// Package tracing provides OpenTelemetry distributed tracing setup with OTLP export support.
//
// This package handles the complete OpenTelemetry SDK initialization including:
//   - Trace provider configuration with batching and resource detection
//   - Log provider setup for structured log export via OTLP
//   - Automatic resource discovery (service name, version, host, container, process info)
//   - Support for both gRPC and HTTP OTLP exporters with TLS configuration
//   - Propagation context setup for distributed tracing across services
//   - Graceful shutdown handling for all telemetry components
//
// The package supports various deployment scenarios:
//   - Development: Local OTLP collectors or observability backends
//   - Production: Secure OTLP export with mutual TLS authentication
//   - Container environments: Automatic container and Kubernetes resource detection
//
// Configuration is primarily handled via standard OpenTelemetry environment variables:
//   - OTEL_SERVICE_NAME: Service identification
//   - OTEL_EXPORTER_OTLP_PROTOCOL: Protocol selection (grpc, http/protobuf)
//   - OTEL_TRACES_EXPORTER: Exporter type (otlp, autoexport)
//   - OTEL_RESOURCE_ATTRIBUTES: Additional resource attributes
//
// Example usage:
//
//	cfg := &tracing.TracerConfig{
//		ServiceName: "my-service",
//		Environment: "production",
//		Endpoint:    "https://otlp.example.com:4317",
//	}
//	shutdown, err := tracing.InitTracer(ctx, cfg)
//	if err != nil {
//		log.Fatal(err)
//	}
//	defer shutdown(ctx)
package tracing

// todo, review:
// https://github.com/ttys3/tracing-go/blob/main/tracing.go#L136

import (
	"context"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"os"
	"slices"
	"time"

	"go.ntppool.org/common/logger"
	"go.ntppool.org/common/version"
	"google.golang.org/grpc/credentials"

	"go.opentelemetry.io/contrib/exporters/autoexport"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp"
	logglobal "go.opentelemetry.io/otel/log/global"
	"go.opentelemetry.io/otel/propagation"
	sdklog "go.opentelemetry.io/otel/sdk/log"
	"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"
)

const (
	// svcNameKey is the environment variable name that Service Name information will be read from.
	svcNameKey = "OTEL_SERVICE_NAME"

	otelExporterOTLPProtoEnvKey       = "OTEL_EXPORTER_OTLP_PROTOCOL"
	otelExporterOTLPTracesProtoEnvKey = "OTEL_EXPORTER_OTLP_TRACES_PROTOCOL"
)

var errInvalidOTLPProtocol = errors.New("invalid OTLP protocol - should be one of ['grpc', 'http/protobuf']")

// https://github.com/open-telemetry/opentelemetry-go/blob/main/exporters/otlp/otlptrace/otlptracehttp/example_test.go

// TpShutdownFunc represents a function that gracefully shuts down telemetry providers.
// It should be called during application shutdown to ensure all telemetry data is flushed
// and exporters are properly closed. The context can be used to set shutdown timeouts.
type TpShutdownFunc func(ctx context.Context) error

// Tracer returns the configured OpenTelemetry tracer for the NTP Pool project.
// This tracer should be used for creating spans and distributed tracing throughout
// the application. It uses the global tracer provider set up by InitTracer/SetupSDK.
func Tracer() trace.Tracer {
	traceProvider := otel.GetTracerProvider()
	return traceProvider.Tracer("ntppool-tracer")
}

// Start creates a new span with the given name and options using the configured tracer.
// This is a convenience function that wraps the standard OpenTelemetry span creation.
// It returns a new context containing the span and the span itself for further configuration.
//
// The returned context should be used for downstream operations to maintain trace correlation.
func Start(ctx context.Context, spanName string, opts ...trace.SpanStartOption) (context.Context, trace.Span) {
	return Tracer().Start(ctx, spanName, opts...)
}

// GetClientCertificate defines a function type for providing client certificates for mutual TLS.
// This is used when exporting telemetry data to secured OTLP endpoints that require
// client certificate authentication.
type GetClientCertificate func(*tls.CertificateRequestInfo) (*tls.Certificate, error)

// TracerConfig provides configuration options for OpenTelemetry tracing setup.
// It supplements standard OpenTelemetry environment variables with additional
// NTP Pool-specific configuration including TLS settings for secure OTLP export.
type TracerConfig struct {
	ServiceName string // Service name for resource identification (overrides OTEL_SERVICE_NAME)
	Environment string // Deployment environment (development, staging, production)
	Endpoint    string // OTLP endpoint hostname/port (e.g., "otlp.example.com:4317")
	EndpointURL string // Complete OTLP endpoint URL (e.g., "https://otlp.example.com:4317/v1/traces")

	CertificateProvider GetClientCertificate // Client certificate provider for mutual TLS
	RootCAs             *x509.CertPool       // CA certificate pool for server verification
}

// InitTracer initializes the OpenTelemetry SDK with the provided configuration.
// This is the main entry point for setting up distributed tracing in applications.
//
// The function configures trace and log providers, sets up OTLP exporters,
// and returns a shutdown function that must be called during application termination.
//
// Returns a shutdown function and an error. The shutdown function should be called
// with a context that has an appropriate timeout for graceful shutdown.
func InitTracer(ctx context.Context, cfg *TracerConfig) (TpShutdownFunc, error) {
	// todo: setup environment from cfg
	return SetupSDK(ctx, cfg)
}

// SetupSDK performs the complete OpenTelemetry SDK initialization including resource
// discovery, exporter configuration, provider setup, and shutdown function creation.
//
// The function automatically discovers system resources (service info, host, container,
// process details) and configures both trace and log exporters. It supports multiple
// OTLP protocols (gRPC, HTTP) and handles TLS configuration for secure deployments.
//
// The returned shutdown function coordinates graceful shutdown of all telemetry
// components in the reverse order of their initialization.
func SetupSDK(ctx context.Context, cfg *TracerConfig) (shutdown TpShutdownFunc, err error) {
	if cfg == nil {
		cfg = &TracerConfig{}
	}

	log := logger.Setup()

	if serviceName := os.Getenv(svcNameKey); len(serviceName) == 0 {
		if len(cfg.ServiceName) > 0 {
			os.Setenv(svcNameKey, cfg.ServiceName)
		}
	}

	resources := []resource.Option{
		resource.WithFromEnv(),      // Discover and provide attributes from OTEL_RESOURCE_ATTRIBUTES and OTEL_SERVICE_NAME environment variables.
		resource.WithTelemetrySDK(), // Discover and provide information about the OpenTelemetry SDK used.
		resource.WithProcess(),      // Discover and provide process information.
		resource.WithOS(),           // Discover and provide OS information.
		resource.WithContainer(),    // Discover and provide container information.
		resource.WithHost(),         // Discover and provide host information.

		// set above via os.Setenv() for WithFromEnv to find
		// resource.WithAttributes(semconv.ServiceNameKey.String(cfg.ServiceName)),

		resource.WithAttributes(semconv.ServiceVersionKey.String(version.Version())),
	}

	if len(cfg.Environment) > 0 {
		resources = append(resources,
			resource.WithAttributes(attribute.String("environment", cfg.Environment)),
		)
	}

	res, err := resource.New(
		context.Background(),
		resources...,
	)
	if errors.Is(err, resource.ErrPartialResource) || errors.Is(err, resource.ErrSchemaURLConflict) {
		log.Warn("otel resource setup", "err", err) // Log non-fatal issues.
	} else if err != nil {
		log.Error("otel resource setup", "err", err)
		return
	}

	var shutdownFuncs []func(context.Context) error
	shutdown = func(ctx context.Context) error {
		var err error
		// need to shutdown the providers first,
		// exporters after which is the opposite
		// order they are setup.
		slices.Reverse(shutdownFuncs)
		for _, fn := range shutdownFuncs {
			// log.Warn("shutting down", "fn", fn)
			err = errors.Join(err, fn(ctx))
		}
		shutdownFuncs = nil
		if err != nil {
			log.Warn("shutdown returned errors", "err", err)
		}
		return err
	}

	// handleErr calls shutdown for cleanup and makes sure that all errors are returned.
	handleErr := func(inErr error) {
		err = errors.Join(inErr, shutdown(ctx))
	}

	prop := newPropagator()
	otel.SetTextMapPropagator(prop)

	var spanExporter sdktrace.SpanExporter

	switch os.Getenv("OTEL_TRACES_EXPORTER") {
	case "":
		spanExporter, err = newOLTPExporter(ctx, cfg)
	case "otlp":
		spanExporter, err = newOLTPExporter(ctx, cfg)
	default:
		// log.Debug("OTEL_TRACES_EXPORTER", "fallback", os.Getenv("OTEL_TRACES_EXPORTER"))
		spanExporter, err = autoexport.NewSpanExporter(ctx)
	}
	if err != nil {
		handleErr(err)
		return
	}
	shutdownFuncs = append(shutdownFuncs, spanExporter.Shutdown)

	logExporter, err := autoexport.NewLogExporter(ctx)
	if err != nil {
		handleErr(err)
		return
	}
	shutdownFuncs = append(shutdownFuncs, logExporter.Shutdown)

	// Set up trace provider.
	tracerProvider, err := newTraceProvider(spanExporter, res)
	if err != nil {
		handleErr(err)
		return
	}
	shutdownFuncs = append(shutdownFuncs, tracerProvider.Shutdown)
	otel.SetTracerProvider(tracerProvider)

	logProvider := sdklog.NewLoggerProvider(sdklog.WithResource(res),
		sdklog.WithProcessor(
			sdklog.NewBatchProcessor(logExporter, sdklog.WithExportBufferSize(10)),
		),
	)

	logglobal.SetLoggerProvider(logProvider)
	shutdownFuncs = append(shutdownFuncs, func(ctx context.Context) error {
		logProvider.ForceFlush(ctx)
		return logProvider.Shutdown(ctx)
	},
	)

	if err != nil {
		handleErr(err)
		return
	}

	return
}

func newOLTPExporter(ctx context.Context, cfg *TracerConfig) (sdktrace.SpanExporter, error) {
	log := logger.Setup()

	var tlsConfig *tls.Config

	if cfg.CertificateProvider != nil {
		tlsConfig = &tls.Config{
			GetClientCertificate: cfg.CertificateProvider,
			RootCAs:              cfg.RootCAs,
		}
	}

	proto := os.Getenv(otelExporterOTLPTracesProtoEnvKey)
	if proto == "" {
		proto = os.Getenv(otelExporterOTLPProtoEnvKey)
	}

	// Fallback to default, http/protobuf.
	if proto == "" {
		proto = "http/protobuf"
	}

	var client otlptrace.Client

	switch proto {
	case "grpc":
		opts := []otlptracegrpc.Option{
			otlptracegrpc.WithCompressor("gzip"),
		}
		if tlsConfig != nil {
			opts = append(opts, otlptracegrpc.WithTLSCredentials(credentials.NewTLS(tlsConfig)))
		}
		if len(cfg.Endpoint) > 0 {
			log.Info("adding option", "Endpoint", cfg.Endpoint)
			opts = append(opts, otlptracegrpc.WithEndpoint(cfg.Endpoint))
		}
		if len(cfg.EndpointURL) > 0 {
			log.Info("adding option", "EndpointURL", cfg.EndpointURL)
			opts = append(opts, otlptracegrpc.WithEndpointURL(cfg.EndpointURL))
		}

		client = otlptracegrpc.NewClient(opts...)
	case "http/protobuf", "http/json":
		opts := []otlptracehttp.Option{
			otlptracehttp.WithCompression(otlptracehttp.GzipCompression),
		}
		if tlsConfig != nil {
			opts = append(opts, otlptracehttp.WithTLSClientConfig(tlsConfig))
		}
		if len(cfg.Endpoint) > 0 {
			opts = append(opts, otlptracehttp.WithEndpoint(cfg.Endpoint))
		}
		if len(cfg.EndpointURL) > 0 {
			opts = append(opts, otlptracehttp.WithEndpointURL(cfg.EndpointURL))
		}

		client = otlptracehttp.NewClient(opts...)
	default:
		return nil, errInvalidOTLPProtocol
	}

	exporter, err := otlptrace.New(ctx, client)
	if err != nil {
		log.ErrorContext(ctx, "creating OTLP trace exporter", "err", err)
	}
	return exporter, err
}

func newTraceProvider(traceExporter sdktrace.SpanExporter, res *resource.Resource) (*sdktrace.TracerProvider, error) {
	traceProvider := sdktrace.NewTracerProvider(
		sdktrace.WithResource(res),
		sdktrace.WithBatcher(traceExporter,
			sdktrace.WithBatchTimeout(time.Second*3),
		),
	)
	return traceProvider, nil
}

func newPropagator() propagation.TextMapPropagator {
	return propagation.NewCompositeTextMapPropagator(
		propagation.TraceContext{},
		propagation.Baggage{},
	)
}