// 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)
}
