and may
// include samples before the window (the newest of them is the baseline, so a
// window's rate is the counter's slope across it rather than a guess).
//
// The counter is never averaged: the increase is summed between consecutive
// samples and divided by the time those samples span. A decrease is a 32-bit
// wrap when wide is false (the increase is d+2^32) and a reset otherwise (the
// counter restarted from zero, so the increase is the new value), which is
// what VictoriaMetrics' increase() assumes too. ok is false when fewer than
// two samples bound the window.
func measureCounter(pts []point, startMs, endMs int64, wide bool) (counterWindow, bool) {
	var base *point
	var in []point
	for i := range pts {
		switch {
		case pts[i].ts <= startMs:
			base = &pts[i]
		case pts[i].ts <= endMs:
			in = append(in, pts[i])
		}
	}
	if len(in) == 0 {
		return counterWindow{}, false
	}
	seq := in
	if base != nil {
		seq = append([]point{*base}, in...)
	}
	if len(seq) < 2 {
		return counterWindow{inside: len(in)}, false
	}
	var inc float64
	for i := 1; i < len(seq); i++ {
		d := seq[i].v - seq[i-1].v
		if d < 0 {
			if wide {
				d = seq[i].v
			} else {
				d += wrap32
			}
		}
		inc += d
	}
	span := float64(seq[len(seq)-1].ts-seq[0].ts) / 1000
	if span <= 0 {
		return counterWindow{inside: len(in)}, false
	}
	rate := inc * 8 / span
	if math.IsInf(rate, 0) || math.IsNaN(rate) {
		return counterWindow{inside: len(in)}, false
	}
	return counterWindow{rateBps: rate, inside: len(in)}, true
}