// Package livenessreply implements the Lobby-facing liveness service-
// layer answer owned by Game Master. It is driven by Game Lobby
// resuming a paused game through
// `GET /api/v1/internal/games/{game_id}/liveness` and reflects GM's
// own view of the runtime without ever calling the engine.
//
// Lifecycle and failure-mode semantics follow `gamemaster/README.md
// §Liveness reply`. The 200 / status="" response on
// `runtime_not_found` is the Stage 17 D5 decision recorded in
// `gamemaster/docs/stage17-admin-operations.md`.
package livenessreply

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"strings"

	"galaxy/gamemaster/internal/domain/runtime"
	"galaxy/gamemaster/internal/ports"
)

// Input stores the per-call arguments for one liveness reply.
type Input struct {
	// GameID identifies the runtime to inspect.
	GameID string
}

// Validate reports whether input carries the structural invariants the
// service requires before any store is touched.
func (input Input) Validate() error {
	if strings.TrimSpace(input.GameID) == "" {
		return fmt.Errorf("game id must not be empty")
	}
	return nil
}

// Result stores the deterministic outcome of one Handle call. The
// endpoint always answers 200; the result fields populate the JSON
// body. ErrorCode / ErrorMessage are reserved for handler-side error
// envelopes and are never set by Handle on a successful read.
type Result struct {
	// Ready is true when the runtime exists and is in `running`.
	Ready bool

	// Status carries the observed runtime status. Empty when the
	// runtime record does not exist (Stage 17 D5).
	Status runtime.Status
}

// Dependencies groups the collaborators required by Service.
type Dependencies struct {
	// RuntimeRecords supplies the runtime status read.
	RuntimeRecords ports.RuntimeRecordStore

	// Logger records structured service-level events. Defaults to
	// `slog.Default()` when nil.
	Logger *slog.Logger
}

// Service executes the liveness reply lookup.
type Service struct {
	runtimeRecords ports.RuntimeRecordStore
	logger         *slog.Logger
}

// NewService constructs one Service from deps.
func NewService(deps Dependencies) (*Service, error) {
	if deps.RuntimeRecords == nil {
		return nil, errors.New("new liveness reply service: nil runtime records")
	}
	logger := deps.Logger
	if logger == nil {
		logger = slog.Default()
	}
	logger = logger.With("service", "gamemaster.livenessreply")

	return &Service{
		runtimeRecords: deps.RuntimeRecords,
		logger:         logger,
	}, nil
}

// Handle executes one liveness reply lookup. The Go-level error return
// is reserved for non-business failures: nil context, nil receiver,
// invalid input (so the handler can answer `invalid_request`), or a
// store read failure (so the handler can answer `service_unavailable`).
// `runtime.ErrNotFound` is intentionally absorbed into Result with
// `Ready=false` and an empty status.
func (service *Service) Handle(ctx context.Context, input Input) (Result, error) {
	if service == nil {
		return Result{}, errors.New("liveness reply: nil service")
	}
	if ctx == nil {
		return Result{}, errors.New("liveness reply: nil context")
	}
	if err := input.Validate(); err != nil {
		return Result{}, fmt.Errorf("%s: %w", ErrorCodeInvalidRequest, err)
	}

	record, err := service.runtimeRecords.Get(ctx, input.GameID)
	switch {
	case err == nil:
		return Result{
			Ready:  record.Status == runtime.StatusRunning,
			Status: record.Status,
		}, nil
	case errors.Is(err, runtime.ErrNotFound):
		return Result{Ready: false, Status: ""}, nil
	default:
		return Result{}, fmt.Errorf("%s: get runtime record: %w", ErrorCodeServiceUnavailable, err)
	}
}