// Package tracing provides distributed tracing for the KnowFoolery application.
package tracing

import (
	"context"
	"fmt"
	"net/url"
	"strconv"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/codes"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
	oteltrace "go.opentelemetry.io/otel/trace"

	"knowfoolery/backend/shared/infra/utils/envutil"
)

// Config holds the configuration for the tracer.
type Config struct {
	ServiceName    string
	ServiceVersion string
	Environment    string
	OTLPEndpoint   string
	// JaegerEndpoint is a deprecated legacy alias kept for backward compatibility.
	JaegerEndpoint string
	SampleRate     float64
	Enabled        bool
}

// DefaultConfig returns a default configuration.
func DefaultConfig() Config {
	return Config{
		ServiceName:    "knowfoolery",
		ServiceVersion: "0.0.0",
		Environment:    "development",
		OTLPEndpoint:   "http://localhost:4318/v1/traces",
		JaegerEndpoint: "http://localhost:14268/api/traces",
		SampleRate:     1.0, // Sample all traces in development
		Enabled:        false,
	}
}

// Tracer provides distributed tracing functionality.
type Tracer struct {
	config   Config
	provider *sdktrace.TracerProvider
	tracer   oteltrace.Tracer
	enabled  bool
}

// NewTracer creates a new Tracer.
func NewTracer(config Config) (*Tracer, error) {
	if !config.Enabled {
		return &Tracer{
			config:  config,
			tracer:  otel.Tracer(config.ServiceName),
			enabled: false,
		}, nil
	}

	sampleRate := config.SampleRate
	if sampleRate < 0 || sampleRate > 1 {
		sampleRate = 1.0
	}

	endpoint := config.OTLPEndpoint
	if endpoint == "" {
		endpoint = config.JaegerEndpoint
	}

	exporter, err := newOTLPHTTPExporter(endpoint)
	if err != nil {
		return nil, fmt.Errorf("create otlp exporter: %w", err)
	}

	res, err := resource.New(
		context.Background(),
		resource.WithAttributes(
			semconv.ServiceName(config.ServiceName),
			semconv.ServiceVersion(config.ServiceVersion),
			attribute.String("deployment.environment", config.Environment),
		),
	)
	if err != nil {
		return nil, fmt.Errorf("create tracer resource: %w", err)
	}

	provider := sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exporter),
		sdktrace.WithSampler(sdktrace.TraceIDRatioBased(sampleRate)),
		sdktrace.WithResource(res),
	)

	otel.SetTracerProvider(provider)

	return &Tracer{
		config:   config,
		provider: provider,
		tracer:   otel.Tracer(config.ServiceName),
		enabled:  true,
	}, nil
}

// Shutdown gracefully shuts down the tracer.
func (t *Tracer) Shutdown(ctx context.Context) error {
	if !t.enabled || t.provider == nil {
		return nil
	}

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

	return nil
}

// StartSpan starts a new span.
func (t *Tracer) StartSpan(ctx context.Context, name string) (context.Context, Span) {
	if !t.enabled || t.tracer == nil {
		return ctx, &noopSpan{}
	}

	spanCtx, span := t.tracer.Start(ctx, name)
	return spanCtx, &otelSpan{span: span}
}

// Span represents a tracing span.
type Span interface {
	// End ends the span.
	End()
	// SetAttribute sets an attribute on the span.
	SetAttribute(key string, value interface{})
	// RecordError records an error on the span.
	RecordError(err error)
}

// noopSpan is a no-op implementation of Span.
type noopSpan struct{}

func (s *noopSpan) End()                                       {}
func (s *noopSpan) SetAttribute(key string, value interface{}) {}
func (s *noopSpan) RecordError(err error)                      {}

// otelSpan wraps an OpenTelemetry span with the local Span interface.
type otelSpan struct {
	span oteltrace.Span
}

func (s *otelSpan) End() {
	s.span.End()
}

func (s *otelSpan) SetAttribute(key string, value interface{}) {
	switch v := value.(type) {
	case string:
		s.span.SetAttributes(attribute.String(key, v))
	case int:
		s.span.SetAttributes(attribute.Int(key, v))
	case int64:
		s.span.SetAttributes(attribute.Int64(key, v))
	case float64:
		s.span.SetAttributes(attribute.Float64(key, v))
	case bool:
		s.span.SetAttributes(attribute.Bool(key, v))
	default:
		s.span.SetAttributes(attribute.String(key, fmt.Sprint(value)))
	}
}

func (s *otelSpan) RecordError(err error) {
	if err == nil {
		return
	}

	s.span.RecordError(err)
	s.span.SetStatus(codes.Error, err.Error())
}

// TraceServiceOperation traces a service operation.
func TraceServiceOperation(ctx context.Context, tracer *Tracer, serviceName,
	operation string, fn func(context.Context) error) error {

	ctx, span := tracer.StartSpan(ctx, fmt.Sprintf("%s.%s", serviceName, operation))
	defer span.End()

	err := fn(ctx)
	if err != nil {
		span.RecordError(err)
		span.SetAttribute("error", true)
	}

	return err
}

// TraceDatabaseOperation traces a database operation.
func TraceDatabaseOperation(ctx context.Context, tracer *Tracer, operation,
	table string, fn func(context.Context) error) error {

	ctx, span := tracer.StartSpan(ctx, fmt.Sprintf("db.%s.%s", operation, table))
	defer span.End()

	span.SetAttribute("db.operation", operation)
	span.SetAttribute("db.table", table)
	span.SetAttribute("db.system", "postgresql")

	err := fn(ctx)
	if err != nil {
		span.RecordError(err)
	}

	return err
}

// ConfigFromEnv creates a Config from environment variables.
func ConfigFromEnv() Config {
	cfg := DefaultConfig()

	cfg.Enabled = parseBoolEnv("TRACING_ENABLED", cfg.Enabled)
	cfg.ServiceName = envutil.String("TRACING_SERVICE_NAME", cfg.ServiceName)
	cfg.ServiceVersion = envutil.String("TRACING_SERVICE_VERSION", cfg.ServiceVersion)
	cfg.Environment = envutil.String("TRACING_ENVIRONMENT", cfg.Environment)
	legacyEndpoint := envutil.String("TRACING_JAEGER_ENDPOINT", "")
	if legacyEndpoint != "" {
		cfg.JaegerEndpoint = legacyEndpoint
	}
	cfg.OTLPEndpoint = envutil.String("TRACING_OTLP_ENDPOINT", "")
	if cfg.OTLPEndpoint == "" && legacyEndpoint != "" {
		cfg.OTLPEndpoint = legacyEndpoint
	}
	if cfg.OTLPEndpoint == "" {
		cfg.OTLPEndpoint = DefaultConfig().OTLPEndpoint
	}
	cfg.SampleRate = parseFloatEnv("TRACING_SAMPLE_RATE", cfg.SampleRate)

	return cfg
}

func newOTLPHTTPExporter(endpoint string) (sdktrace.SpanExporter, error) {
	parsed, err := url.Parse(endpoint)
	if err != nil {
		return nil, fmt.Errorf("parse endpoint: %w", err)
	}
	if parsed.Scheme == "" || parsed.Host == "" {
		return nil, fmt.Errorf("endpoint must be absolute URL, got %q", endpoint)
	}
	if parsed.Scheme != "http" && parsed.Scheme != "https" {
		return nil, fmt.Errorf("unsupported endpoint scheme %q", parsed.Scheme)
	}

	urlPath := parsed.EscapedPath()
	if urlPath == "" || urlPath == "/" {
		urlPath = "/v1/traces"
	}

	options := []otlptracehttp.Option{
		otlptracehttp.WithEndpoint(parsed.Host),
		otlptracehttp.WithURLPath(urlPath),
	}
	if parsed.Scheme == "http" {
		options = append(options, otlptracehttp.WithInsecure())
	}

	exporter, err := otlptracehttp.New(context.Background(), options...)
	if err != nil {
		return nil, err
	}

	return exporter, nil
}

func parseBoolEnv(key string, fallback bool) bool {
	value := envutil.String(key, "")
	if value == "" {
		return fallback
	}

	parsed, err := strconv.ParseBool(value)
	if err != nil {
		return fallback
	}

	return parsed
}

func parseFloatEnv(key string, fallback float64) float64 {
	value := envutil.String(key, "")
	if value == "" {
		return fallback
	}

	parsed, err := strconv.ParseFloat(value, 64)
	if err != nil {
		return fallback
	}

	return parsed
}