package obs

import (
	"context"
	"fmt"
	"net/url"
	"os"
	"time"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp"
	"go.opentelemetry.io/otel/propagation"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
)

// TracingOptions says where traces go.
type TracingOptions struct {
	Endpoint    string  // http(s)://host:4318; empty reads OTEL_EXPORTER_OTLP_ENDPOINT
	SampleRatio float64 // of traces the caller did not already decide; zero takes 1
	Service     string
	Version     string
}

// SetupTracing installs the global tracer provider and the W3C propagator,
// and returns what flushes it on the way out. With no endpoint it installs
// nothing, and every span is the no-op one: tracing that is off costs a
// function call per span.
func SetupTracing(ctx context.Context, o TracingOptions) (func(context.Context) error, error) {
	noop := func(context.Context) error { return nil }
	endpoint := o.Endpoint
	if endpoint == "" {
		endpoint = os.Getenv("OTEL_EXPORTER_OTLP_ENDPOINT")
	}
	if endpoint == "" {
		return noop, nil
	}
	u, err := url.Parse(endpoint)
	if err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
		return noop, fmt.Errorf("tracing endpoint %q is not an http(s) address", endpoint)
	}
	opts := []otlptracehttp.Option{otlptracehttp.WithEndpoint(u.Host)}
	if u.Scheme == "http" {
		opts = append(opts, otlptracehttp.WithInsecure())
	}
	if u.Path != "" && u.Path != "/" {
		opts = append(opts, otlptracehttp.WithURLPath(u.Path))
	}
	exp, err := otlptracehttp.New(ctx, opts...)
	if err != nil {
		return noop, err
	}
	ratio := o.SampleRatio
	if ratio <= 0 || ratio > 1 {
		ratio = 1
	}
	res := resource.NewSchemaless(
		attribute.String("service.name", o.Service),
		attribute.String("service.version", o.Version),
	)
	tp := sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exp, sdktrace.WithBatchTimeout(5*time.Second)),
		sdktrace.WithResource(res),
		// A caller that sent a sampled traceparent has already decided; the
		// ratio is for requests that arrive without one.
		sdktrace.WithSampler(sdktrace.ParentBased(sdktrace.TraceIDRatioBased(ratio))),
	)
	otel.SetTracerProvider(tp)
	otel.SetTextMapPropagator(propagation.TraceContext{})
	return tp.Shutdown, nil
}