// Package telemetry provides OpenTelemetry integration for the rdev API.
//
// It initializes a tracer provider with OTLP exporter for distributed tracing.
// Traces are exported to an OpenTelemetry collector (e.g., otel-collector in k8s).
//
// Configuration via environment variables:
//   - OTEL_EXPORTER_OTLP_ENDPOINT: Collector endpoint (default: otel-collector.observability.svc:4317)
//   - OTEL_SERVICE_NAME: Service name for traces (default: rdev-api)
//   - OTEL_SERVICE_VERSION: Service version (default: unknown)
//   - OTEL_SERVICE_NAMESPACE: Namespace (default: rdev)
//   - OTEL_ENABLED: Enable/disable telemetry (default: true)
package telemetry

import (
	"context"
	"fmt"
	"log/slog"
	"os"
	"strings"
	"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"
	"go.opentelemetry.io/otel/trace"
	"go.opentelemetry.io/otel/trace/noop"
)

// Config holds telemetry configuration.
type Config struct {
	// Endpoint is the OTLP collector endpoint (gRPC).
	// Default: otel-collector.observability.svc:4317
	Endpoint string

	// ServiceName identifies this service in traces.
	// Default: rdev-api
	ServiceName string

	// ServiceVersion is the version of this service.
	// Default: unknown
	ServiceVersion string

	// ServiceNamespace groups related services.
	// Default: rdev
	ServiceNamespace string

	// Enabled controls whether telemetry is active.
	// Default: true
	Enabled bool

	// Insecure disables TLS for the gRPC connection.
	// Default: true (for internal k8s communication)
	Insecure bool

	// Logger for telemetry initialization messages.
	Logger *slog.Logger
}

// DefaultConfig returns configuration with defaults applied.
func DefaultConfig() Config {
	return Config{
		Endpoint:         getEnv("OTEL_EXPORTER_OTLP_ENDPOINT", "otel-collector.observability.svc:4317"),
		ServiceName:      getEnv("OTEL_SERVICE_NAME", "rdev-api"),
		ServiceVersion:   getEnv("OTEL_SERVICE_VERSION", "unknown"),
		ServiceNamespace: getEnv("OTEL_SERVICE_NAMESPACE", "rdev"),
		Enabled:          getEnvBool("OTEL_ENABLED", true),
		Insecure:         true,
	}
}

// Telemetry manages OpenTelemetry resources.
type Telemetry struct {
	config         Config
	tracerProvider *sdktrace.TracerProvider
	tracer         trace.Tracer
	logger         *slog.Logger
}

// New creates and initializes a new Telemetry instance.
// Call Shutdown() when done to flush pending traces.
func New(ctx context.Context, cfg Config) (*Telemetry, error) {
	logger := cfg.Logger
	if logger == nil {
		logger = slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
			Level: slog.LevelInfo,
		}))
	}

	t := &Telemetry{
		config: cfg,
		logger: logger,
	}

	if !cfg.Enabled {
		logger.Info("telemetry disabled, using noop tracer")
		t.tracer = noop.NewTracerProvider().Tracer(cfg.ServiceName)
		return t, nil
	}

	// Create OTLP exporter
	opts := []otlptracegrpc.Option{
		otlptracegrpc.WithEndpoint(cfg.Endpoint),
	}
	if cfg.Insecure {
		opts = append(opts, otlptracegrpc.WithInsecure())
	}

	exporter, err := otlptracegrpc.New(ctx, opts...)
	if err != nil {
		return nil, fmt.Errorf("failed to create OTLP exporter: %w", err)
	}

	// Create resource with service information
	// Note: We create a new resource instead of merging with Default() to avoid
	// schema URL conflicts between different semconv versions
	res := resource.NewWithAttributes(
		semconv.SchemaURL,
		semconv.ServiceName(cfg.ServiceName),
		semconv.ServiceVersion(cfg.ServiceVersion),
		semconv.ServiceNamespace(cfg.ServiceNamespace),
		attribute.String("deployment.environment", getEnv("ENVIRONMENT", "production")),
	)

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

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

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

	t.tracerProvider = tp
	t.tracer = tp.Tracer(cfg.ServiceName)

	logger.Info("telemetry initialized",
		"endpoint", cfg.Endpoint,
		"service", cfg.ServiceName,
		"version", cfg.ServiceVersion,
		"namespace", cfg.ServiceNamespace,
	)

	return t, nil
}

// Tracer returns the tracer for creating spans.
func (t *Telemetry) Tracer() trace.Tracer {
	return t.tracer
}

// Shutdown gracefully shuts down the telemetry, flushing any pending traces.
// Should be called during application shutdown.
func (t *Telemetry) Shutdown(ctx context.Context) error {
	if t.tracerProvider == nil {
		return nil
	}

	t.logger.Info("shutting down telemetry")

	// Create a timeout context if none provided
	if _, hasDeadline := ctx.Deadline(); !hasDeadline {
		var cancel context.CancelFunc
		ctx, cancel = context.WithTimeout(ctx, 10*time.Second)
		defer cancel()
	}

	if err := t.tracerProvider.Shutdown(ctx); err != nil {
		return fmt.Errorf("telemetry shutdown failed: %w", err)
	}

	t.logger.Info("telemetry shutdown complete")
	return nil
}

// StartSpan starts a new span with the given name.
// Returns the span and a new context containing the span.
func (t *Telemetry) StartSpan(ctx context.Context, name string, opts ...trace.SpanStartOption) (context.Context, trace.Span) {
	return t.tracer.Start(ctx, name, opts...)
}

// AddSpanEvent adds an event to the current span in the context.
func AddSpanEvent(ctx context.Context, name string, attrs ...attribute.KeyValue) {
	span := trace.SpanFromContext(ctx)
	span.AddEvent(name, trace.WithAttributes(attrs...))
}

// SetSpanError records an error on the current span.
func SetSpanError(ctx context.Context, err error) {
	span := trace.SpanFromContext(ctx)
	span.RecordError(err)
}

// SetSpanAttributes sets attributes on the current span.
func SetSpanAttributes(ctx context.Context, attrs ...attribute.KeyValue) {
	span := trace.SpanFromContext(ctx)
	span.SetAttributes(attrs...)
}

// getEnv returns the environment variable value or the default.
func getEnv(key, defaultVal string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return defaultVal
}

// getEnvBool returns the environment variable as bool or the default.
func getEnvBool(key string, defaultVal bool) bool {
	v := os.Getenv(key)
	if v == "" {
		return defaultVal
	}
	v = strings.ToLower(v)
	return v == "true" || v == "1" || v == "yes"
}