// Package ingest turns broker messages into database rows.
//
// The whole tenancy decision happens here, in one place, so it can be read and
// tested as a unit rather than being spread across handlers.
package ingest

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log"

	"github.com/loyaly/behavision-server/internal/contract"
)

// Store is the persistence the consumer needs. An interface so the routing and
// tenancy rules below can be tested without a database.
type Store interface {
	// ResolveSite maps an authenticated MQTT username to a tenant. It must
	// NEVER create anything: see the comment in Handle.
	ResolveSite(ctx context.Context, mqttUsername string) (Site, error)
	RecordVisit(ctx context.Context, site Site, v *contract.Visit) (inserted bool, err error)
	RecordHeartbeat(ctx context.Context, site Site, h *contract.Heartbeat) error
}

// Site is a resolved tenant.
type Site struct {
	ClientID string
	SiteID   string
	AgentID  string
	Slug     string
}

// ErrUnknownSite means no provisioned site matches the username.
var ErrUnknownSite = errors.New("unknown site")

// Consumer applies one message at a time.
type Consumer struct {
	Store Store
	Log   *log.Logger
	// Metrics, read by /healthz. Counting drops matters as much as counting
	// successes: a consumer silently discarding a tenth of its traffic looks
	// identical to a quiet week.
	Accepted  uint64
	Duplicate uint64
	Dropped   uint64

	// Notify, when set, is rung once per visit that was genuinely new, so live
	// listeners re-query instead of waiting out their fallback tick. It carries
	// a client id and nothing else - the listeners run the same query a polling
	// client would, so there is one definition of what an arrival looks like.
	//
	// It MUST NOT block. This runs on the consumer's own goroutine, and one
	// slow subscriber holding it up would stall ingest for every tenant on the
	// server, turning a cosmetic delay on one shop screen into lost throughput
	// across the estate.
	Notify func(clientID string)
}

// Handle processes one broker message.
//
// Returning nil means "done with this message, do not redeliver". A permanent
// failure returns nil too, on purpose: retrying a malformed payload forever
// would stop every good message behind it, which is exactly the failure the
// agent's spool quarantine exists to avoid. Only a transient error asks for
// redelivery.
func (c *Consumer) Handle(ctx context.Context, topic string, payload []byte) error {
	t, err := contract.ParseTopic(topic)
	if err != nil {
		c.drop("topic %q: %v", topic, err)
		return nil
	}

	// The tenant comes from the topic prefix, which the broker enforces with
	// `pattern write bv/%u/...` - a site physically cannot publish under
	// another site's prefix. So this lookup is a resolution, not a trust
	// decision.
	//
	// It must never auto-create. A site typo'd into existence would silently
	// become a tenant with its own visitors and its own footfall report, and
	// nobody would notice until the numbers did not add up. Provisioning is a
	// deliberate act.
	site, err := c.Store.ResolveSite(ctx, t.Username)
	if err != nil {
		if errors.Is(err, ErrUnknownSite) {
			c.drop("no provisioned site for %q (broker credential exists but "+
				"the site was never registered)", t.Username)
			return nil
		}
		return fmt.Errorf("resolve site %q: %w", t.Username, err)
	}

	switch t.Kind {
	case "visit":
		return c.handleVisit(ctx, site, payload)
	case "heartbeat":
		return c.handleHeartbeat(ctx, site, payload)
	case "status":
		// Same shape as a heartbeat, sent on change rather than on a timer.
		return c.handleHeartbeat(ctx, site, payload)
	default:
		c.drop("unknown message kind %q from %s", t.Kind, t.Username)
		return nil
	}
}

func (c *Consumer) handleVisit(ctx context.Context, site Site, payload []byte) error {
	var v contract.Visit
	if err := json.Unmarshal(payload, &v); err != nil {
		c.drop("visit from %s is not valid json: %v", site.Slug, err)
		return nil
	}
	if err := v.Validate(); err != nil {
		if errors.Is(err, contract.ErrPermanent) {
			c.drop("visit %q from %s rejected: %v", v.EventID, site.Slug, err)
			return nil
		}
		return err
	}
	inserted, err := c.Store.RecordVisit(ctx, site, &v)
	if err != nil {
		// A database failure IS transient - the broker should redeliver rather
		// than the event being lost. This is the one path that returns an error.
		return fmt.Errorf("record visit %q: %w", v.EventID, err)
	}
	if inserted {
		c.Accepted++
		// Only on a genuine insert. At-least-once delivery makes redelivery
		// normal, and ringing for a duplicate would wake every live stream on
		// the estate to re-query for a row they already have.
		if c.Notify != nil {
			c.Notify(site.ClientID)
		}
	} else {
		// Not an error and not a warning: at-least-once delivery makes this
		// normal after any reconnect. It is counted so that a sudden rise -
		// which would mean the agent is not acking - is visible.
		c.Duplicate++
	}
	return nil
}

func (c *Consumer) handleHeartbeat(ctx context.Context, site Site, payload []byte) error {
	var h contract.Heartbeat
	if err := json.Unmarshal(payload, &h); err != nil {
		c.drop("heartbeat from %s is not valid json: %v", site.Slug, err)
		return nil
	}
	if err := c.Store.RecordHeartbeat(ctx, site, &h); err != nil {
		return fmt.Errorf("record heartbeat for %s: %w", site.Slug, err)
	}
	if h.Dropped > 0 {
		// The site's own queue overflowed, so it genuinely lost footfall.
		// Surfaced loudly - this is data the customer paid for and will never
		// get back, and it must not be discoverable only by reading a graph.
		c.logf("WARNING: site %s reports %d events dropped from its local "+
			"queue (it was offline long enough to overflow)", site.Slug, h.Dropped)
	}
	c.Accepted++
	return nil
}

func (c *Consumer) drop(format string, args ...any) {
	c.Dropped++
	c.logf("dropped: "+format, args...)
}

func (c *Consumer) logf(format string, args ...any) {
	if c.Log != nil {
		c.Log.Printf(format, args...)
	}
}