package correlation

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"net/http"
	"regexp"
)

const Header = "X-Correlation-ID"

type contextKey struct{}

var validID = regexp.MustCompile(`^[A-Za-z0-9._:-]{8,64}$`)

func New() string {
	bytes := make([]byte, 16)
	if _, err := rand.Read(bytes); err != nil {
		return "correlation-unavailable"
	}
	return hex.EncodeToString(bytes)
}

func FromContext(ctx context.Context) string {
	value, _ := ctx.Value(contextKey{}).(string)
	return value
}

func WithContext(ctx context.Context, id string) context.Context {
	return context.WithValue(ctx, contextKey{}, id)
}

func Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(response http.ResponseWriter, request *http.Request) {
		id := request.Header.Get(Header)
		if !validID.MatchString(id) {
			id = New()
		}
		response.Header().Set(Header, id)
		next.ServeHTTP(response, request.WithContext(WithContext(request.Context(), id)))
	})
}