package crypto

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

const encPrefix = "enc:"

// Cipher encrypts and decrypts short string values using AES-256-GCM.
// The encrypted format is: "enc:" + hex(nonce + ciphertext + tag).
//
// Values that do not start with "enc:" are treated as plain text —
// this allows safe migration of existing unencrypted records.
type Cipher struct {
	gcm cipher.AEAD
}

// NewCipher creates a Cipher from a 64-hex-character (32-byte) key.
func NewCipher(keyHex string) (*Cipher, error) {
	key, err := hex.DecodeString(keyHex)
	if err != nil {
		return nil, fmt.Errorf("crypto: invalid hex key: %w", err)
	}
	if len(key) != 32 {
		return nil, fmt.Errorf("crypto: key must be 32 bytes (64 hex chars), got %d bytes", len(key))
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("crypto: failed to create AES cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("crypto: failed to create GCM: %w", err)
	}
	return &Cipher{gcm: gcm}, nil
}

// Encrypt encrypts plaintext and returns "enc:<hex(nonce+ciphertext+tag)>".
// Empty input is returned as-is (no encryption needed for empty/unset fields).
func (c *Cipher) Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	nonce := make([]byte, c.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", fmt.Errorf("crypto: failed to generate nonce: %w", err)
	}
	ciphertext := c.gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return encPrefix + hex.EncodeToString(ciphertext), nil
}

// Decrypt decrypts a value produced by Encrypt.
// If the value does not start with "enc:", it is returned unchanged (plain text migration path).
func (c *Cipher) Decrypt(value string) (string, error) {
	if value == "" {
		return "", nil
	}
	if !strings.HasPrefix(value, encPrefix) {
		// Plain text — not yet encrypted; return as-is.
		return value, nil
	}
	data, err := hex.DecodeString(strings.TrimPrefix(value, encPrefix))
	if err != nil {
		return "", fmt.Errorf("crypto: malformed ciphertext hex: %w", err)
	}
	nonceSize := c.gcm.NonceSize()
	if len(data) < nonceSize {
		return "", errors.New("crypto: ciphertext too short")
	}
	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
	plaintext, err := c.gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", fmt.Errorf("crypto: decryption failed: %w", err)
	}
	return string(plaintext), nil
}