// Copyright 2016 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package prometheus

import "time"

// Observer is the interface that wraps the Observe method, which is used by
// Histogram and Summary to add observations.
type Observer interface {
	Observe(float64)
}

// The ObserverFunc type is an adapter to allow the use of ordinary
// functions as Observers. If f is a function with the appropriate
// signature, ObserverFunc(f) is an Observer that calls f.
//
// This adapter is usually used in connection with the Timer type, and there are
// two general use cases:
//
// The most common one is to use a Gauge as the Observer for a Timer.
// See the "Gauge" Timer example.
//
// The more advanced use case is to create a function that dynamically decides
// which Observer to use for observing the duration. See the "Complex" Timer
// example.
type ObserverFunc func(float64)

// Observe calls f(value). It implements Observer.
func (f ObserverFunc) Observe(value float64) {
	f(value)
}

// Timer is a helper type to time functions. Use NewTimer to create new
// instances.
type Timer struct {
	begin    time.Time
	observer Observer
}

// NewTimer creates a new Timer. The provided Observer is used to observe a
// duration in seconds. Timer is usually used to time a function call in the
// following way:
//    func TimeMe() {
//        timer := NewTimer(myHistogram)
//        defer timer.ObserveDuration()
//        // Do actual work.
//    }
func NewTimer(o Observer) *Timer {
	return &Timer{
		begin:    time.Now(),
		observer: o,
	}
}

// ObserveDuration records the duration passed since the Timer was created with
// NewTimer. It calls the Observe method of the Observer provided during
// construction with the duration in seconds as an argument. ObserveDuration is
// usually called with a defer statement.
func (t *Timer) ObserveDuration() {
	if t.observer != nil {
		t.observer.Observe(time.Since(t.begin).Seconds())
	}
}