// Package telemetry owns the OpenTelemetry runtime for the backend process.
//
// New constructs the configured tracer and meter providers, registers them as
// the OpenTelemetry globals, and exposes Shutdown for orderly exit. The MVP
// supports the `none` and `stdout` exporters; OTLP export and dashboards arrive
// in a later stage. The per-request timing middleware lives in middleware.go and
// uses the registered global tracer, so requests are timed and logged even when
// the exporter is `none`.
package telemetry

import (
	"context"
	"errors"
	"fmt"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/stdout/stdoutmetric"
	"go.opentelemetry.io/otel/exporters/stdout/stdouttrace"
	"go.opentelemetry.io/otel/metric"
	"go.opentelemetry.io/otel/propagation"
	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	"go.opentelemetry.io/otel/trace"
	"go.uber.org/zap"
)

// Exporter selectors supported by the backend.
const (
	ExporterNone   = "none"
	ExporterStdout = "stdout"
)

// DefaultServiceName labels traces and metrics when BACKEND_SERVICE_NAME is
// unset.
const DefaultServiceName = "scrabble-backend"

// Config selects the telemetry providers' service name and exporters.
type Config struct {
	// ServiceName is reported as the OpenTelemetry service.name resource.
	ServiceName string
	// TracesExporter is one of ExporterNone or ExporterStdout.
	TracesExporter string
	// MetricsExporter is one of ExporterNone or ExporterStdout.
	MetricsExporter string
}

// DefaultConfig returns the MVP telemetry configuration: named service, no
// exporters (so no collector is required locally or in CI).
func DefaultConfig() Config {
	return Config{
		ServiceName:     DefaultServiceName,
		TracesExporter:  ExporterNone,
		MetricsExporter: ExporterNone,
	}
}

// Validate reports whether the configuration selects supported exporters.
func (c Config) Validate() error {
	if c.ServiceName == "" {
		return errors.New("telemetry: ServiceName must not be empty")
	}
	if err := validateExporter("traces", c.TracesExporter); err != nil {
		return err
	}
	return validateExporter("metrics", c.MetricsExporter)
}

func validateExporter(kind, value string) error {
	switch value {
	case ExporterNone, ExporterStdout:
		return nil
	default:
		return fmt.Errorf("telemetry: unsupported %s exporter %q", kind, value)
	}
}

// Runtime owns the shared OpenTelemetry providers.
type Runtime struct {
	tracerProvider *sdktrace.TracerProvider
	meterProvider  *sdkmetric.MeterProvider
}

// New constructs the telemetry runtime, registers the global providers and the
// W3C trace-context/baggage propagators, and returns the Runtime. Callers must
// invoke Runtime.Shutdown during process exit.
func New(ctx context.Context, cfg Config) (*Runtime, error) {
	if err := cfg.Validate(); err != nil {
		return nil, err
	}

	res, err := resource.New(ctx, resource.WithAttributes(
		attribute.String("service.name", cfg.ServiceName),
	))
	if err != nil {
		return nil, fmt.Errorf("telemetry: build resource: %w", err)
	}

	tracerProvider, err := newTracerProvider(cfg, res)
	if err != nil {
		return nil, fmt.Errorf("telemetry: build tracer provider: %w", err)
	}
	meterProvider, err := newMeterProvider(cfg, res)
	if err != nil {
		_ = tracerProvider.Shutdown(ctx)
		return nil, fmt.Errorf("telemetry: build meter provider: %w", err)
	}

	otel.SetTracerProvider(tracerProvider)
	otel.SetMeterProvider(meterProvider)
	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
		propagation.TraceContext{},
		propagation.Baggage{},
	))

	return &Runtime{tracerProvider: tracerProvider, meterProvider: meterProvider}, nil
}

// TracerProvider returns the runtime tracer provider, or the global one when r
// is not initialised.
func (r *Runtime) TracerProvider() trace.TracerProvider {
	if r == nil || r.tracerProvider == nil {
		return otel.GetTracerProvider()
	}
	return r.tracerProvider
}

// MeterProvider returns the runtime meter provider, or the global one when r is
// not initialised.
func (r *Runtime) MeterProvider() metric.MeterProvider {
	if r == nil || r.meterProvider == nil {
		return otel.GetMeterProvider()
	}
	return r.meterProvider
}

// Shutdown flushes both providers within ctx.
func (r *Runtime) Shutdown(ctx context.Context) error {
	if r == nil {
		return nil
	}
	var err error
	if r.meterProvider != nil {
		err = errors.Join(err, r.meterProvider.Shutdown(ctx))
	}
	if r.tracerProvider != nil {
		err = errors.Join(err, r.tracerProvider.Shutdown(ctx))
	}
	return err
}

// TraceFieldsFromContext returns zap fields identifying the active span, or nil
// when ctx carries no valid span context. Collocated here so callers do not
// import the OpenTelemetry API directly.
func TraceFieldsFromContext(ctx context.Context) []zap.Field {
	if ctx == nil {
		return nil
	}
	sc := trace.SpanContextFromContext(ctx)
	if !sc.IsValid() {
		return nil
	}
	return []zap.Field{
		zap.String("otel_trace_id", sc.TraceID().String()),
		zap.String("otel_span_id", sc.SpanID().String()),
	}
}

func newTracerProvider(cfg Config, res *resource.Resource) (*sdktrace.TracerProvider, error) {
	switch cfg.TracesExporter {
	case ExporterNone:
		return sdktrace.NewTracerProvider(sdktrace.WithResource(res)), nil
	case ExporterStdout:
		exporter, err := stdouttrace.New()
		if err != nil {
			return nil, fmt.Errorf("stdout trace exporter: %w", err)
		}
		return sdktrace.NewTracerProvider(
			sdktrace.WithBatcher(exporter),
			sdktrace.WithResource(res),
		), nil
	default:
		return nil, fmt.Errorf("unsupported traces exporter %q", cfg.TracesExporter)
	}
}

func newMeterProvider(cfg Config, res *resource.Resource) (*sdkmetric.MeterProvider, error) {
	switch cfg.MetricsExporter {
	case ExporterNone:
		return sdkmetric.NewMeterProvider(sdkmetric.WithResource(res)), nil
	case ExporterStdout:
		exporter, err := stdoutmetric.New()
		if err != nil {
			return nil, fmt.Errorf("stdout metric exporter: %w", err)
		}
		return sdkmetric.NewMeterProvider(
			sdkmetric.WithResource(res),
			sdkmetric.WithReader(sdkmetric.NewPeriodicReader(exporter)),
		), nil
	default:
		return nil, fmt.Errorf("unsupported metrics exporter %q", cfg.MetricsExporter)
	}
}