package crypto

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

	"golang.org/x/crypto/argon2"
)

// Params holds Argon2id parameters and the salt used for key derivation.
type Params struct {
	Salt    []byte
	Time    uint32
	Memory  uint32
	Threads uint8
}

// DefaultParams generates new Params with a random 16-byte salt and
// reasonable Argon2id defaults.
func DefaultParams() (*Params, error) {
	salt := make([]byte, 16)
	if _, err := io.ReadFull(rand.Reader, salt); err != nil {
		return nil, fmt.Errorf("generate salt: %w", err)
	}
	return &Params{
		Salt:    salt,
		Time:    3,
		Memory:  64 * 1024,
		Threads: 4,
	}, nil
}

// DeriveKey derives a 32-byte AES-256 key from a passphrase using Argon2id.
func DeriveKey(passphrase string, p *Params) []byte {
	return argon2.IDKey([]byte(passphrase), p.Salt, p.Time, p.Memory, p.Threads, 32)
}

// Encryptor provides AES-256-GCM encryption and decryption.
type Encryptor struct {
	aead cipher.AEAD
}

// NewEncryptor creates an Encryptor from a 32-byte key.
func NewEncryptor(key []byte) (*Encryptor, error) {
	if len(key) != 32 {
		return nil, errors.New("key must be 32 bytes")
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("new cipher: %w", err)
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("new gcm: %w", err)
	}
	return &Encryptor{aead: aead}, nil
}

// Encrypt encrypts plaintext. The returned ciphertext has the nonce prepended.
func (e *Encryptor) Encrypt(plaintext []byte) ([]byte, error) {
	nonce := make([]byte, e.aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("generate nonce: %w", err)
	}
	return e.aead.Seal(nonce, nonce, plaintext, nil), nil
}

// Decrypt decrypts ciphertext produced by Encrypt.
func (e *Encryptor) Decrypt(ciphertext []byte) ([]byte, error) {
	nonceSize := e.aead.NonceSize()
	if len(ciphertext) < nonceSize {
		return nil, errors.New("ciphertext too short")
	}
	nonce, ct := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := e.aead.Open(nil, nonce, ct, nil)
	if err != nil {
		return nil, fmt.Errorf("decrypt: %w", err)
	}
	return plaintext, nil
}

// EncryptString encrypts a string and returns base64-encoded ciphertext.
func (e *Encryptor) EncryptString(plaintext string) (string, error) {
	ct, err := e.Encrypt([]byte(plaintext))
	if err != nil {
		return "", err
	}
	return base64.StdEncoding.EncodeToString(ct), nil
}

// DecryptString decodes base64 ciphertext and returns the decrypted string.
func (e *Encryptor) DecryptString(encoded string) (string, error) {
	ct, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", fmt.Errorf("decode base64: %w", err)
	}
	pt, err := e.Decrypt(ct)
	if err != nil {
		return "", err
	}
	return string(pt), nil
}