package logger

import (
	"log/slog"

	slogjournal "github.com/systemd/slog-journal"
)

// newJournalHandler returns a slog.Handler that delivers records to the
// systemd journal using the native protocol. It returns nil when stderr
// is not connected to journald, in which case the caller falls back to
// the stderr text handler.
func newJournalHandler(opts *slog.HandlerOptions) slog.Handler {
	if !stderrIsJournal() {
		return nil
	}
	h, err := slogjournal.NewHandler(&slogjournal.Options{
		Level:        opts.Level,
		ReplaceAttr:  journalReplaceAttr,
		ReplaceGroup: sanitizeJournalKey,
	})
	if err != nil {
		return nil
	}
	return h
}

// journalReplaceAttr sanitizes slog attribute keys so they satisfy the
// journal's ^[A-Z_][A-Z0-9_]*$ constraint. Without this, otherwise
// valid attributes from third-party handlers (e.g. the trace_id/span_id
// injected by slogtraceid) are silently dropped by the journal.
func journalReplaceAttr(groups []string, a slog.Attr) slog.Attr {
	a.Key = sanitizeJournalKey(a.Key)
	return a
}

// sanitizeJournalKey maps an arbitrary string to a journal-legal key:
// upper-cases ASCII letters, replaces everything outside [A-Z0-9_] with
// '_', and prefixes '_' if the first byte is a digit.
func sanitizeJournalKey(k string) string {
	if k == "" {
		return k
	}
	b := make([]byte, 0, len(k)+1)
	for i := 0; i < len(k); i++ {
		c := k[i]
		switch {
		case c >= 'A' && c <= 'Z', c == '_':
			b = append(b, c)
		case c >= 'a' && c <= 'z':
			b = append(b, c-'a'+'A')
		case c >= '0' && c <= '9':
			if i == 0 {
				b = append(b, '_')
			}
			b = append(b, c)
		default:
			b = append(b, '_')
		}
	}
	return string(b)
}