package tracing

import (
	"context"
	"fmt"
	"io"
	"sync"
	"sync/atomic"

	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	"go.opentelemetry.io/otel/exporters/stdout/stdouttrace"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	semconv "go.opentelemetry.io/otel/semconv/v1.17.0"
	"go.opentelemetry.io/otel/trace"
	"go.opentelemetry.io/otel/trace/noop"
)

var (
	// tracingLock protects provider, done, config and tracer from concurrent update.
	// These fields change when the config is updated or the application is shutdown.
	tracingLock = &sync.Mutex{}

	provider *sdktrace.TracerProvider
	done     bool

	config = Config{}
	tracer = getDefaultTracer()
)

// Setup initializes global tracer.
// Returns true if global tracer was updated.
// Shutdown method must be called for graceful shutdown.
func Setup(ctx context.Context, cfg Config) (bool, error) {
	if err := cfg.validate(); err != nil {
		return false, err
	}

	tracingLock.Lock()
	defer tracingLock.Unlock()

	if done {
		return false, fmt.Errorf("failed to setup tracing: already shutdown")
	}

	if !config.hasChange(&cfg) {
		return false, nil
	}

	if !cfg.Enabled {
		config = cfg
		tracer.Store(&tracerHolder{Tracer: noop.NewTracerProvider().Tracer("")})
		return true, flushAndShutdown(ctx)
	}

	exp, err := getExporter(ctx, &cfg)
	if err != nil {
		return false, err
	}

	prevProvider := provider

	provider = sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exp),
		sdktrace.WithResource(newResource(&cfg)),
	)

	config = cfg
	tracer.Store(&tracerHolder{Tracer: provider.Tracer(cfg.Service)})

	var retErr error
	if prevProvider != nil {
		retErr = prevProvider.ForceFlush(ctx)
		if err := prevProvider.Shutdown(ctx); err != nil {
			if retErr == nil {
				retErr = err
			} else {
				retErr = fmt.Errorf("%v ; %v", retErr, err)
			}
		}
	}

	return true, retErr
}

// Shutdown shutdowns tracing.
func Shutdown(ctx context.Context) error {
	tracingLock.Lock()
	defer tracingLock.Unlock()

	if done {
		return nil
	}

	done = true

	config = Config{}
	tracer.Store(&tracerHolder{Tracer: noop.NewTracerProvider().Tracer("")})

	return flushAndShutdown(ctx)
}

func getDefaultTracer() *atomic.Pointer[tracerHolder] {
	v := new(atomic.Pointer[tracerHolder])
	v.Store(&tracerHolder{Tracer: noop.NewTracerProvider().Tracer("")})
	return v
}

func flushAndShutdown(ctx context.Context) error {
	if provider == nil {
		return nil
	}

	tmp := provider
	provider = nil
	var retErr error
	retErr = tmp.ForceFlush(ctx)
	if err := tmp.Shutdown(ctx); err != nil {
		if retErr == nil {
			retErr = err
		} else {
			retErr = fmt.Errorf("%v ; %v", retErr, err)
		}
	}
	return retErr
}

func getExporter(ctx context.Context, cfg *Config) (sdktrace.SpanExporter, error) {
	switch cfg.Exporter {
	default:
		return nil, fmt.Errorf("failed to setup tracing: unknown tracing exporter (%s)", cfg.Exporter)
	case StdoutExporter:
		return stdouttrace.New()
	case NoOpExporter:
		return stdouttrace.New(stdouttrace.WithWriter(io.Discard))
	case OTLPgRPCExporter:
		return otlptracegrpc.New(ctx, otlptracegrpc.WithEndpoint(cfg.Endpoint), otlptracegrpc.WithInsecure())
	}
}

func newResource(cfg *Config) *resource.Resource {
	attrs := []attribute.KeyValue{
		semconv.ServiceName(cfg.Service),
	}
	if len(cfg.Version) > 0 {
		attrs = append(attrs, semconv.ServiceVersion(cfg.Version))
	}
	if len(cfg.InstanceID) > 0 {
		attrs = append(attrs, semconv.ServiceInstanceID(cfg.InstanceID))
	}
	return resource.NewWithAttributes(
		semconv.SchemaURL,
		attrs...,
	)
}

type tracerHolder struct {
	Tracer trace.Tracer
}