mqtt-5-learning/internal/contracts/message.go

package contracts

import (
	"fmt"
	"time"
)

const SchemaVersionOne = 1

// SchemaVersion keeps the teaching examples' original package-level name.
const SchemaVersion = SchemaVersionOne

// MQTTMessage is implemented by each versioned message contract.
// Validation uses the parsed route and caller-supplied time for deterministic checks.
type MQTTMessage interface {
	Validate(topic Topic, now time.Time) error
}

// These types define the JSON fields shared by backend and simulator.
type Heartbeat struct {
	SchemaVersion int      `json:"schema_version"`
	MessageID     string   `json:"message_id"`
	DeviceID      string   `json:"device_id"`
	OccurredAt    string   `json:"occurred_at"`
	Temperature   *float64 `json:"temperature,omitempty"`
}

type Command struct {
	SchemaVersion int    `json:"schema_version"`
	MessageID     string `json:"message_id"`
	DeviceID      string `json:"device_id"`
	OccurredAt    string `json:"occurred_at"`
	CommandID     string `json:"command_id"`
	Command       string `json:"command"`
}

type CommandResult struct {
	SchemaVersion int    `json:"schema_version"`
	MessageID     string `json:"message_id"`
	DeviceID      string `json:"device_id"`
	OccurredAt    string `json:"occurred_at"`
	CommandID     string `json:"command_id"`
	Status        string `json:"status"`
	Detail        string `json:"detail,omitempty"`
}

type SalesLine struct {
	SKU         string `json:"sku"`
	Quantity    int64  `json:"quantity"`
	AmountMinor int64  `json:"amount_minor"`
}

type SalesReport struct {
	SchemaVersion int         `json:"schema_version"`
	ReportID      string      `json:"report_id"`
	DeviceID      string      `json:"device_id"`
	OccurredAt    string      `json:"occurred_at"`
	Items         []SalesLine `json:"items"`
}

// validateEnvelope checks fields shared by every versioned message.
func validateEnvelope(topic Topic, version int, messageID, deviceID, occurredAt string, now time.Time) error {
	if version != SchemaVersionOne {
		return fmt.Errorf("unsupported schema_version %d", version)
	}
	if !validID(messageID) || !validID(deviceID) {
		return fmt.Errorf("message_id and device_id must be valid IDs")
	}
	if topic.DeviceID != deviceID {
		return fmt.Errorf("topic device %q does not match payload device_id %q", topic.DeviceID, deviceID)
	}
	stamp, err := time.Parse(time.RFC3339, occurredAt)
	if err != nil {
		return fmt.Errorf("occurred_at must be RFC3339: %w", err)
	}
	if stamp.After(now.Add(30 * time.Second)) {
		return fmt.Errorf("occurred_at is more than 30 seconds in the future")
	}
	return nil
}

// Each Validate method adds checks specific to one message kind.
func (m Heartbeat) Validate(topic Topic, now time.Time) error {
	if topic.Kind != KindTelemetry {
		return fmt.Errorf("heartbeat requires telemetry topic")
	}
	if err := validateEnvelope(topic, m.SchemaVersion, m.MessageID, m.DeviceID, m.OccurredAt, now); err != nil {
		return err
	}
	if m.Temperature != nil && (*m.Temperature < -50 || *m.Temperature > 100) {
		return fmt.Errorf("temperature is outside the supported range")
	}
	return nil
}

func (m Command) Validate(topic Topic, now time.Time) error {
	if topic.Kind != KindCommands {
		return fmt.Errorf("command requires commands topic")
	}
	if err := validateEnvelope(topic, m.SchemaVersion, m.MessageID, m.DeviceID, m.OccurredAt, now); err != nil {
		return err
	}
	if !validID(m.CommandID) || m.Command != "ping" {
		return fmt.Errorf("command_id is invalid or command is unsupported")
	}
	return nil
}

func (m CommandResult) Validate(topic Topic, now time.Time) error {
	if topic.Kind != KindResponses {
		return fmt.Errorf("command result requires responses topic")
	}
	if err := validateEnvelope(topic, m.SchemaVersion, m.MessageID, m.DeviceID, m.OccurredAt, now); err != nil {
		return err
	}
	if !validID(m.CommandID) || (m.Status != "accepted" && m.Status != "completed" && m.Status != "rejected") {
		return fmt.Errorf("command result has invalid command_id or status")
	}
	return nil
}

func (m SalesReport) Validate(topic Topic, now time.Time) error {
	if topic.Kind != KindSales {
		return fmt.Errorf("sales report requires sales topic")
	}
	if err := validateEnvelope(topic, m.SchemaVersion, m.ReportID, m.DeviceID, m.OccurredAt, now); err != nil {
		return err
	}
	if len(m.Items) == 0 || len(m.Items) > 100 {
		return fmt.Errorf("sales report must contain 1 to 100 items")
	}
	for i, item := range m.Items {
		if !validID(item.SKU) || item.Quantity <= 0 || item.AmountMinor < 0 {
			return fmt.Errorf("item %d has invalid SKU, quantity, or amount_minor", i)
		}
	}
	return nil
}