// Package tracing wires OpenTelemetry tracing into the operator: SDK setup
// gated on standard OTEL_* environment variables, span helpers that keep the
// logr/zap logging enriched with trace context, and decorators for the
// reconciler, providers, HTTP server, and Kubernetes API transport.
package tracing

import (
	"context"
	"fmt"
	"os"
	"strings"

	"github.com/go-logr/logr"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp"
	"go.opentelemetry.io/otel/exporters/stdout/stdouttrace"
	"go.opentelemetry.io/otel/propagation"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	semconv "go.opentelemetry.io/otel/semconv/v1.43.0"
)

// maxQueueSize bounds the batch processor's span buffer; the default 2048
// is oversized for this process (3 concurrent reconciles, tickers) and the
// manager pod runs with a 128Mi memory limit.
const maxQueueSize = 512

// Setup initializes the global tracing pipeline from OTEL_* environment
// variables and returns a shutdown func that flushes pending spans.
//
// Tracing stays fully off — no exporter, no global provider, spans are
// no-ops, logs carry no traceID — unless the environment opts in by setting
// OTEL_TRACES_EXPORTER or an OTLP endpoint. OTEL_SDK_DISABLED=true and
// OTEL_TRACES_EXPORTER=none force it off (the Go SDK does not implement
// OTEL_SDK_DISABLED itself). Sampling, endpoints, TLS, and headers follow
// the standard SDK/exporter env vars (OTEL_TRACES_SAMPLER, OTEL_EXPORTER_OTLP_*).
func Setup(ctx context.Context, log logr.Logger, service, version string) (func(context.Context) error, error) {
	noop := func(context.Context) error { return nil }

	exporterEnv := strings.ToLower(strings.TrimSpace(os.Getenv("OTEL_TRACES_EXPORTER")))
	endpointSet := os.Getenv("OTEL_EXPORTER_OTLP_ENDPOINT") != "" ||
		os.Getenv("OTEL_EXPORTER_OTLP_TRACES_ENDPOINT") != ""
	sdkDisabled := strings.EqualFold(strings.TrimSpace(os.Getenv("OTEL_SDK_DISABLED")), "true")

	if sdkDisabled || exporterEnv == "none" || (exporterEnv == "" && !endpointSet) {
		log.Info("tracing disabled",
			"reason", disabledReason(sdkDisabled, exporterEnv))
		return noop, nil
	}

	exp, expName, err := newExporter(ctx, exporterEnv)
	if err != nil {
		return noop, fmt.Errorf("tracing setup: %w", err)
	}

	res, err := buildResource(ctx, service, version)
	if err != nil {
		if res == nil {
			return noop, fmt.Errorf("tracing setup: building resource: %w", err)
		}
		// Schema-URL conflicts between semconv versions still yield a usable
		// merged resource; keep it rather than losing tracing over metadata.
		log.V(1).Info("tracing resource merge conflict; continuing", "err", err.Error())
	}

	tp := sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exp, sdktrace.WithMaxQueueSize(maxQueueSize)),
		sdktrace.WithResource(res),
	)
	otel.SetTracerProvider(tp)
	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
		propagation.TraceContext{}, propagation.Baggage{}))
	otelLog := log.WithName("otel")
	otel.SetLogger(otelLog)
	// V(1), not Error: an unreachable collector fails every export cycle and
	// would otherwise spam the error log every few seconds.
	otel.SetErrorHandler(otel.ErrorHandlerFunc(func(err error) {
		otelLog.V(1).Info("otel error", "err", err.Error())
	}))

	log.Info("tracing enabled", "exporter", expName, "service", service)
	return tp.Shutdown, nil
}

func disabledReason(sdkDisabled bool, exporterEnv string) string {
	switch {
	case sdkDisabled:
		return "OTEL_SDK_DISABLED=true"
	case exporterEnv == "none":
		return "OTEL_TRACES_EXPORTER=none"
	default:
		return "no OTEL_TRACES_EXPORTER or OTLP endpoint configured"
	}
}

// newExporter hand-rolls the OTEL_TRACES_EXPORTER / OTEL_EXPORTER_OTLP_*
// protocol selection instead of pulling in contrib's autoexport, which drags
// metric/log/prometheus exporters into a trace-only binary.
func newExporter(ctx context.Context, kind string) (sdktrace.SpanExporter, string, error) {
	switch kind {
	case "", "otlp":
		proto := strings.ToLower(strings.TrimSpace(os.Getenv("OTEL_EXPORTER_OTLP_TRACES_PROTOCOL")))
		if proto == "" {
			proto = strings.ToLower(strings.TrimSpace(os.Getenv("OTEL_EXPORTER_OTLP_PROTOCOL")))
		}
		switch proto {
		case "", "http/protobuf":
			exp, err := otlptracehttp.New(ctx)
			return exp, "otlp/http", err
		case "grpc":
			exp, err := otlptracegrpc.New(ctx)
			return exp, "otlp/grpc", err
		default:
			return nil, "", fmt.Errorf("unsupported OTLP protocol %q (supported: http/protobuf, grpc)", proto)
		}
	case "console":
		exp, err := stdouttrace.New()
		return exp, "console", err
	default:
		return nil, "", fmt.Errorf("unsupported OTEL_TRACES_EXPORTER %q (supported: otlp, console, none)", kind)
	}
}

// buildResource layers defaults < service identity < environment, so
// OTEL_SERVICE_NAME / OTEL_RESOURCE_ATTRIBUTES always win.
func buildResource(ctx context.Context, service, version string) (*resource.Resource, error) {
	base, err := resource.Merge(resource.Default(), resource.NewWithAttributes(semconv.SchemaURL,
		semconv.ServiceName(service),
		semconv.ServiceVersion(version),
	))
	if err != nil {
		return base, err
	}
	env, err := resource.New(ctx, resource.WithFromEnv())
	if err != nil {
		return base, err
	}
	return resource.Merge(base, env)
}