package journslog

import (
	_ "log/slog"
	"strings"
	"sync"

	"src.lwithers.me.uk/go/journal"
)

// KeyMapper maps [slog.Attr] keys onto [journal.AttrKey] objects. Such a mapper
// is necessary because systemd-journald is very restrictive about suitable
// attribute keys.
type KeyMapper interface {
	KeyMap(slogAttrKey string) journal.AttrKey
}

// DefaultKeyMapper returns a [KeyMapper] with the package's default behaviour.
// The returned mapper contains an internal, persistent map and a [sync.RWMutex]
// to cache map results.
//
// The default behaviour is to strip leading underscores; to turn lowercase
// ASCII ‘a’–‘z’ chars into uppercase ‘A’–‘Z’ (uppercase is passed through
// verbatim), to accept underscores, and to map all other chars to underscores.
// If the result is an empty string, it instead returns the key "BAD_ATTR_KEY".
func DefaultKeyMapper() KeyMapper {
	return &defaultKeyMapper{
		badKey: journal.MustAttrKey("BAD_ATTR_KEY"),
		keys:   make(map[string]journal.AttrKey),
	}
}

type defaultKeyMapper struct {
	badKey journal.AttrKey
	keys   map[string]journal.AttrKey
	lock   sync.RWMutex
}

func (dkm *defaultKeyMapper) KeyMap(slogAttrKey string) journal.AttrKey {
	if key, ok := dkm.mapFast(slogAttrKey); ok {
		return key
	}
	return dkm.mapSlow(slogAttrKey)
}

func (dkm *defaultKeyMapper) mapFast(slogAttrKey string) (journal.AttrKey, bool) {
	dkm.lock.RLock()
	defer dkm.lock.RUnlock()
	key, ok := dkm.keys[slogAttrKey]
	return key, ok
}

func (dkm *defaultKeyMapper) mapSlow(slogAttrKey string) journal.AttrKey {
	key := dkm.mapFunc(slogAttrKey)

	// it's OK if another goroutine in the slow path just wrote the key, we
	// will overwrite its result but the result will still be correct
	dkm.lock.Lock()
	defer dkm.lock.Unlock()
	dkm.keys[slogAttrKey] = key
	return key
}

func (dkm *defaultKeyMapper) mapFunc(slogAttrKey string) journal.AttrKey {
	key, err := journal.NewAttrKey(slogAttrKey)
	if err == nil {
		return key
	}

	var s strings.Builder
	for _, r := range slogAttrKey {
		switch {
		case r >= 'a' && r <= 'z':
			s.WriteRune(r - 'a' + 'A')
		case r >= '0' && r <= '9',
			r >= 'A' && r <= 'Z':
			s.WriteRune(r)
		default:
			if s.Len() > 0 {
				s.WriteRune('_')
			}
		}
	}
	if s.Len() == 0 {
		return dkm.badKey
	}

	skey := s.String()
	if len(skey) > 255 {
		skey = skey[:255] // known to only contain ASCII chars
	}

	if key, err = journal.NewAttrKey(skey); err != nil {
		return dkm.badKey
	}
	return key
}