// Package cookie implements a specialized version of a context
// - It is cancellable
// - It has a channel associated with it to reply to state callbacks
// - It can be marshalled by having a cgo Handle attached to it
package cookie

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"runtime/cgo"
)

// Cookie is the cookie which is just a context with some other data associated with it
// We could potentially only uses contexts with values, but this is nicer for type checking
type Cookie struct {
	c         chan string
	ctx       context.Context
	ctxCancel context.CancelFunc
	H         cgo.Handle
}

// contextt is the context type for the value
type contextt int8

// CONTEXTK is the key of the cookie
const CONTEXTK contextt = 0

// NewWithContext creates a new cookie with a context
// It stores the cancel and channel inside of the struct
func NewWithContext(ctx context.Context) *Cookie {
	// if the context already has a handle, return that cookie
	if h, ok := ctx.Value(CONTEXTK).(cgo.Handle); ok {
		if ck, ok := h.Value().(*Cookie); ok {
			return ck
		}
	}
	ctx, cancel := context.WithCancel(ctx)
	return &Cookie{
		c:         make(chan string),
		ctx:       ctx,
		ctxCancel: cancel,
	}
}

// MarshalJSON marshals the cookie to JSON
func (c *Cookie) MarshalJSON() ([]byte, error) {
	if c.H == 0 {
		return nil, errors.New("no associated handle found")
	}
	return json.Marshal(c.H)
}

// Receive receives a value from the cookie up until the context is done or errchan gets an error
// This error chan is used for goroutines to signal errors that we have to exit early
func (c *Cookie) Receive(errchan chan error) (string, error) {
	select {
	case r := <-c.c:
		return r, nil
	case e := <-errchan:
		return "", e
	case <-c.ctx.Done():
		return "", fmt.Errorf("receive cookie done: %w", context.Canceled)
	}
}

// Cancel cancels the cookie by calling the context cancel function
// It returns an error if no such function exists
func (c *Cookie) Cancel() error {
	if c.ctxCancel == nil {
		return errors.New("no cancel function found")
	}
	c.ctxCancel()
	return nil
}

// Send sends data to the cookie channel if the context is not canceled
func (c *Cookie) Send(data string) error {
	select {
	case <-c.ctx.Done():
		return fmt.Errorf("send cookie done: %w", context.Canceled)
	default:
		if c.c == nil {
			return errors.New("channel is nil")
		}
		c.c <- data
		return nil
	}
}

// Context gets the underlying context of the cookie
func (c *Cookie) Context() context.Context {
	return context.WithValue(c.ctx, CONTEXTK, c.H)
}