// Package lease is crossbar's sticky placement table. It remembers which host each
// conversation (and each route) is on and keeps it there unless the host is
// unhealthy, the lease has been idle past lease_idle, or an operator releases or
// pins the route. Every change is written through to a Persister and replayed back
// at start so a restart does not reshuffle sessions.
package lease

import (
	"errors"
	"fmt"
	"sort"
	"sync"
	"time"

	"git.wntrmute.dev/kyle/crossbar/internal/store"
)

var (
	ErrNoHost      = errors.New("lease: no usable host")
	ErrPinnedDown  = errors.New("lease: pinned host is not healthy")
	ErrUnknownHost = errors.New("lease: unknown host")
)

// Key identifies one conversation: a route, the fingerprint of its first request
// (empty for a request with no user message, which leases the route itself), and
// the model it wants.
type Key struct {
	Route, FP, Model string
}

// Lease is one routed model on one host, in memory.
type Lease struct {
	Key
	Host              string
	State             store.State
	Created, LastUsed time.Time
}

// Persister is the durable half of the table: leases, the pin row, and the event
// log. *store.Store satisfies it.
type Persister interface {
	SaveLease(store.Lease) error
	DeleteLease(route, fp, model string) error
	ListLeases() ([]store.Lease, error)
	RecordEvent(store.LeaseEvent) error
}

// Hosts is the live health view the table consults before placing a lease.
type Hosts interface {
	Healthy(name string) bool
	Draining(name string) bool
}

// Chooser picks a host for a new lease among the eligible candidates.
type Chooser interface {
	Choose(candidates []string, model string) (string, bool)
}

type Table struct {
	mu     sync.Mutex
	leases map[Key]*Lease             // active leases, keyed by (route, fp, model)
	pins   map[string]string          // route -> pinned host
	seen   map[string]map[string]bool // route -> candidate hosts ever asked for or stored
	p      Persister
	hosts  Hosts
	choose Chooser
	idle   time.Duration
}

// New builds a table and loads its state. Rows with FP=="" && Model=="" &&
// State==Pinned are pins; every other row becomes an active lease and its host
// counts as a candidate already seen for the route.
func New(p Persister, h Hosts, c Chooser, idle time.Duration) (*Table, error) {
	loads, err := p.ListLeases()
	if err != nil {
		return nil, fmt.Errorf("lease: list leases: %w", err)
	}
	t := &Table{
		leases: make(map[Key]*Lease),
		pins:   make(map[string]string),
		seen:   make(map[string]map[string]bool),
		p:      p,
		hosts:  h,
		choose: c,
		idle:   idle,
	}
	for _, l := range loads {
		if l.FP == "" && l.Model == "" && l.State == store.Pinned {
			t.pins[l.Route] = l.Host
		} else {
			t.leases[Key{l.Route, l.FP, l.Model}] = &Lease{Key{l.Route, l.FP, l.Model}, l.Host, l.State, l.Created, l.LastUsed}
		}
		if t.seen[l.Route] == nil {
			t.seen[l.Route] = make(map[string]bool)
		}
		t.seen[l.Route][l.Host] = true
	}
	return t, nil
}

func (l *Lease) store() store.Lease {
	return store.Lease{Route: l.Route, FP: l.FP, Model: l.Model, Host: l.Host, State: l.State, Created: l.Created, LastUsed: l.LastUsed}
}

// Acquire places k, keeping it sticky. See the task's Acquire ordering: pinned
// route, existing lease, inherit the route's host, else choose. Only one lease is
// created per call, and a failed save is rolled back in memory.
func (t *Table) Acquire(k Key, candidates []string, now time.Time) (host string, reused bool, err error) {
	t.mu.Lock()
	defer t.mu.Unlock()

	if t.seen[k.Route] == nil {
		t.seen[k.Route] = make(map[string]bool)
	}
	for _, c := range candidates {
		t.seen[k.Route][c] = true
	}

	// unhealthyFrom is set when an existing lease sat on a dead host; the next
	// create records an unhealthy move instead of a fresh new.
	var unhealthyFrom string
	reason := store.ReasonNew

	// 1. Pinned route.
	if pin, ok := t.pins[k.Route]; ok {
		if !t.hosts.Healthy(pin) {
			return "", false, ErrPinnedDown
		}
		if _, exists := t.leases[k]; exists {
			return pin, true, nil
		}
		l := &Lease{k, pin, store.Active, now, now}
		t.leases[k] = l
		if err := t.save(l); err != nil {
			delete(t.leases, k)
			return "", false, err
		}
		t.event(now, k, store.ReasonNew, "", pin)
		return pin, false, nil
	}

	// 2. Existing lease for k.
	if l, exists := t.leases[k]; exists {
		if t.hosts.Healthy(l.Host) {
			l.LastUsed = now
			if err := t.save(l); err != nil {
				return "", false, err
			}
			return l.Host, true, nil
		}
		unhealthyFrom = l.Host
	}

	// 3. Inherit the route's own host when a fingerprinted request can start
	//    where the route already lives.
	if k.FP != "" {
		rk := Key{k.Route, "", k.Model}
		if rl, exists := t.leases[rk]; exists && t.hosts.Healthy(rl.Host) {
			l := &Lease{k, rl.Host, store.Active, now, now}
			t.leases[k] = l
			if err := t.save(l); err != nil {
				delete(t.leases, k)
				return "", false, err
			}
			if unhealthyFrom != "" {
				reason = store.ReasonUnhealthy
			}
			t.event(now, k, reason, unhealthyFrom, rl.Host)
			return rl.Host, true, nil
		}
	}

	// 4. Choose among healthy, non-draining candidates.
	filtered := make([]string, 0, len(candidates))
	for _, c := range candidates {
		if t.hosts.Healthy(c) && !t.hosts.Draining(c) {
			filtered = append(filtered, c)
		}
	}
	host, ok := t.choose.Choose(filtered, k.Model)
	if !ok {
		return "", false, ErrNoHost
	}
	l := &Lease{k, host, store.Active, now, now}
	t.leases[k] = l
	if err := t.save(l); err != nil {
		delete(t.leases, k)
		return "", false, err
	}
	if unhealthyFrom != "" {
		reason = store.ReasonUnhealthy
	}
	t.event(now, k, reason, unhealthyFrom, host)
	return host, false, nil
}

// save writes a lease through, failing the call on a persister error.
func (t *Table) save(l *Lease) error {
	if err := t.p.SaveLease(l.store()); err != nil {
		return fmt.Errorf("lease: %w", err)
	}
	return nil
}

// event appends a lease event. from is empty for a fresh placement.
func (t *Table) event(now time.Time, k Key, reason, from, to string) {
	_ = t.p.RecordEvent(store.LeaseEvent{TS: now, Route: k.Route, Model: k.Model, FromHost: from, ToHost: to, Reason: reason})
}

// ExpireIdle removes leases idle longer than lease_idle (never pins, which are
// not in the lease map) and records an idle event for each. It returns how many
// it removed.
func (t *Table) ExpireIdle(now time.Time) int {
	t.mu.Lock()
	defer t.mu.Unlock()

	var idle []Key
	for k, l := range t.leases {
		if now.Sub(l.LastUsed) > t.idle {
			idle = append(idle, k)
		}
	}
	for _, k := range idle {
		l := t.leases[k]
		delete(t.leases, k)
		_ = t.p.DeleteLease(k.Route, k.FP, k.Model)
		t.event(now, k, store.ReasonIdle, l.Host, "")
	}
	return len(idle)
}

// Pin routes route to host. host must be a candidate the table has seen for the
// route, else ErrUnknownHost. It records a pin event, stores the pin row, and
// deletes the route's existing leases on other hosts so the next turn lands on
// the pin.
func (t *Table) Pin(route, host string, now time.Time) error {
	t.mu.Lock()
	defer t.mu.Unlock()

	if t.seen[route] == nil || !t.seen[route][host] {
		return ErrUnknownHost
	}
	t.event(now, Key{Route: route}, store.ReasonPin, "", host)
	if err := t.p.SaveLease(store.Lease{Route: route, FP: "", Model: "", Host: host, State: store.Pinned, Created: now, LastUsed: now}); err != nil {
		return fmt.Errorf("lease: %w", err)
	}
	t.pins[route] = host
	for k, l := range t.leases {
		if k.Route == route && l.Host != host {
			delete(t.leases, k)
			_ = t.p.DeleteLease(k.Route, k.FP, k.Model)
			t.event(now, k, store.ReasonPin, l.Host, host)
		}
	}
	return nil
}

// Unpin clears the pin for route and records a release. Existing leases stay put.
func (t *Table) Unpin(route string) {
	t.mu.Lock()
	defer t.mu.Unlock()

	delete(t.pins, route)
	_ = t.p.DeleteLease(route, "", "")
	t.event(time.Now(), Key{Route: route}, store.ReasonRelease, "", "")
}

// Pinned returns the pinned host for route, or "" if it is not pinned.
func (t *Table) Pinned(route string) string {
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.pins[route]
}

// Release drops every lease (not the pin) of route and records a release event
// per dropped lease. It returns how many were removed.
func (t *Table) Release(route string) int {
	t.mu.Lock()
	defer t.mu.Unlock()

	var keys []Key
	for k := range t.leases {
		if k.Route == route {
			keys = append(keys, k)
		}
	}
	for _, k := range keys {
		delete(t.leases, k)
		_ = t.p.DeleteLease(k.Route, k.FP, k.Model)
		t.event(time.Now(), k, store.ReasonRelease, "", "")
	}
	return len(keys)
}

// Snapshot returns copies of the active leases, sorted by route, fp, model. Pins
// are excluded.
func (t *Table) Snapshot() []Lease {
	t.mu.Lock()
	defer t.mu.Unlock()

	out := make([]Lease, 0, len(t.leases))
	for _, l := range t.leases {
		out = append(out, *l)
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Route != out[j].Route {
			return out[i].Route < out[j].Route
		}
		if out[i].FP != out[j].FP {
			return out[i].FP < out[j].FP
		}
		return out[i].Model < out[j].Model
	})
	return out
}