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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))
}