package protocol

import (
	"encoding/json"
	"fmt"
	"time"
)

// Message contains validated routing, metadata, and a typed payload. Retained
// is the delivery flag. Live delivery of retained-topic data can be false.
type Message[T any] struct {
	Address  Address
	Metadata Metadata
	Payload  T
	Retained bool
}

// Publication is a broker-independent outgoing message. The transport sets
// UTF-8 payload format and application/json content type. Commands and hub
// status also need MQTT expiry computed from their envelope deadlines.
type Publication struct {
	Topic  string
	Data   []byte
	QoS    byte
	Retain bool
	// ExpiresAt is the command, earliest batch member, or hub deadline.
	ExpiresAt *time.Time
	// ResponseTopic and CorrelationData apply to individual commands only.
	ResponseTopic   string
	CorrelationData string
}

// Parse routes an incoming publication to a typed payload without caller topic
// operations. Payload is DescriptorPayload, AvailabilityPayload, ValuePayload,
// EventPayload, BatchPayload, ResultPayload, or HubStatusPayload. Address.Kind
// distinguishes command, reported, and telemetry value payloads.
func Parse(topic string, data []byte, retained bool) (Message[any], error) {
	address, err := parseTopic(topic)
	if err != nil {
		return Message[any]{}, fmt.Errorf("parse address: %w", err)
	}

	switch address.Kind {
	case Descriptor:
		return eraseMessage(ParseDescriptor(topic, data, retained))
	case Availability:
		return eraseMessage(ParseAvailability(topic, data, retained))
	case Reported:
		return eraseMessage(ParseReported(topic, data, retained))
	case Telemetry:
		return eraseMessage(ParseTelemetry(topic, data, retained))
	case Event:
		return eraseMessage(ParseEvent(topic, data, retained))
	case Command:
		return eraseMessage(ParseCommand(topic, data, retained))
	case CommandBatch:
		return eraseMessage(ParseCommandBatch(topic, data, retained))
	case Result:
		return eraseMessage(ParseResult(topic, data, retained))
	case HubStatus:
		return eraseMessage(ParseHubStatus(topic, data, retained))
	default:
		return Message[any]{}, fmt.Errorf("unsupported message kind")
	}
}

func eraseMessage[T any](message Message[T], err error) (Message[any], error) {
	if err != nil {
		return Message[any]{}, fmt.Errorf("parse message: %w", err)
	}

	return Message[any]{
		Address:  message.Address,
		Metadata: message.Metadata,
		Payload:  message.Payload,
		Retained: message.Retained,
	}, nil
}

func buildMessage[T any](address Address, metadata Metadata, payload T) (Publication, error) {
	topic, err := address.topic()
	if err != nil {
		return Publication{}, fmt.Errorf("build address: %w", err)
	}

	data, err := encodeEnvelope(metadata, payload)
	if err != nil {
		return Publication{}, fmt.Errorf("build envelope: %w", err)
	}

	retain := address.Kind == Descriptor || address.Kind == Availability || address.Kind == Reported || address.Kind == HubStatus
	_, err = parseMessage[T](address.Kind, topic, data, retain)
	if err != nil {
		return Publication{}, fmt.Errorf("validate publication: %w", err)
	}

	qos := byte(1)
	if address.Kind == Telemetry {
		qos = 0
	}

	publication := Publication{
		Topic:  topic,
		Data:   data,
		QoS:    qos,
		Retain: retain,
	}

	switch address.Kind {
	case Command:
		publication.ExpiresAt = metadata.ExpiresAt
		publication.CorrelationData = metadata.MessageID
		publication.ResponseTopic, err = (Address{
			Kind:   Result,
			ID:     address.ID,
			Target: metadata.MessageID,
		}).topic()
		if err != nil {
			return Publication{}, fmt.Errorf("build command response topic: %w", err)
		}
	case CommandBatch:
		batch, ok := any(payload).(BatchPayload)
		if !ok {
			return Publication{}, fmt.Errorf("batch requires command members")
		}

		for _, command := range batch.Commands {
			if publication.ExpiresAt == nil || command.ExpiresAt.Before(*publication.ExpiresAt) {
				publication.ExpiresAt = command.ExpiresAt
			}
		}
	case HubStatus:
		publication.ExpiresAt = metadata.ExpiresAt
	}

	return publication, nil
}

func parseMessage[T any](kind Kind, topic string, data []byte, retained bool) (Message[T], error) {
	address, err := parseTopic(topic)
	if err != nil {
		return Message[T]{}, fmt.Errorf("parse address: %w", err)
	}

	if address.Kind != kind {
		return Message[T]{}, fmt.Errorf("message topic does not match parser")
	}

	if retained && (kind == Telemetry || kind == Event || kind == Command || kind == CommandBatch || kind == Result) {
		return Message[T]{}, fmt.Errorf("message must not be retained")
	}

	envelope, err := parseEnvelope(data)
	if err != nil {
		return Message[T]{}, fmt.Errorf("parse envelope: %w", err)
	}

	var payload T

	err = decodeObject(envelope.Payload, &payload)
	if err != nil {
		return Message[T]{}, fmt.Errorf("decode payload: %w", err)
	}

	err = validatePayload(address, envelope, payload)
	if err != nil {
		return Message[T]{}, fmt.Errorf("validate payload: %w", err)
	}

	return Message[T]{
		Address:  address,
		Metadata: envelope.Metadata,
		Payload:  payload,
		Retained: retained,
	}, nil
}

func validatePayload(address Address, envelope wireEnvelope, payload any) error {
	switch value := payload.(type) {
	case DescriptorPayload:
		err := validateDescriptor(value, envelope.Payload)
		if err != nil {
			return fmt.Errorf("validate descriptor: %w", err)
		}
	case AvailabilityPayload:
		if value.Status != "online" && value.Status != "offline" {
			return fmt.Errorf("availability must be online or offline")
		}
	case ValuePayload:
		if !json.Valid(value.Value) {
			return fmt.Errorf("value is required and must be valid JSON")
		}

		if address.Kind == Command && envelope.ExpiresAt == nil {
			return fmt.Errorf("command must have an expiry")
		}
	case EventPayload:
		// An event object has application-defined fields.
	case ResultPayload:
		if envelope.CorrelationID != address.Target {
			return fmt.Errorf("result correlation must match its command ID")
		}

		switch value.Status {
		case "accepted", "completed":
			if value.Reason != "" {
				return fmt.Errorf("successful result must not have a reason")
			}
		case "rejected", "failed":
			if value.Reason == "" {
				return fmt.Errorf("unsuccessful result must have a reason")
			}
		default:
			return fmt.Errorf("result status is invalid")
		}
	case BatchPayload:
		err := validateBatch(value, envelope)
		if err != nil {
			return fmt.Errorf("validate command batch: %w", err)
		}
	case HubStatusPayload:
		err := validateHub(value, address, envelope)
		if err != nil {
			return fmt.Errorf("validate hub status: %w", err)
		}
	default:
		return fmt.Errorf("unsupported payload type")
	}

	return nil
}

func validateDescriptor(payload DescriptorPayload, raw json.RawMessage) error {
	if payload.Manufacturer == "" || payload.Model == "" || payload.Firmware == "" || len(payload.Attribution) > 2000 || len(payload.Capabilities) == 0 {
		return fmt.Errorf("descriptor metadata or capabilities are invalid")
	}

	var fields struct {
		Capabilities []map[string]json.RawMessage `json:"capabilities"`
	}

	err := json.Unmarshal(raw, &fields)
	if err != nil {
		return fmt.Errorf("read descriptor access flags: %w", err)
	}

	seen := make(map[string]bool, len(payload.Capabilities))

	for index, capability := range payload.Capabilities {
		if !ValidIdentifier(capability.ID) || !ValidIdentifier(capability.Type) || seen[capability.ID] {
			return fmt.Errorf("capability IDs and types must be valid and unique")
		}

		if !capability.ReportsState && !capability.AcceptsCommands {
			return fmt.Errorf("capability must have at least one access mode")
		}

		for _, name := range []string{"reports_state", "accepts_commands"} {
			field := fields.Capabilities[index][name]
			if string(field) != "true" && string(field) != "false" {
				return fmt.Errorf("capability access flags must be present booleans")
			}
		}

		seen[capability.ID] = true
	}

	return nil
}

func validateBatch(payload BatchPayload, envelope wireEnvelope) error {
	if len(payload.Commands) < 2 || len(payload.Commands) > MaximumCommandBatchSize {
		return fmt.Errorf("batch size must be 2 through %d", MaximumCommandBatchSize)
	}

	if envelope.MessageID != payload.Commands[0].MessageID {
		return fmt.Errorf("batch ID must equal its first command ID")
	}

	ids := make(map[string]bool, len(payload.Commands))
	targets := make(map[string]bool, len(payload.Commands))

	for _, command := range payload.Commands {
		if !ValidIdentifier(command.Capability) || ids[command.MessageID] || targets[command.Capability] {
			return fmt.Errorf("batch targets and IDs must be valid and distinct")
		}

		_, err := encodeEnvelope(command.Metadata, command.Payload)
		if err != nil {
			return fmt.Errorf("validate batch command envelope: %w", err)
		}

		if command.ExpiresAt == nil || !json.Valid(command.Payload.Value) {
			return fmt.Errorf("batch commands require expiry and a JSON value")
		}

		ids[command.MessageID] = true
		targets[command.Capability] = true
	}

	return nil
}

func validateHub(payload HubStatusPayload, address Address, envelope wireEnvelope) error {
	if payload.ID != address.ID || envelope.ExpiresAt == nil {
		return fmt.Errorf("hub status requires a matching ID and an expiry")
	}

	if payload.Status != "ready" && payload.Status != "unavailable" {
		return fmt.Errorf("hub status must be ready or unavailable")
	}

	if payload.Address == "" || payload.ConfiguredDevices < 0 || payload.PairedDevices != nil && *payload.PairedDevices < 0 {
		return fmt.Errorf("hub address or device counts are invalid")
	}

	timestamps := []*time.Time{payload.UpdatedAt}
	if payload.Diagnostics != nil {
		timestamps = append(timestamps, &payload.Diagnostics.ObservedAt)
	}

	if payload.Pairing != nil {
		timestamps = append(timestamps, &payload.Pairing.RequestedAt, &payload.Pairing.Deadline)
	}

	for _, timestamp := range timestamps {
		if timestamp != nil {
			_, offset := timestamp.Zone()
			if offset != 0 || timestamp.IsZero() {
				return fmt.Errorf("hub timestamps must be nonzero UTC times")
			}
		}
	}

	return nil
}