package event

import "fmt"

func addFields(event map[string]interface{}, fields map[string]interface{}, protected ...string) error {
	if len(fields) == 0 {
		return nil
	}
	block := map[string]struct{}{}
	for _, key := range protected {
		block[key] = struct{}{}
	}
	for k, v := range fields {
		if _, ok := block[k]; ok {
			return fmt.Errorf("fields must not override %s", k)
		}
		event[k] = v
	}
	return nil
}