// Copyright 2016 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 go1.7

package context

import (

	"context" // standard library's context, as of Go 1.7

	"time"

)

var (

	todo       = context.TODO()

	background = context.Background()

)

// Canceled is the error returned by Context.Err when the context is canceled.

var Canceled = context.Canceled

// DeadlineExceeded is the error returned by Context.Err when the context's

// deadline passes.

var DeadlineExceeded = context.DeadlineExceeded

// WithCancel returns a copy of parent with a new Done channel. The returned

// context's Done channel is closed when the returned cancel function is called

// or when the parent context's Done channel is closed, whichever happens first.

//

// Canceling this context releases resources associated with it, so code should

// call cancel as soon as the operations running in this Context complete.

func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {

	ctx, f := context.WithCancel(parent)

	return ctx, CancelFunc(f)

}

// WithDeadline returns a copy of the parent context with the deadline adjusted

// to be no later than d. If the parent's deadline is already earlier than d,

// WithDeadline(parent, d) is semantically equivalent to parent. The returned

// context's Done channel is closed when the deadline expires, when the returned

// cancel function is called, or when the parent context's Done channel is

// closed, whichever happens first.

//

// Canceling this context releases resources associated with it, so code should

// call cancel as soon as the operations running in this Context complete.

func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {

	ctx, f := context.WithDeadline(parent, deadline)

	return ctx, CancelFunc(f)

}

// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).

//

// Canceling this context releases resources associated with it, so code should

// call cancel as soon as the operations running in this Context complete:

//

// 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {

// 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)

// 		defer cancel()  // releases resources if slowOperation completes before timeout elapses

// 		return slowOperation(ctx)

// 	}

func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {

	return WithDeadline(parent, time.Now().Add(timeout))

}

// WithValue returns a copy of parent in which the value associated with key is

// val.

//

// Use context Values only for request-scoped data that transits processes and

// APIs, not for passing optional parameters to functions.

func WithValue(parent Context, key interface{}, val interface{}) Context {

	return context.WithValue(parent, key, val)

}