/*
   Copyright 2014 CoreOS, Inc.

   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 metrics provides metrics view of variables which is exposed through
// expvar package.
//
// Naming conventions:
// 1. volatile path components should be kept as deep into the hierarchy as possible
// 2. each path component should have a clear and well-defined purpose
// 3. components.separated.with.dot, and put package prefix at the head
// 4. words_separated_with_underscore, and put clarifiers last, e.g., requests_total
package metrics

import (
	"bytes"
	"expvar"
	"fmt"
	"sort"
	"sync"
)

// Counter is a number that increases over time monotonically.
type Counter struct{ i *expvar.Int }

func (c *Counter) Add() { c.i.Add(1) }

func (c *Counter) AddBy(delta int64) { c.i.Add(delta) }

func (c *Counter) String() string { return c.i.String() }

// Gauge returns instantaneous value that is expected to fluctuate over time.
type Gauge struct{ i *expvar.Int }

func (g *Gauge) Set(value int64) { g.i.Set(value) }

func (g *Gauge) String() string { return g.i.String() }

type nilVar struct{}

func (v *nilVar) String() string { return "nil" }

// Map aggregates Counters and Gauges.
type Map struct{ *expvar.Map }

func (m *Map) NewCounter(key string) *Counter {
	c := &Counter{i: new(expvar.Int)}
	m.Set(key, c)
	return c
}

func (m *Map) NewGauge(key string) *Gauge {
	g := &Gauge{i: new(expvar.Int)}
	m.Set(key, g)
	return g
}

// TODO: remove the var from the map to avoid memory boom
func (m *Map) Delete(key string) { m.Set(key, &nilVar{}) }

// String returns JSON format string that represents the group.
// It does not print out nilVar.
func (m *Map) String() string {
	var b bytes.Buffer
	fmt.Fprintf(&b, "{")
	first := true
	m.Do(func(kv expvar.KeyValue) {
		v := kv.Value.String()
		if v == "nil" {
			return
		}
		if !first {
			fmt.Fprintf(&b, ", ")
		}
		fmt.Fprintf(&b, "%q: %v", kv.Key, v)
		first = false
	})
	fmt.Fprintf(&b, "}")
	return b.String()
}

// All published variables.
var (
	mutex   sync.RWMutex
	vars    = make(map[string]expvar.Var)
	varKeys []string // sorted
)

// Publish declares a named exported variable.
// If the name is already registered then this will overwrite the old one.
func Publish(name string, v expvar.Var) {
	mutex.Lock()
	defer mutex.Unlock()
	if _, existing := vars[name]; !existing {
		varKeys = append(varKeys, name)
	}
	sort.Strings(varKeys)
	vars[name] = v
	return
}

// Get retrieves a named exported variable.
func Get(name string) expvar.Var {
	mutex.RLock()
	defer mutex.RUnlock()
	return vars[name]
}

// Convenience functions for creating new exported variables.
func NewCounter(name string) *Counter {
	c := &Counter{i: new(expvar.Int)}
	Publish(name, c)
	return c
}

func NewGauge(name string) *Gauge {
	g := &Gauge{i: new(expvar.Int)}
	Publish(name, g)
	return g
}

func NewMap(name string) *Map {
	m := &Map{Map: new(expvar.Map).Init()}
	Publish(name, m)
	return m
}

// GetMap returns the map if it exists, or inits the given name map if it does
// not exist.
func GetMap(name string) *Map {
	v := Get(name)
	if v == nil {
		return NewMap(name)
	}
	return v.(*Map)
}

// Do calls f for each exported variable.
// The global variable map is locked during the iteration,
// but existing entries may be concurrently updated.
func Do(f func(expvar.KeyValue)) {
	mutex.RLock()
	defer mutex.RUnlock()
	for _, k := range varKeys {
		f(expvar.KeyValue{k, vars[k]})
	}
}

// for test only
func reset() {
	vars = make(map[string]expvar.Var)
	varKeys = nil
}