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// Copyright 2009 The Go Authors. All rights reserved.

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

// Package expvar provides a standardized interface to public variables, such

// as operation counters in servers. It exposes these variables via HTTP at

// /debug/vars in JSON format.

//

// Operations to set or modify these public variables are atomic.

//

// In addition to adding the HTTP handler, this package registers the

// following variables:

//

//      cmdline   os.Args

//      memstats  runtime.Memstats

//

// The package is sometimes only imported for the side effect of

// registering its HTTP handler and the above variables.  To use it

// this way, link this package into your program:

//      import _ "expvar"

//

package expvar

import (

        "bytes"

        "encoding/json"

        "fmt"

        "log"

        "net/http"

        "os"

        "runtime"

        "strconv"

        "sync"

)

// Var is an abstract type for all exported variables.

type Var interface {

        String() string

}

// Int is a 64-bit integer variable that satisfies the Var interface.

type Int struct {

        i  int64

        mu sync.Mutex

}

func (v *Int) String() string { return strconv.FormatInt(v.i, 10) }

func (v *Int) Add(delta int64) {

        v.mu.Lock()

        defer v.mu.Unlock()

        v.i += delta

}

func (v *Int) Set(value int64) {

        v.mu.Lock()

        defer v.mu.Unlock()

        v.i = value

}

// Float is a 64-bit float variable that satisfies the Var interface.

type Float struct {

        f  float64

        mu sync.Mutex

}

func (v *Float) String() string { return strconv.FormatFloat(v.f, 'g', -1, 64) }

// Add adds delta to v.

func (v *Float) Add(delta float64) {

        v.mu.Lock()

        defer v.mu.Unlock()

        v.f += delta

}

// Set sets v to value.

func (v *Float) Set(value float64) {

        v.mu.Lock()

        defer v.mu.Unlock()

        v.f = value

}

// Map is a string-to-Var map variable that satisfies the Var interface.

type Map struct {

        m  map[string]Var

        mu sync.RWMutex

}

// KeyValue represents a single entry in a Map.

type KeyValue struct {

        Key   string

        Value Var

}

func (v *Map) String() string {

        v.mu.RLock()

        defer v.mu.RUnlock()

        b := new(bytes.Buffer)

        fmt.Fprintf(b, "{")

        first := true

        for key, val := range v.m {

                if !first {

                        fmt.Fprintf(b, ", ")

                }

                fmt.Fprintf(b, "\"%s\": %v", key, val)

                first = false

        }

        fmt.Fprintf(b, "}")

        return b.String()

}

func (v *Map) Init() *Map {

        v.m = make(map[string]Var)

        return v

}

func (v *Map) Get(key string) Var {

        v.mu.RLock()

        defer v.mu.RUnlock()

        return v.m[key]

}

func (v *Map) Set(key string, av Var) {

        v.mu.Lock()

        defer v.mu.Unlock()

        v.m[key] = av

}

func (v *Map) Add(key string, delta int64) {

        v.mu.RLock()

        av, ok := v.m[key]

        v.mu.RUnlock()

        if !ok {

                // check again under the write lock

                v.mu.Lock()

                if _, ok = v.m[key]; !ok {

                        av = new(Int)

                        v.m[key] = av

                }

                v.mu.Unlock()

        }

        // Add to Int; ignore otherwise.

        if iv, ok := av.(*Int); ok {

                iv.Add(delta)

        }

}

// AddFloat adds delta to the *Float value stored under the given map key.

func (v *Map) AddFloat(key string, delta float64) {

        v.mu.RLock()

        av, ok := v.m[key]

        v.mu.RUnlock()

        if !ok {

                // check again under the write lock

                v.mu.Lock()

                if _, ok = v.m[key]; !ok {

                        av = new(Float)

                        v.m[key] = av

                }

                v.mu.Unlock()

        }

        // Add to Float; ignore otherwise.

        if iv, ok := av.(*Float); ok {

                iv.Add(delta)

        }

}

// Do calls f for each entry in the map.

// The map is locked during the iteration,

// but existing entries may be concurrently updated.

func (v *Map) Do(f func(KeyValue)) {

        v.mu.RLock()

        defer v.mu.RUnlock()

        for k, v := range v.m {

                f(KeyValue{k, v})

        }

}

// String is a string variable, and satisfies the Var interface.

type String struct {

        s string

}

func (v *String) String() string { return strconv.Quote(v.s) }

func (v *String) Set(value string) { v.s = value }

// Func implements Var by calling the function

// and formatting the returned value using JSON.

type Func func() interface{}

func (f Func) String() string {

        v, _ := json.Marshal(f())

        return string(v)

}

// All published variables.

var (

        mutex sync.RWMutex

        vars  map[string]Var = make(map[string]Var)

)

// Publish declares a named exported variable. This should be called from a

// package's init function when it creates its Vars. If the name is already

// registered then this will log.Panic.

func Publish(name string, v Var) {

        mutex.Lock()

        defer mutex.Unlock()

        if _, existing := vars[name]; existing {

                log.Panicln("Reuse of exported var name:", name)

        }

        vars[name] = v

}

// Get retrieves a named exported variable.

func Get(name string) Var {

        mutex.RLock()

        defer mutex.RUnlock()

        return vars[name]

}

// Convenience functions for creating new exported variables.

func NewInt(name string) *Int {

        v := new(Int)

        Publish(name, v)

        return v

}

func NewFloat(name string) *Float {

        v := new(Float)

        Publish(name, v)

        return v

}

func NewMap(name string) *Map {

        v := new(Map).Init()

        Publish(name, v)

        return v

}

func NewString(name string) *String {

        v := new(String)

        Publish(name, v)

        return v

}

// 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(KeyValue)) {

        mutex.RLock()

        defer mutex.RUnlock()

        for k, v := range vars {

                f(KeyValue{k, v})

        }

}

func expvarHandler(w http.ResponseWriter, r *http.Request) {

        w.Header().Set("Content-Type", "application/json; charset=utf-8")

        fmt.Fprintf(w, "{\n")

        first := true

        Do(func(kv KeyValue) {

                if !first {

                        fmt.Fprintf(w, ",\n")

                }

                first = false

                fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)

        })

        fmt.Fprintf(w, "\n}\n")

}

func cmdline() interface{} {

        return os.Args

}

func memstats() interface{} {

        stats := new(runtime.MemStats)

        runtime.ReadMemStats(stats)

        return *stats

}

func init() {

        http.HandleFunc("/debug/vars", expvarHandler)

        Publish("cmdline", Func(cmdline))

        Publish("memstats", Func(memstats))

}