// Package tracing provides OpenTelemetry instrumentation for the API gateway.
package tracing

import (
	"context"
	"os"
	"time"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	"go.opentelemetry.io/otel/propagation"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
)

// Config holds tracing configuration.
type Config struct {
	ServiceName    string
	ServiceVersion string
	Environment    string
	OTLPEndpoint   string
}

// DefaultConfig returns configuration from environment variables.
func DefaultConfig() Config {
	endpoint := os.Getenv("OTEL_EXPORTER_OTLP_ENDPOINT")
	if endpoint == "" {
		endpoint = "otel-collector.tracing.svc.cluster.local:4317"
	}
	return Config{
		ServiceName:    getEnvOrDefault("OTEL_SERVICE_NAME", "api-gateway"),
		ServiceVersion: getEnvOrDefault("OTEL_SERVICE_VERSION", "1.0.0"),
		Environment:    getEnvOrDefault("OTEL_ENVIRONMENT", "production"),
		OTLPEndpoint:   endpoint,
	}
}

func getEnvOrDefault(key, defaultVal string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return defaultVal
}

// Init initializes the OpenTelemetry tracer provider.
// Returns a shutdown function that should be called on application exit.
func Init(ctx context.Context, cfg Config) (func(context.Context) error, error) {
	// Create OTLP exporter
	exporter, err := otlptracegrpc.New(ctx,
		otlptracegrpc.WithEndpoint(cfg.OTLPEndpoint),
		otlptracegrpc.WithInsecure(),
		otlptracegrpc.WithTimeout(5*time.Second),
	)
	if err != nil {
		return nil, err
	}

	// Create resource with service information
	res, err := resource.Merge(
		resource.Default(),
		resource.NewWithAttributes(
			semconv.SchemaURL,
			semconv.ServiceName(cfg.ServiceName),
			semconv.ServiceVersion(cfg.ServiceVersion),
			attribute.String("deployment.environment", cfg.Environment),
		),
	)
	if err != nil {
		return nil, err
	}

	// Create tracer provider with batch processor
	tp := sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exporter,
			sdktrace.WithBatchTimeout(5*time.Second),
			sdktrace.WithMaxExportBatchSize(512),
		),
		sdktrace.WithResource(res),
		sdktrace.WithSampler(sdktrace.AlwaysSample()),
	)

	// Set global tracer provider
	otel.SetTracerProvider(tp)

	// Set global propagator (W3C Trace Context + Baggage)
	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
		propagation.TraceContext{},
		propagation.Baggage{},
	))

	return tp.Shutdown, nil
}