package multicast

import (
	"fmt"
	"math"
	"runtime"
	"sync"
	"sync/atomic"
	"time"
)

const _PADDING = 1	// 0 turns padding off, 1 turns it on.

const _EXTRA_PADDING = 0 * 64	// multiples of 64, benefits inconclusive.

type pad60 [_PADDING * (_EXTRA_PADDING + 60)]byte

type pad56 [_PADDING * (_EXTRA_PADDING + 56)]byte

type pad48 [_PADDING * (_EXTRA_PADDING + 48)]byte

type pad40 [_PADDING * (_EXTRA_PADDING + 40)]byte

type pad32 [_PADDING * (_EXTRA_PADDING + 32)]byte

// Activity of committer
const (
	resting	uint32	= iota
	working
)

// Activity of endpoints
const (
	idling	uint32	= iota
	enumerating
	creating
)

// State of endpoint and channel
const (
	active	uint64	= iota
	canceled
	closed
)

// Cursor is parked so it does not influence advancing the commit index.
const (
	parked uint64 = math.MaxUint64
)

const (
	// ReplayAll can be passed to NewEndpoint to retain as many of the
	// previously sent messages as possible that are still in the buffer.
	ReplayAll uint64 = math.MaxUint64
)

// Chan is a fast, concurrent multi-(casting,sending,receiving) buffered
// channel. It is implemented using only sync/atomic operations. Spinlocks using
// runtime.Gosched() are used in situations where goroutines are waiting or
// contending for resources.
type Chan[T any] struct {
	buffer		[]T
	_________a	pad40
	begin		uint64
	_________b	pad56
	end		uint64
	_________c	pad56
	commit		uint64
	_________d	pad56
	mod		uint64
	_________e	pad56
	endpoints	endpoints[T]

	err		error
	____________f	pad48
	channelState	uint64	// active, closed
	____________g	pad56

	write			uint64
	_________________h	pad56
	start			time.Time
	_________________i	pad40
	written			[]int64	// nanoseconds since start
	_________________j	pad40
	committerActivity	uint32	// resting, working
	_________________k	pad60

	receivers		*sync.Cond
	_________________l	pad56
}

type endpoints[T any] struct {
	entry			[]Endpoint[T]
	len			uint32
	endpointsActivity	uint32	// idling, enumerating, creating
	________		pad32
}

type ChannelError string

func (e ChannelError) Error() string	{ return string(e) }

// ErrOutOfEndpoints is returned by NewEndpoint when the maximum number of
// endpoints has already been created.
const ErrOutOfEndpoints = ChannelError("out of endpoints")

func (e *endpoints[T]) NewForChan(c *Chan[T], keep uint64) (*Endpoint[T], error) {
	for !atomic.CompareAndSwapUint32(&e.endpointsActivity, idling, creating) {
		runtime.Gosched()
	}
	defer atomic.StoreUint32(&e.endpointsActivity, idling)
	var start uint64
	commit := c.commitData()
	begin := atomic.LoadUint64(&c.begin)
	if commit-begin <= keep {
		start = begin
	} else {
		start = commit - keep
	}
	if int(e.len) == len(e.entry) {
		for index := uint32(0); index < e.len; index++ {
			ep := &e.entry[index]
			if atomic.CompareAndSwapUint64(&ep.cursor, parked, start) {
				ep.endpointState = atomic.LoadUint64(&c.channelState)
				ep.lastActive = time.Now()
				return ep, nil
			}
		}
		return nil, ErrOutOfEndpoints
	}
	ep := &e.entry[e.len]
	ep.Chan = c
	ep.cursor = start
	ep.endpointState = atomic.LoadUint64(&c.channelState)
	ep.lastActive = time.Now()
	e.len++
	return ep, nil
}

func (e *endpoints[T]) Access(access func(*endpoints[T])) bool {
	contention := false
	for !atomic.CompareAndSwapUint32(&e.endpointsActivity, idling, enumerating) {
		runtime.Gosched()
		contention = true
	}
	access(e)
	atomic.StoreUint32(&e.endpointsActivity, idling)
	return !contention
}

// NewChan creates a new channel. The parameters bufferCapacity and
// endpointCapacity determine the size of the message buffer and maximum
// number of concurrent receiving endpoints respectively.
//
// Note that bufferCapacity is always scaled up to a power of 2 so e.g.
// specifying 400 will create a buffer of 512 (2^9). Also because of this a
// bufferCapacity of 0 is scaled up to 1 (2^0).
func NewChan[T any](bufferCapacity int, endpointCapacity int) *Chan[T] {
	size := uint64(1) << uint(math.Ceil(math.Log2(float64(bufferCapacity))))
	c := &Chan[T]{
		end:		size,
		mod:		size - 1,
		buffer:		make([]T, size),
		start:		time.Now(),
		written:	make([]int64, size),
		endpoints: endpoints[T]{
			entry: make([]Endpoint[T], endpointCapacity),
		},
	}
	c.receivers = sync.NewCond(c)
	return c
}

// Lock, empty method so we can pass *Chan to sync.NewCond as a Locker.
func (c *Chan[T]) Lock()	{}

// Unlock, empty method so we can pass *Chan to sync.NewCond as a Locker.
func (c *Chan[T]) Unlock()	{}

// Endpoint is returned by a call to NewEndpoint on the channel. Every
// endpoint should be used by only a single goroutine, so no sharing between
// goroutines.
type Endpoint[T any] struct {
	*Chan[T]
	_____________a	pad56
	cursor		uint64
	_____________b	pad56
	endpointState	uint64	// active, canceled, closed
	_____________c	pad56
	lastActive	time.Time	// track activity to deterime when to sleep
	_____________d	pad40
	endpointClosed	uint64	// active, closed
	_____________e	pad56
}

func (c *Chan[T]) commitData() uint64 {
	commit := atomic.LoadUint64(&c.commit)
	if commit >= atomic.LoadUint64(&c.write) {
		return commit
	}
	if !atomic.CompareAndSwapUint32(&c.committerActivity, resting, working) {
		return commit
	}
	commit = atomic.LoadUint64(&c.commit)
	newcommit := commit
	for ; atomic.LoadInt64(&c.written[newcommit&c.mod])&1 == 1; newcommit++ {
		atomic.AddInt64(&c.written[newcommit&c.mod], -1)
		if newcommit >= atomic.LoadUint64(&c.end) {
			break
		}
	}
	write := atomic.LoadUint64(&c.write)
	if newcommit > write {
		panic(fmt.Sprintf("commitData: range error (commit=%d,write=%d,newcommit=%d)", commit, write, newcommit))
	}
	if newcommit > commit {
		if !atomic.CompareAndSwapUint64(&c.commit, commit, newcommit) {
			panic(fmt.Sprintf("commitData; swap error (c.commit=%d,%d,%d)", c.commit, commit, newcommit))
		}
		c.receivers.Broadcast()
	}
	atomic.StoreUint32(&c.committerActivity, resting)
	return atomic.LoadUint64(&c.commit)
}

func (c *Chan[T]) slideBuffer() bool {
	slowestCursor := parked
	spinlock := c.endpoints.Access(func(endpoints *endpoints[T]) {
		for i := uint32(0); i < endpoints.len; i++ {
			cursor := atomic.LoadUint64(&endpoints.entry[i].cursor)
			if cursor < slowestCursor {
				slowestCursor = cursor
			}
		}
		if atomic.LoadUint64(&c.begin) < slowestCursor && slowestCursor <= atomic.LoadUint64(&c.end) {
			if c.mod < 16 {
				atomic.AddUint64(&c.begin, 1)
				atomic.AddUint64(&c.end, 1)
			} else {
				atomic.StoreUint64(&c.begin, slowestCursor)
				atomic.StoreUint64(&c.end, slowestCursor+c.mod+1)
			}
		} else {
			slowestCursor = parked
		}
	})
	if slowestCursor == parked {
		if spinlock {
			runtime.Gosched()
		}
		if atomic.LoadUint64(&c.channelState) != active {
			return false
		}
	}
	return true
}

// FastSend can be used to send values to the channel from a SINGLE goroutine.
// Also, this does not record the time a message was sent, so the maxAge value
// passed to Range will be ignored.
//
// Note, that when the number of unread messages has reached bufferCapacity, then
// the call to FastSend will block until the slowest Endpoint has read another
// message.
func (c *Chan[T]) FastSend(value T) {
	for c.commit == c.end {
		if !c.slideBuffer() {
			return
		}
	}
	c.buffer[c.commit&c.mod] = value
	atomic.AddUint64(&c.commit, 1)
	c.receivers.Broadcast()
}

// Send can be used by concurrent goroutines to send values to the channel.
//
// Note, that when the number of unread messages has reached bufferCapacity, then
// the call to Send will block until the slowest Endpoint has read another
// message.
func (c *Chan[T]) Send(value T) {
	write := atomic.AddUint64(&c.write, 1) - 1
	for write >= atomic.LoadUint64(&c.end) {
		if !c.slideBuffer() {
			return
		}
	}
	c.buffer[write&c.mod] = value
	updated := time.Since(c.start).Nanoseconds()
	if updated == 0 {
		panic("clock failure; zero duration measured")
	}
	atomic.StoreInt64(&c.written[write&c.mod], updated<<1+1)
	c.receivers.Broadcast()
}

// Close will close the channel. Pass in an error or nil. Endpoints  continue to
// receive data until the buffer is empty. Only then will the close notification
// be delivered to the Range function.
func (c *Chan[T]) Close(err error) {
	if atomic.CompareAndSwapUint64(&c.channelState, active, closed) {
		c.err = err
		c.endpoints.Access(func(endpoints *endpoints[T]) {
			for i := uint32(0); i < endpoints.len; i++ {
				atomic.CompareAndSwapUint64(&endpoints.entry[i].endpointState, active, closed)
			}
		})
	}
	c.receivers.Broadcast()
}

// Closed returns true when the channel was closed using the Close method.
func (c *Chan[T]) Closed() bool {
	return atomic.LoadUint64(&c.channelState) >= closed
}

// NewEndpoint will create a new channel endpoint that can be used to receive
// from the channel. The argument keep specifies how many entries of the
// existing channel buffer to keep.
//
// After Close is called on the channel, any endpoints created after that
// will still receive the number of messages as indicated in the keep parameter
// and then subsequently the close.
//
// An endpoint that is canceled or read until it is exhausted (after channel was
// closed) will be reused by NewEndpoint.
func (c *Chan[T]) NewEndpoint(keep uint64) (*Endpoint[T], error) {
	return c.endpoints.NewForChan(c, keep)
}

// Range will call the passed in foreach function with all the messages in
// the buffer, followed by all the messages received. When the foreach function
// returns true Range will continue, when you return false this is the same as
// calling Cancel. When canceled the foreach will never be called again.
// Passing a maxAge duration other than 0 will skip messages that are older
// than maxAge.
//
// When the channel is closed, eventually when the buffer is exhausted the close
// with optional error will be notified by calling foreach one last time with
// the closed parameter set to true.
func (e *Endpoint[T]) Range(next func(value T, closed bool) bool, err func(err error, closed bool) bool, maxAge time.Duration) {
	e.lastActive = time.Now()
	for {
		commit := e.commitData()
		for ; e.cursor == commit; commit = e.commitData() {
			if atomic.CompareAndSwapUint64(&e.endpointState, canceled, canceled) {
				atomic.StoreUint64(&e.cursor, parked)
				return
			}
			if atomic.LoadUint64(&e.commit) < atomic.LoadUint64(&e.write) {
				if e.endpointClosed == 1 {
					panic(fmt.Sprintf("data written after closing endpoint; commit(%d) write(%d)",
						atomic.LoadUint64(&e.commit), atomic.LoadUint64(&e.write)))
				}
				runtime.Gosched()
				e.lastActive = time.Now()
			} else {
				now := time.Now()
				if now.Before(e.lastActive.Add(1 * time.Millisecond)) {
					if atomic.CompareAndSwapUint64(&e.endpointState, closed, closed) {
						e.endpointClosed = 1
					}
					runtime.Gosched()
				} else if now.Before(e.lastActive.Add(250 * time.Millisecond)) {
					if atomic.CompareAndSwapUint64(&e.endpointState, closed, closed) {
						err(e.err, true)
						atomic.StoreUint64(&e.cursor, parked)
						return
					}
					runtime.Gosched()
				} else {
					e.receivers.Wait()
					e.lastActive = time.Now()
				}
			}
		}

		for ; e.cursor != commit; atomic.AddUint64(&e.cursor, 1) {
			item := e.buffer[e.cursor&e.mod]
			emit := true
			if maxAge != 0 {
				stale := time.Since(e.start).Nanoseconds() - maxAge.Nanoseconds()
				updated := atomic.LoadInt64(&e.written[e.cursor&e.mod]) >> 1
				if updated != 0 && updated <= stale {
					emit = false
				}
			}
			if emit && !next(item, false) {
				atomic.StoreUint64(&e.endpointState, canceled)
			}
			if atomic.LoadUint64(&e.endpointState) == canceled {
				atomic.StoreUint64(&e.cursor, parked)
				return
			}
		}
		e.lastActive = time.Now()
	}
}

// Cancel cancels the endpoint, making it available to be reused when
// NewEndpoint is called on the channel. When canceled the foreach function
// passed to Range is not notified, instead just never called again.
func (e *Endpoint[T]) Cancel() {
	atomic.CompareAndSwapUint64(&e.endpointState, active, canceled)
	e.receivers.Broadcast()
}