package harness

import (
	"context"
	"encoding/json"
	"errors"
	"sync"
	"time"

	"keel/internal/evidence"
	"keel/internal/mode"
)

var (
	ErrBusy        = errors.New("harness: another mode is active")
	ErrUnknownMode = errors.New("harness: unknown mode")
	ErrIdle        = errors.New("harness: no active mode")
)

// Factory builds a mode from the shared services, in its initial active state.
type Factory func(Services) mode.Mode

type Harness struct {
	mu        sync.Mutex
	active    mode.Mode
	factories map[string]Factory
	services  Services
	onChange  []func()
}

// New wires Services.Notify to the harness so every mode's async completion
// reaches all listeners, then stores the services for the factories to receive.
func New(services Services, factories map[string]Factory) *Harness {
	h := &Harness{factories: factories}
	services.Notify = h.notify
	h.services = services
	return h
}

// AddOnChange registers a change listener (web broadcaster, status writer).
func (h *Harness) AddOnChange(f func()) {
	h.mu.Lock()
	h.onChange = append(h.onChange, f)
	h.mu.Unlock()
}

// SetServices swaps the services future Start calls hand to factories. In v1 it
// affects the NEXT mode start only; an already-active mode keeps the services it
// was built with (settings changes take effect on the next session).
func (h *Harness) SetServices(s Services) {
	s.Notify = h.notify
	h.mu.Lock()
	h.services = s
	h.mu.Unlock()
}

// Services returns the shared services the harness hands to factories, with
// Notify wired to the harness change fan-out. Use it to build a mode for Adopt
// (e.g. startup restoration) so the restored mode's async work reaches the
// harness's onChange listeners.
func (h *Harness) Services() Services {
	h.mu.Lock()
	defer h.mu.Unlock()
	return h.services
}

func (h *Harness) notify() {
	h.mu.Lock()
	fs := append([]func(){}, h.onChange...)
	h.mu.Unlock()
	for _, f := range fs {
		if f != nil {
			f()
		}
	}
}

// Adopt installs an already-constructed mode as active (startup restoration).
// It fails if a mode is already active.
func (h *Harness) Adopt(m mode.Mode) error {
	h.mu.Lock()
	if h.active != nil {
		h.mu.Unlock()
		return ErrBusy
	}
	h.active = m
	h.mu.Unlock()
	h.notify()
	return nil
}

// Start activates a mode by kind. Idempotent if that kind is already active;
// ErrBusy if a different kind is active.
func (h *Harness) Start(kind string) error {
	h.mu.Lock()
	if h.active != nil {
		busy := h.active.Kind() != kind
		h.mu.Unlock()
		if busy {
			return ErrBusy
		}
		return nil
	}
	f, ok := h.factories[kind]
	if !ok {
		h.mu.Unlock()
		return ErrUnknownMode
	}
	h.active = f(h.services)
	h.mu.Unlock()
	h.notify()
	return nil
}

// Command routes to the active mode, then releases it if it went inactive.
func (h *Harness) Command(ctx context.Context, name string, payload json.RawMessage) error {
	h.mu.Lock()
	m := h.active
	h.mu.Unlock()
	if m == nil {
		return ErrIdle
	}
	if err := m.Command(ctx, name, payload); err != nil {
		return err
	}
	h.releaseIfDone()
	h.notify()
	return nil
}

func (h *Harness) releaseIfDone() {
	h.mu.Lock()
	if h.active != nil && !h.active.Active() {
		h.active = nil
	}
	h.mu.Unlock()
}

// State returns the surfacing envelope.
func (h *Harness) State() mode.Envelope {
	h.mu.Lock()
	m := h.active
	h.mu.Unlock()
	if m == nil {
		return mode.Envelope{}
	}
	return mode.Envelope{ActiveMode: m.Kind(), Mode: m.View()}
}

// RecordWindow forwards a window snapshot iff the active mode consumes evidence.
func (h *Harness) RecordWindow(w evidence.WindowSnapshot) {
	h.mu.Lock()
	m := h.active
	h.mu.Unlock()
	if c, ok := m.(mode.EvidenceConsumer); ok {
		c.OnWindow(w)
	}
}

// Deadline returns the active mode's deadline iff it is an Expirer.
func (h *Harness) Deadline() time.Time {
	h.mu.Lock()
	m := h.active
	h.mu.Unlock()
	if e, ok := m.(mode.Expirer); ok {
		return e.Deadline()
	}
	return time.Time{}
}

// Expire fires the active mode's expiry iff it is an Expirer, then releases it
// if it went inactive.
func (h *Harness) Expire() error {
	h.mu.Lock()
	m := h.active
	h.mu.Unlock()
	e, ok := m.(mode.Expirer)
	if !ok {
		return ErrIdle
	}
	if err := e.Expire(); err != nil {
		return err
	}
	h.releaseIfDone()
	h.notify()
	return nil
}