package metrics

import (
	"fmt"
	"io"
)

// NewGauge registers and returns gauge with the given name, which calls f
// to obtain gauge value.
//
// name must be valid Prometheus-compatible metric with possible labels.
// For instance,
//
//     * foo
//     * foo{bar="baz"}
//     * foo{bar="baz",aaa="b"}
//
// f must be safe for concurrent calls.
//
// The returned gauge is safe to use from concurrent goroutines.
func NewGauge(name string, f func() float64) *Gauge {
	g := &Gauge{
		f: f,
	}
	registerMetric(name, g)
	return g
}

// Gauge is a float64 gauge.
type Gauge struct {
	f func() float64
}

// Get returns the current value for g.
func (g *Gauge) Get() float64 {
	return g.f()
}

func (g *Gauge) marshalTo(prefix string, w io.Writer) {
	v := g.f()
	if float64(int64(v)) == v {
		// Marshal integer values without scientific notations
		fmt.Fprintf(w, "%s %d\n", prefix, int64(v))
	} else {
		fmt.Fprintf(w, "%s %g\n", prefix, v)
	}
}