gauge.go: add Inc, Dec and Add methods to Gauge

gauge.go

@@ -17,7 +17,7 @@ import (
 //   - foo{bar="baz",aaa="b"}
 //
 // f must be safe for concurrent calls.
-// if f is nil, then it is expected that the gauge value is changed via Gauge.Set() call.
+// if f is nil, then it is expected that the gauge value is changed via Set(), Inc(), Dec() and Add() calls.
 //
 // The returned gauge is safe to use from concurrent goroutines.
 //
@@ -55,6 +55,38 @@ func (g *Gauge) Set(v float64) {
 	atomic.StoreUint64(&g.valueBits, n)
 }
 
+// Inc increments g by 1.
+//
+// The g must be created with nil callback in order to be able to call this function.
+func (g *Gauge) Inc() {
+	g.Add(1)
+}
+
+// Dec decrements g by 1.
+//
+// The g must be created with nil callback in order to be able to call this function.
+func (g *Gauge) Dec() {
+	g.Add(-1)
+}
+
+// Add adds fAdd to g. fAdd may be positive and negative.
+//
+// The g must be created with nil callback in order to be able to call this function.
+func (g *Gauge) Add(fAdd float64) {
+	if g.f != nil {
+		panic(fmt.Errorf("cannot call Set on gauge created with non-nil callback"))
+	}
+	for {
+		n := atomic.LoadUint64(&g.valueBits)
+		f := math.Float64frombits(n)
+		fNew := f + fAdd
+		nNew := math.Float64bits(fNew)
+		if atomic.CompareAndSwapUint64(&g.valueBits, n, nNew) {
+			break
+		}
+	}
+}
+
 func (g *Gauge) marshalTo(prefix string, w io.Writer) {
 	v := g.Get()
 	if float64(int64(v)) == v {

gauge_test.go

@@ -15,6 +15,18 @@ func TestGaugeError(t *testing.T) {
 		g := GetOrCreateGauge("GetOrCreateGauge_nil_callback", func() float64 { return 123 })
 		g.Set(42)
 	})
+	expectPanic(t, "GetOrCreateGauge_Add_non-nil-callback", func() {
+		g := GetOrCreateGauge("GetOrCreateGauge_nil_callback", func() float64 { return 123 })
+		g.Add(42)
+	})
+	expectPanic(t, "GetOrCreateGauge_Inc_non-nil-callback", func() {
+		g := GetOrCreateGauge("GetOrCreateGauge_nil_callback", func() float64 { return 123 })
+		g.Inc()
+	})
+	expectPanic(t, "GetOrCreateGauge_Dec_non-nil-callback", func() {
+		g := GetOrCreateGauge("GetOrCreateGauge_nil_callback", func() float64 { return 123 })
+		g.Dec()
+	})
 }
 
 func TestGaugeSet(t *testing.T) {
@@ -29,6 +41,49 @@ func TestGaugeSet(t *testing.T) {
 	}
 }
 
+func TestGaugeIncDec(t *testing.T) {
+	s := NewSet()
+	g := s.NewGauge("foo", nil)
+	if n := g.Get(); n != 0 {
+		t.Fatalf("unexpected gauge value: %g; expecting 0", n)
+	}
+	for i := 1; i <= 100; i++ {
+		g.Inc()
+		if n := g.Get(); n != float64(i) {
+			t.Fatalf("unexpected gauge value %g; expecting %d", n, i)
+		}
+	}
+	for i := 99; i >= 0; i-- {
+		g.Dec()
+		if n := g.Get(); n != float64(i) {
+			t.Fatalf("unexpected gauge value %g; expecting %d", n, i)
+		}
+	}
+}
+
+func TestGaugeIncDecConcurrenc(t *testing.T) {
+	s := NewSet()
+	g := s.NewGauge("foo", nil)
+
+	workers := 5
+	var wg sync.WaitGroup
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			for i := 0; i < 100; i++ {
+				g.Inc()
+				g.Dec()
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+
+	if n := g.Get(); n != 0 {
+		t.Fatalf("unexpected gauge value %g; want 0", n)
+	}
+}
+
 func TestGaugeSerial(t *testing.T) {
 	name := "GaugeSerial"
 	n := 1.23