package credentials

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"strings"
)

const prefix = "UMC1|"

type Manager struct {
	activeKeyID string
	keys        map[string][]byte
}

type payload struct {
	Username string `json:"username"`
	Password string `json:"password"`
}

func NewManager(keysSpec, activeKeyID string) (*Manager, error) {
	keys, err := parseKeys(keysSpec)
	if err != nil {
		return nil, err
	}
	if len(keys) == 0 {
		return nil, errors.New("mail credential keys are required")
	}

	if activeKeyID == "" {
		for keyID := range keys {
			activeKeyID = keyID
			break
		}
	}
	if _, ok := keys[activeKeyID]; !ok {
		return nil, fmt.Errorf("active credential key id %q not found", activeKeyID)
	}

	return &Manager{
		activeKeyID: activeKeyID,
		keys:        keys,
	}, nil
}

func (m *Manager) Encrypt(username, password string) ([]byte, error) {
	key := m.keys[m.activeKeyID]
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("new cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("new gcm: %w", err)
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("nonce: %w", err)
	}

	rawPayload, err := json.Marshal(payload{Username: username, Password: password})
	if err != nil {
		return nil, fmt.Errorf("marshal payload: %w", err)
	}

	ciphertext := gcm.Seal(nil, nonce, rawPayload, nil)

	serialized := strings.Join([]string{
		prefix[:len(prefix)-1],
		m.activeKeyID,
		base64.StdEncoding.EncodeToString(nonce),
		base64.StdEncoding.EncodeToString(ciphertext),
	}, "|")
	return []byte(serialized), nil
}

func (m *Manager) Decrypt(blob []byte) (string, string, error) {
	parts := strings.Split(string(blob), "|")
	if len(parts) != 4 || parts[0] != strings.TrimSuffix(prefix, "|") {
		return "", "", errors.New("credentials payload is not encrypted with supported format")
	}

	keyID := parts[1]
	key, ok := m.keys[keyID]
	if !ok {
		return "", "", fmt.Errorf("unknown credential key id %q", keyID)
	}

	nonce, err := base64.StdEncoding.DecodeString(parts[2])
	if err != nil {
		return "", "", fmt.Errorf("decode nonce: %w", err)
	}
	ciphertext, err := base64.StdEncoding.DecodeString(parts[3])
	if err != nil {
		return "", "", fmt.Errorf("decode ciphertext: %w", err)
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return "", "", fmt.Errorf("new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", "", fmt.Errorf("new gcm: %w", err)
	}

	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", "", fmt.Errorf("decrypt: %w", err)
	}

	var p payload
	if err := json.Unmarshal(plaintext, &p); err != nil {
		return "", "", fmt.Errorf("unmarshal payload: %w", err)
	}
	if p.Username == "" || p.Password == "" {
		return "", "", errors.New("decrypted credentials are incomplete")
	}
	return p.Username, p.Password, nil
}

func IsEncrypted(blob []byte) bool {
	return strings.HasPrefix(string(blob), prefix)
}

func parseKeys(spec string) (map[string][]byte, error) {
	entries := strings.Split(spec, ",")
	keys := make(map[string][]byte, len(entries))
	for _, entry := range entries {
		entry = strings.TrimSpace(entry)
		if entry == "" {
			continue
		}
		pair := strings.SplitN(entry, ":", 2)
		if len(pair) != 2 {
			return nil, fmt.Errorf("invalid key entry %q", entry)
		}
		keyID := strings.TrimSpace(pair[0])
		if keyID == "" {
			return nil, errors.New("key id cannot be empty")
		}
		rawKey, err := base64.StdEncoding.DecodeString(strings.TrimSpace(pair[1]))
		if err != nil {
			return nil, fmt.Errorf("decode key %s: %w", keyID, err)
		}
		if l := len(rawKey); l != 16 && l != 24 && l != 32 {
			return nil, fmt.Errorf("key %s must be AES-128/192/256", keyID)
		}
		keys[keyID] = rawKey
	}
	return keys, nil
}