package contracts

import (
	"fmt"
	"strings"
)

// TopicName is a concrete MQTT topic used by PUBLISH. It never contains MQTT
// subscription wildcards.
type TopicName string

// TopicFilter is an MQTT subscription pattern. This lab deliberately supports
// one device-level wildcard rather than accepting arbitrary filters.
type TopicFilter string

// Kind identifies the message flow in the final topic level.
type Kind string

const (
	KindTelemetry Kind = "telemetry"
	KindCommands  Kind = "commands"
	KindResponses Kind = "responses"
	KindSales     Kind = "sales"
)

const topicPrefix = "lab/v1/devices/"

// Topic is the parsed device identity and flow from a concrete topic name.
type Topic struct {
	DeviceID string
	Kind     Kind
}

func (t TopicName) String() string   { return string(t) }
func (f TopicFilter) String() string { return string(f) }

// DeviceTopic builds the concrete topic name for one device and supported flow.
func DeviceTopic(deviceID string, kind Kind) (TopicName, error) {
	if !validID(deviceID) {
		return "", fmt.Errorf("invalid device ID %q", deviceID)
	}
	if !supportedKind(kind) {
		return "", fmt.Errorf("unsupported topic kind %q", kind)
	}
	return TopicName(topicPrefix + deviceID + "/" + string(kind)), nil
}

// DeviceFilter builds a subscription filter for one flow across lab devices.
func DeviceFilter(kind Kind) (TopicFilter, error) {
	if !supportedKind(kind) {
		return "", fmt.Errorf("unsupported topic kind %q", kind)
	}
	return TopicFilter(topicPrefix + "+/" + string(kind)), nil
}

// DeviceSubscription builds an exact-topic filter for one device and flow.
func DeviceSubscription(deviceID string, kind Kind) (TopicFilter, error) {
	topic, err := DeviceTopic(deviceID, kind)
	if err != nil {
		return "", err
	}
	return TopicFilter(topic.String()), nil
}

// ParseTopic turns broker routing metadata into values handlers can compare.
func ParseTopic(value string) (Topic, error) {
	// Builders and parser share topicPrefix, so changing the route prefix does
	// not require changing the levels that identify a device and message flow.
	if !strings.HasPrefix(value, topicPrefix) {
		return Topic{}, fmt.Errorf("topic must be %s{deviceID}/{kind}", topicPrefix)
	}
	parts := strings.Split(strings.TrimPrefix(value, topicPrefix), "/")
	if len(parts) != 2 || !validID(parts[0]) {
		return Topic{}, fmt.Errorf("topic must be %s{deviceID}/{kind}", topicPrefix)
	}
	kind := Kind(parts[1])
	if !supportedKind(kind) {
		return Topic{}, fmt.Errorf("unsupported topic kind %q", kind)
	}
	return Topic{DeviceID: parts[0], Kind: kind}, nil
}

func supportedKind(kind Kind) bool {
	switch kind {
	case KindTelemetry, KindCommands, KindResponses, KindSales:
		return true
	default:
		return false
	}
}

// validID limits identifiers to safe, single-segment topic values.
func validID(value string) bool {
	if value == "" || len(value) > 128 {
		return false
	}
	for _, r := range value {
		if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || strings.ContainsRune("._-", r)) {
			return false
		}
	}
	return true
}