// Copyright 2026 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package auth

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"time"

	"gitea.dev/tea/internal/settings"
	"github.com/zalando/go-keyring"
)

// keyringService namespaces credentials in the platform credential manager.
const keyringService = "tea-cli-v2"

// Credential holds the secret token material kept outside instance metadata.
type Credential struct {
	AccessToken  string    `json:"access_token"`
	RefreshToken string    `json:"refresh_token,omitempty"`
	ExpiresAt    time.Time `json:"expires_at,omitzero"`
}

// Keyring is the operating-system credential storage boundary.
type Keyring interface {
	Get(string, string) (string, error)
	Set(string, string, string) error
	Delete(string, string) error
}

// SystemKeyring delegates to the platform credential manager.
type SystemKeyring struct{}

// Get retrieves a secret from the platform credential manager.
func (SystemKeyring) Get(service, user string) (string, error) { return keyring.Get(service, user) }

// Set persists a secret in the platform credential manager.
func (SystemKeyring) Set(service, user, value string) error { return keyring.Set(service, user, value) }

// Delete removes only the selected instance/account credential.
func (SystemKeyring) Delete(service, user string) error { return keyring.Delete(service, user) }

// errNoKeyring rejects the keyring backend on a Store without a Keyring.
var errNoKeyring = errors.New("keyring storage unavailable")

// Store selects secure storage or explicitly opted-in private files.
type Store struct {
	Dir     string
	Keyring Keyring
}

// Identity builds a stable, opaque key from an instance URL and account.
func Identity(instance, user string) string {
	sum := sha256.Sum256([]byte(instance + "\x00" + user))
	return hex.EncodeToString(sum[:])
}

func (s *Store) filename(instance, user string) string {
	return filepath.Join(s.Dir, "credentials", Identity(instance, user)+".json")
}

// Load returns a credential without exposing storage contents in errors.
func (s *Store) Load(instance, user, backend string) (Credential, error) {
	var data []byte
	var err error
	switch backend {
	case settings.StoreKeyring:
		if s.Keyring == nil {
			return Credential{}, errNoKeyring
		}
		value, loadErr := s.Keyring.Get(keyringService, Identity(instance, user))
		data, err = []byte(value), loadErr
	case settings.StoreFile:
		data, err = os.ReadFile(s.filename(instance, user))
	default:
		err = errors.New("unsupported credential store")
	}
	if errors.Is(err, keyring.ErrNotFound) || errors.Is(err, os.ErrNotExist) {
		return Credential{}, errors.New("credential unavailable; run tea auth login for this instance and account")
	}
	if err != nil {
		return Credential{}, fmt.Errorf("credential storage unavailable: %w", err)
	}
	var credential Credential
	if err := json.Unmarshal(data, &credential); err != nil {
		return Credential{}, errors.New("invalid stored credential; run tea auth login")
	}
	if credential.AccessToken == "" {
		return Credential{}, errors.New("stored credential has no access token")
	}
	return credential, nil
}

// Save persists credentials without falling back to a different backend.
func (s *Store) Save(instance, user, backend string, credential Credential) error {
	if credential.AccessToken == "" {
		return errors.New("access token cannot be empty")
	}
	data, err := json.Marshal(credential)
	if err != nil {
		return err
	}
	switch backend {
	case settings.StoreKeyring:
		if s.Keyring == nil {
			return errNoKeyring
		}
		if err := s.Keyring.Set(keyringService, Identity(instance, user), string(data)); err != nil {
			return errors.New("secure credential storage unavailable; use --credential-store file only if you accept private file storage")
		}
		return nil
	case settings.StoreFile:
		if err := os.MkdirAll(filepath.Dir(s.filename(instance, user)), 0o700); err != nil {
			return err
		}
		return settings.AtomicWrite(s.filename(instance, user), data)
	default:
		return errors.New("unsupported credential store")
	}
}

// Delete removes only the selected instance/account credential.
func (s *Store) Delete(instance, user, backend string) error {
	switch backend {
	case settings.StoreKeyring:
		if s.Keyring == nil {
			return errNoKeyring
		}
		err := s.Keyring.Delete(keyringService, Identity(instance, user))
		if errors.Is(err, keyring.ErrNotFound) {
			return nil
		}
		if err != nil {
			return errors.New("failed to remove credential from secure storage")
		}
		return nil
	case settings.StoreFile:
		err := os.Remove(s.filename(instance, user))
		if errors.Is(err, os.ErrNotExist) {
			return nil
		}
		return err
	default:
		return fmt.Errorf("unsupported credential store %q", backend)
	}
}