// Copyright 2019 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.

// +build !disable_events

package event

import (
	"context"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"
)

// Exporter synchronizes the delivery of events to handlers.
type Exporter struct {
	Now func() time.Time

	mu        sync.Mutex
	handler   Handler
	lastEvent uint64
}

// contextKey is used as the key for storing a contextValue on the context.
type contextKeyType struct{}

var contextKey interface{} = contextKeyType{}

// contextValue is stored by value in the context to track the exporter and
// current parent event.
type contextValue struct {
	exporter *Exporter
	parent   uint64
}

var (
	defaultExporter unsafe.Pointer
)

// NewExporter creates an Exporter using the supplied handler.
// Event delivery is serialized to enable safe atomic handling.
func NewExporter(h Handler) *Exporter {
	return &Exporter{
		Now:     time.Now,
		handler: h,
	}
}

func setDefaultExporter(e *Exporter) {
	atomic.StorePointer(&defaultExporter, unsafe.Pointer(e))
}

func getDefaultExporter() *Exporter {
	return (*Exporter)(atomic.LoadPointer(&defaultExporter))
}

func newContext(ctx context.Context, exporter *Exporter, parent uint64) context.Context {
	return context.WithValue(ctx, contextKey, contextValue{exporter: exporter, parent: parent})
}

func fromContext(ctx context.Context) (*Exporter, uint64) {
	if v, ok := ctx.Value(contextKey).(contextValue); ok {
		return v.exporter, v.parent
	}
	return getDefaultExporter(), 0
}

// prepare events before delivering to the underlying handler.
// The event will be assigned a new ID.
// If the event does not have a timestamp, and the exporter has a Now function
// then the timestamp will be updated.
// prepare must be called with the export mutex held.
func (e *Exporter) prepare(ev *Event) {
	e.lastEvent++
	id := e.lastEvent
	ev.ID = id
	if e.Now != nil && ev.At.IsZero() {
		ev.At = e.Now()
	}
}