package auth

import (
	"crypto/rand"
	"fmt"
	"math/big"
)

// GenerateDatadogHeaders generates Datadog APM tracing headers
// matching the format used by the OpenAI frontend.
func GenerateDatadogHeaders() map[string]string {
	traceID := randomDigits(19)
	parentID := randomDigits(16)
	traceHex := fmt.Sprintf("%016x", mustParseUint(traceID))
	parentHex := fmt.Sprintf("%016x", mustParseUint(parentID))

	return map[string]string{
		"traceparent":                  fmt.Sprintf("00-0000000000000000%s-%s-01", traceHex, parentHex),
		"tracestate":                   "dd=s:1;o:rum",
		"x-datadog-origin":             "rum",
		"x-datadog-parent-id":          parentID,
		"x-datadog-sampling-priority":  "1",
		"x-datadog-trace-id":           traceID,
	}
}

func randomDigits(n int) string {
	result := make([]byte, n)
	for i := range result {
		v, _ := rand.Int(rand.Reader, big.NewInt(10))
		result[i] = '0' + byte(v.Int64())
	}
	// Ensure first digit is not 0
	if result[0] == '0' {
		v, _ := rand.Int(rand.Reader, big.NewInt(9))
		result[0] = '1' + byte(v.Int64())
	}
	return string(result)
}

func mustParseUint(s string) uint64 {
	var result uint64
	for _, c := range s {
		result = result*10 + uint64(c-'0')
	}
	return result
}