// Copyright 2014 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 (

	"errors"

	"fmt"

	"sync"

	"time"

)

// An emptyCtx is never canceled, has no values, and has no deadline. It is not

// struct{}, since vars of this type must have distinct addresses.

type emptyCtx int

func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {

	return

}

func (*emptyCtx) Done() <-chan struct{} {

	return nil

}

func (*emptyCtx) Err() error {

	return nil

}

func (*emptyCtx) Value(key interface{}) interface{} {

	return nil

}

func (e *emptyCtx) String() string {

	switch e {

	case background:

		return "context.Background"

	case todo:

		return "context.TODO"

	}

	return "unknown empty Context"

}

var (

	background = new(emptyCtx)

	todo       = new(emptyCtx)

)

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

var Canceled = errors.New("context canceled")

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

// deadline passes.

var DeadlineExceeded = errors.New("context deadline exceeded")

// 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) {

	c := newCancelCtx(parent)

	propagateCancel(parent, c)

	return c, func() { c.cancel(true, Canceled) }

}

// newCancelCtx returns an initialized cancelCtx.

func newCancelCtx(parent Context) *cancelCtx {

	return &cancelCtx{

		Context: parent,

		done:    make(chan struct{}),

	}

}

// propagateCancel arranges for child to be canceled when parent is.

func propagateCancel(parent Context, child canceler) {

	if parent.Done() == nil {

		return // parent is never canceled

	}

	if p, ok := parentCancelCtx(parent); ok {

		p.mu.Lock()

		if p.err != nil {

			// parent has already been canceled

			child.cancel(false, p.err)

		} else {

			if p.children == nil {

				p.children = make(map[canceler]bool)

			}

			p.children[child] = true

		}

		p.mu.Unlock()

	} else {

		go func() {

			select {

			case <-parent.Done():

				child.cancel(false, parent.Err())

			case <-child.Done():

			}

		}()

	}

}

// parentCancelCtx follows a chain of parent references until it finds a

// *cancelCtx. This function understands how each of the concrete types in this

// package represents its parent.

func parentCancelCtx(parent Context) (*cancelCtx, bool) {

	for {

		switch c := parent.(type) {

		case *cancelCtx:

			return c, true

		case *timerCtx:

			return c.cancelCtx, true

		case *valueCtx:

			parent = c.Context

		default:

			return nil, false

		}

	}

}

// removeChild removes a context from its parent.

func removeChild(parent Context, child canceler) {

	p, ok := parentCancelCtx(parent)

	if !ok {

		return

	}

	p.mu.Lock()

	if p.children != nil {

		delete(p.children, child)

	}

	p.mu.Unlock()

}

// A canceler is a context type that can be canceled directly. The

// implementations are *cancelCtx and *timerCtx.

type canceler interface {

	cancel(removeFromParent bool, err error)

	Done() <-chan struct{}

}

// A cancelCtx can be canceled. When canceled, it also cancels any children

// that implement canceler.

type cancelCtx struct {

	Context

	done chan struct{} // closed by the first cancel call.

	mu       sync.Mutex

	children map[canceler]bool // set to nil by the first cancel call

	err      error             // set to non-nil by the first cancel call

}

func (c *cancelCtx) Done() <-chan struct{} {

	return c.done

}

func (c *cancelCtx) Err() error {

	c.mu.Lock()

	defer c.mu.Unlock()

	return c.err

}

func (c *cancelCtx) String() string {

	return fmt.Sprintf("%v.WithCancel", c.Context)

}

// cancel closes c.done, cancels each of c's children, and, if

// removeFromParent is true, removes c from its parent's children.

func (c *cancelCtx) cancel(removeFromParent bool, err error) {

	if err == nil {

		panic("context: internal error: missing cancel error")

	}

	c.mu.Lock()

	if c.err != nil {

		c.mu.Unlock()

		return // already canceled

	}

	c.err = err

	close(c.done)

	for child := range c.children {

		// NOTE: acquiring the child's lock while holding parent's lock.

		child.cancel(false, err)

	}

	c.children = nil

	c.mu.Unlock()

	if removeFromParent {

		removeChild(c.Context, c)

	}

}

// 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) {

	if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {

		// The current deadline is already sooner than the new one.

		return WithCancel(parent)

	}

	c := &timerCtx{

		cancelCtx: newCancelCtx(parent),

		deadline:  deadline,

	}

	propagateCancel(parent, c)

	d := deadline.Sub(time.Now())

	if d <= 0 {

		c.cancel(true, DeadlineExceeded) // deadline has already passed

		return c, func() { c.cancel(true, Canceled) }

	}

	c.mu.Lock()

	defer c.mu.Unlock()

	if c.err == nil {

		c.timer = time.AfterFunc(d, func() {

			c.cancel(true, DeadlineExceeded)

		})

	}

	return c, func() { c.cancel(true, Canceled) }

}

// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to

// implement Done and Err. It implements cancel by stopping its timer then

// delegating to cancelCtx.cancel.

type timerCtx struct {

	*cancelCtx

	timer *time.Timer // Under cancelCtx.mu.

	deadline time.Time

}

func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {

	return c.deadline, true

}

func (c *timerCtx) String() string {

	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, c.deadline.Sub(time.Now()))

}

func (c *timerCtx) cancel(removeFromParent bool, err error) {

	c.cancelCtx.cancel(false, err)

	if removeFromParent {

		// Remove this timerCtx from its parent cancelCtx's children.

		removeChild(c.cancelCtx.Context, c)

	}

	c.mu.Lock()

	if c.timer != nil {

		c.timer.Stop()

		c.timer = nil

	}

	c.mu.Unlock()

}

// 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 &valueCtx{parent, key, val}

}

// A valueCtx carries a key-value pair. It implements Value for that key and

// delegates all other calls to the embedded Context.

type valueCtx struct {

	Context

	key, val interface{}

}

func (c *valueCtx) String() string {

	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)

}

func (c *valueCtx) Value(key interface{}) interface{} {

	if c.key == key {

		return c.val

	}

	return c.Context.Value(key)

}