// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package vpp

import (
	"context"
	"errors"
	"log/slog"

	"git.ipng.ch/ipng/vpp-maglev/internal/checker"
)

// EventSource is the subset of checker.Checker that Reconciler needs.
// Decoupling via an interface keeps the dependency direction
// vpp → checker (checker never imports vpp).
type EventSource interface {
	Subscribe() (<-chan checker.Event, func())
}

// Reconciler bridges checker state transitions to VPP dataplane changes.
// It subscribes to the checker's event channel and, for every transition,
// runs SyncLBStateVIP for the frontend the backend belongs to. This is
// the ONLY place in the codebase where backend state transitions cause
// VPP calls — every LB change flows through Client.SyncLBStateVIP.
//
// The reconciler carries no state of its own. Idempotency is guaranteed
// by SyncLBStateVIP itself (diff-based, driven by the pure asFromBackend
// mapping in lbsync.go).
type Reconciler struct {
	client   *Client
	events   EventSource
	stateSrc StateSource
}

// NewReconciler creates a Reconciler. client is the VPP client, events is
// the checker (or anything that implements Subscribe), and stateSrc provides
// the live config for SyncLBStateVIP calls. All three are normally the
// checker/vpp client pair constructed at daemon startup.
func NewReconciler(client *Client, events EventSource, stateSrc StateSource) *Reconciler {
	return &Reconciler{client: client, events: events, stateSrc: stateSrc}
}

// Run subscribes to the checker and loops until ctx is cancelled. Each
// received event fires a single-VIP sync for the frontend the transitioned
// backend belongs to.
func (r *Reconciler) Run(ctx context.Context) {
	ch, unsub := r.events.Subscribe()
	defer unsub()

	slog.Info("vpp-reconciler-start")
	defer slog.Info("vpp-reconciler-stop")

	for {
		select {
		case <-ctx.Done():
			return
		case ev, ok := <-ch:
			if !ok {
				return
			}
			r.handle(ev)
		}
	}
}

// handle reconciles one event. Operates only on backend-transition events
// that carry a frontend name (the checker emits one event per frontend that
// references the backend, so a backend shared across multiple frontends
// produces multiple events and all relevant VIPs are reconciled).
// Frontend-transition events are observational only — the dataplane work
// they would imply has already been done by the backend-transition event
// that triggered them.
func (r *Reconciler) handle(ev checker.Event) {
	if ev.FrontendTransition != nil {
		return // frontend-only event; no dataplane work
	}
	if ev.FrontendName == "" {
		return
	}
	cfg := r.stateSrc.Config()
	if cfg == nil {
		return
	}
	slog.Debug("vpp-reconciler-event",
		"frontend", ev.FrontendName,
		"backend", ev.BackendName,
		"from", ev.Transition.From.String(),
		"to", ev.Transition.To.String())

	if err := r.client.SyncLBStateVIP(cfg, ev.FrontendName, ""); err != nil {
		if errors.Is(err, ErrFrontendNotFound) {
			// Frontend was removed between the event being emitted and
			// us handling it; a periodic SyncLBStateAll will clean it up.
			return
		}
		slog.Warn("vpp-reconciler-error",
			"frontend", ev.FrontendName,
			"backend", ev.BackendName,
			"from", ev.Transition.From.String(),
			"to", ev.Transition.To.String(),
			"err", err)
	}
}