A struct to hold a number of current and peak listeners was added.

internal/radio/listeners.go

@@ -0,0 +1,50 @@
+package radio
+
+import (
+	"errors"
+	"fmt"
+	"math"
+)
+
+// Listeners stores the current and peak numbers of listeners.
+type Listeners struct {
+	current, peak int
+}
+
+// NewListeners returns a new Listeners struct instance.
+func NewListeners() *Listeners {
+	return &Listeners{}
+}
+
+// Current returns a number of current listeners.
+func (l *Listeners) Current() int {
+	return l.current
+}
+
+// Current returns a number of peak listeners.
+func (l *Listeners) Peak() int {
+	return l.peak
+}
+
+// Inc increments by 1 a current number of listeners and updates a peak number.
+func (l *Listeners) Inc() int {
+	if l.current == math.MaxInt {
+		// We panic here because if this will ever happen, then something's going certainly wrong.
+		panic(fmt.Sprint("a current number of listeners exceeded MaxInt which is", math.MaxInt))
+	}
+	l.current++
+	if l.current > l.peak {
+		l.peak = l.current
+	}
+	return l.current
+}
+
+// Dec decrements by 1 a current number of listeners. An error will occur if
+// a resulting number is less than 0.
+func (l *Listeners) Dec() (int, error) {
+	if l.current == 0 {
+		return l.current, errors.New("an attempt to decrement a number of current listeners down to less than 0")
+	}
+	l.current--
+	return l.current, nil
+}