// Package protocol defines the messages agents and the master exchange over
// their WebSocket connection. Every message is an Envelope whose Payload is
// one of the payload types below, selected by Type.
//
// Both sides close the connection on a message they cannot parse (bad JSON,
// or a known Type whose payload does not decode). A message with an unknown
// Type is logged and ignored, so a newer peer can add message types without
// breaking an older one.
package protocol

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

// Version is the protocol version this build speaks. Agents send it in
// Hello. Phase-1 agents do not send the field, which reads as 0.
const Version = 2

// MinJobVersion is the lowest agent protocol version the master sends jobs to.
const MinJobVersion = 2

// MaxJobOutput caps the job output an agent keeps and the master stores, in
// bytes. Only the tail is kept. With JSON escaping (worst case 6 bytes per
// byte) a JobResult carrying this much output still fits the 1 MiB read limit.
const MaxJobOutput = 128 << 10

// Message types sent from agent to master.
const (
	TypeHello       = "hello"
	TypeSnapshot    = "snapshot"
	TypeUpsert      = "upsert"
	TypeRemove      = "remove"
	TypeJobProgress = "job.progress"
	TypeJobResult   = "job.result"
)

// Message types sent from master to agent.
const (
	TypeJobStart = "job.start"
	TypeJobAck   = "job.ack"
)

// Envelope wraps every message on the wire. ID is the job ID (decimal) on
// job.* messages and empty on inventory messages.
type Envelope struct {
	Type    string          `json:"type"`
	ID      string          `json:"id,omitempty"`
	Payload json.RawMessage `json:"payload,omitempty"`
}

// Hello is the first message an agent sends on every connection.
type Hello struct {
	AgentVersion    string `json:"agent_version"`
	ProtocolVersion int    `json:"protocol_version,omitempty"`
	Hostname        string `json:"hostname"`
	SwarmManager    bool   `json:"swarm_manager"`
	SwarmClusterID  string `json:"swarm_cluster_id,omitempty"`
	// KnownJobs lists every job ID the agent holds: queued, running, or
	// finished but not yet acknowledged. The master fails its queued and
	// running jobs for this agent that are not listed ("lost").
	KnownJobs []string `json:"known_jobs,omitempty"`
}

// Kind says how a service is deployed.
type Kind string

// Service kinds.
const (
	KindCompose   Kind = "compose"
	KindSwarm     Kind = "swarm"
	KindContainer Kind = "container"
)

// Service is one deployable unit as an agent sees it. Which location fields
// are set depends on Kind.
type Service struct {
	Kind           Kind   `json:"kind"`
	ComposeProject string `json:"compose_project,omitempty"`
	ComposeService string `json:"compose_service,omitempty"`
	WorkingDir     string `json:"working_dir,omitempty"`
	// ConfigFiles is the raw com.docker.compose.project.config_files label:
	// absolute host paths joined by ",". Kept as a string so Service stays
	// comparable with ==.
	ConfigFiles   string `json:"config_files,omitempty"`
	SwarmService  string `json:"swarm_service,omitempty"`
	ContainerName string `json:"container_name,omitempty"`
	Image         string `json:"image"`
	Digest        string `json:"digest,omitempty"`
	Status        string `json:"status"`
	Self          bool   `json:"self,omitempty"`
}

// Key identifies the service within one agent's report. It is stable across
// restarts and is what Remove refers to.
func (s Service) Key() string {
	switch s.Kind {
	case KindCompose:
		return "compose|" + s.ComposeProject + "|" + s.ComposeService
	case KindSwarm:
		return "swarm|" + s.SwarmService
	default:
		return "container|" + s.ContainerName
	}
}

// Snapshot is the agent's complete inventory. Sent after every Hello.
type Snapshot struct {
	Services []Service `json:"services"`
}

// Upsert reports a service that appeared or changed.
type Upsert struct {
	Service Service `json:"service"`
}

// Remove reports that the service with Key is gone.
type Remove struct {
	Key string `json:"key"`
}

// JobStart asks the agent to deploy Ref to one service. Ref is "repo:tag";
// the agent resolves the tag's current digest and deploys "repo:tag@digest".
// The agent reads the Compose working directory and config files from its
// own Docker daemon, not from this message.
type JobStart struct {
	Kind           Kind   `json:"kind"`
	ComposeProject string `json:"compose_project,omitempty"`
	ComposeService string `json:"compose_service,omitempty"`
	SwarmClusterID string `json:"swarm_cluster_id,omitempty"`
	SwarmService   string `json:"swarm_service,omitempty"`
	Ref            string `json:"ref"`
}

// JobProgress carries output produced since the previous JobProgress for the
// same job. The first JobProgress of a job (Output may be empty) tells the
// master the job is running.
type JobProgress struct {
	Output string `json:"output"`
}

// Job result statuses.
const (
	ResultSucceeded = "succeeded"
	ResultFailed    = "failed"
)

// JobResult is a job's outcome. The agent keeps it until the master sends
// JobAck for the same ID, and resends it after every reconnect until then.
type JobResult struct {
	Status string `json:"status"`        // ResultSucceeded or ResultFailed
	Ref    string `json:"ref,omitempty"` // deployed "repo:tag@digest", set on success
	Error  string `json:"error,omitempty"`
	// Output is the job's complete output, tail-capped at MaxJobOutput. It
	// replaces whatever the master assembled from JobProgress messages.
	Output          string    `json:"output"`
	OutputTruncated bool      `json:"output_truncated,omitempty"`
	StartedAt       time.Time `json:"started_at"`
	FinishedAt      time.Time `json:"finished_at"`
}

// JobAck tells the agent the master has stored the JobResult with the same
// ID. It has no fields.
type JobAck struct{}

// Encode wraps payload in an Envelope of the given type.
func Encode(typ string, payload any) (Envelope, error) {
	raw, err := json.Marshal(payload)
	if err != nil {
		return Envelope{}, fmt.Errorf("encode %s: %w", typ, err)
	}
	return Envelope{Type: typ, Payload: raw}, nil
}

// EncodeJob wraps a job message's payload in an Envelope carrying the job ID.
func EncodeJob(typ, jobID string, payload any) (Envelope, error) {
	env, err := Encode(typ, payload)
	if err != nil {
		return Envelope{}, err
	}
	env.ID = jobID
	return env, nil
}

// Decode unpacks env's payload into T.
func Decode[T any](env Envelope) (T, error) {
	var v T
	if err := json.Unmarshal(env.Payload, &v); err != nil {
		return v, fmt.Errorf("decode %s: %w", env.Type, err)
	}
	return v, nil
}