// Package session owns transport-neutral server session scheduling and fencing.
package session

import (
	"context"
	"errors"
	"sync"
	"time"
)

var (
	ErrControlLaneFull = errors.New("session control lane is full")
	ErrSessionClosed   = errors.New("session is closed")
)

type FrameKind uint8

const (
	FrameControl FrameKind = iota + 1
	FrameData
)

type Frame struct {
	Kind    FrameKind
	Payload []byte
	// Written is closed by the sole socket writer after a successful write.
	// It is used only for protocol barriers such as reconciliation-before-work.
	Written chan<- struct{}
}

// WriterQueue is owned by one socket writer. Data saturation leaves the work
// durable; control saturation is a session-fatal invariant.
type WriterQueue struct {
	control      chan Frame
	data         chan Frame
	controlBurst uint32
	controlRun   uint32
	closed       chan struct{}
	closeOnce    sync.Once
}

func NewWriterQueue(controlCapacity, dataCapacity, controlBurst uint32) *WriterQueue {
	if controlCapacity == 0 || dataCapacity == 0 || controlBurst == 0 {
		panic("session queue capacities must be positive")
	}
	return &WriterQueue{
		control: make(chan Frame, controlCapacity), data: make(chan Frame, dataCapacity),
		controlBurst: controlBurst, closed: make(chan struct{}),
	}
}

func (queue *WriterQueue) EnqueueControl(frame Frame) error {
	if frame.Kind != FrameControl {
		return ErrControlLaneFull
	}
	select {
	case <-queue.closed:
		return ErrSessionClosed
	default:
	}
	select {
	case queue.control <- frame:
		return nil
	default:
		return ErrControlLaneFull
	}
}

// EnqueueData reports false when the bounded data lane is full. It never
// creates a fallback goroutine or buffer.
func (queue *WriterQueue) EnqueueData(frame Frame) bool {
	if frame.Kind != FrameData {
		return false
	}
	select {
	case <-queue.closed:
		return false
	default:
	}
	select {
	case queue.data <- frame:
		return true
	default:
		return false
	}
}

func (queue *WriterQueue) Next(ctx context.Context) (Frame, error) {
	for {
		if queue.controlRun < queue.controlBurst {
			select {
			case frame := <-queue.control:
				queue.controlRun++
				return frame, nil
			default:
			}
		}
		select {
		case frame := <-queue.data:
			queue.controlRun = 0
			return frame, nil
		default:
		}
		select {
		case frame := <-queue.control:
			queue.controlRun++
			return frame, nil
		default:
		}
		select {
		case <-ctx.Done():
			return Frame{}, ctx.Err()
		case <-queue.closed:
			return Frame{}, ErrSessionClosed
		case frame := <-queue.control:
			queue.controlRun++
			return frame, nil
		case frame := <-queue.data:
			queue.controlRun = 0
			return frame, nil
		}
	}
}

func (queue *WriterQueue) Close() { queue.closeOnce.Do(func() { close(queue.closed) }) }

type HeartbeatAction uint8

const (
	HeartbeatNone HeartbeatAction = iota
	HeartbeatPing
	HeartbeatClose
)

type Heartbeat struct {
	idle        time.Duration
	timeout     time.Duration
	lastInbound time.Duration
	pingSent    bool
}

func NewHeartbeat(idle, timeout, now time.Duration) *Heartbeat {
	if idle <= 0 || timeout <= idle {
		panic("invalid heartbeat timings")
	}
	return &Heartbeat{idle: idle, timeout: timeout, lastInbound: now}
}

func (heartbeat *Heartbeat) ObserveInbound(now time.Duration) {
	if now >= heartbeat.lastInbound {
		heartbeat.lastInbound = now
		heartbeat.pingSent = false
	}
}

func (heartbeat *Heartbeat) Check(now time.Duration) HeartbeatAction {
	if now < heartbeat.lastInbound {
		return HeartbeatNone
	}
	elapsed := now - heartbeat.lastInbound
	if elapsed >= heartbeat.timeout {
		return HeartbeatClose
	}
	if elapsed >= heartbeat.idle && !heartbeat.pingSent {
		heartbeat.pingSent = true
		return HeartbeatPing
	}
	return HeartbeatNone
}

type DispatchLane uint8

const (
	DispatchNone DispatchLane = iota
	DispatchRunning
	DispatchQueued
)

type CapacityShadow struct {
	running, queued       uint32
	maxRunning, maxQueued uint32
	shadowRunning         uint32
	shadowQueued          uint32
}

func (shadow *CapacityShadow) UpdateAdvertised(running, queued, maxRunning, maxQueued uint32) bool {
	if maxRunning == 0 || maxQueued == 0 || running > maxRunning || queued > maxQueued {
		return false
	}
	shadow.running, shadow.queued = running, queued
	shadow.maxRunning, shadow.maxQueued = maxRunning, maxQueued
	return true
}

func (shadow *CapacityShadow) Reserve() DispatchLane {
	if shadow.running+shadow.shadowRunning < shadow.maxRunning {
		shadow.shadowRunning++
		return DispatchRunning
	}
	if shadow.queued+shadow.shadowQueued < shadow.maxQueued {
		shadow.shadowQueued++
		return DispatchQueued
	}
	return DispatchNone
}

func (shadow *CapacityShadow) Release(lane DispatchLane) bool {
	switch lane {
	case DispatchRunning:
		if shadow.shadowRunning == 0 {
			return false
		}
		shadow.shadowRunning--
	case DispatchQueued:
		if shadow.shadowQueued == 0 {
			return false
		}
		shadow.shadowQueued--
	default:
		return false
	}
	return true
}

func (shadow CapacityShadow) Pending() (running, queued uint32) {
	return shadow.shadowRunning, shadow.shadowQueued
}

type Handle struct {
	ClientID   string
	SessionID  [16]byte
	Generation uint64
	Context    context.Context
	cancel     context.CancelFunc
}

type Registry struct {
	mu      sync.Mutex
	clients map[string]*Handle
}

func NewRegistry() *Registry { return &Registry{clients: make(map[string]*Handle)} }

// Install replaces only an older in-memory handle for the same client. The
// caller must have already durably fenced it in the store.
func (registry *Registry) Install(clientID string, sessionID [16]byte, generation uint64) (*Handle, error) {
	if clientID == "" || sessionID == [16]byte{} || generation == 0 {
		return nil, ErrSessionClosed
	}
	registry.mu.Lock()
	defer registry.mu.Unlock()
	if old := registry.clients[clientID]; old != nil {
		old.cancel()
	}
	ctx, cancel := context.WithCancel(context.Background())
	handle := &Handle{ClientID: clientID, SessionID: sessionID, Generation: generation, Context: ctx, cancel: cancel}
	registry.clients[clientID] = handle
	return handle, nil
}

func (registry *Registry) Remove(handle *Handle) bool {
	if handle == nil {
		return false
	}
	registry.mu.Lock()
	defer registry.mu.Unlock()
	current := registry.clients[handle.ClientID]
	if current != handle || current.Generation != handle.Generation || current.SessionID != handle.SessionID {
		return false
	}
	delete(registry.clients, handle.ClientID)
	handle.cancel()
	return true
}

func (registry *Registry) Get(clientID string) *Handle {
	registry.mu.Lock()
	defer registry.mu.Unlock()
	return registry.clients[clientID]
}