package wallet

import (
	"fmt"

	"github.com/btcsuite/btcd/btcutil/hdkeychain"
	"github.com/btcsuite/btcd/chaincfg"
	bip39 "github.com/tyler-smith/go-bip39"
)

// ⚠️ OFFLINE ONLY. Everything in this file touches the BIP39 seed / private keys.
// It exists for (a) generating the account xpub to hand to the watcher, and
// (b) deriving per-address private keys for offline sweep signing (Phase D).
// It must NEVER be linked into or run on the internet-facing pangolin-pay
// watcher — the hot service only ever handles the account xpub (see derive.go).

const (
	purposeBIP44 = 44
	coinTypeTRON = 195
	// hardenedOffset marks a derivation index as hardened (requires the private
	// key). BIP44's first three levels (purpose'/coin'/account') are hardened.
	hardenedOffset = hdkeychain.HardenedKeyStart // 0x80000000
)

// AccountKeyFromMnemonic derives the account-level extended *private* key at
// m/44'/195'/<account>' from a BIP39 mnemonic (+ optional passphrase).
// OFFLINE ONLY.
func AccountKeyFromMnemonic(mnemonic, passphrase string, account uint32) (*hdkeychain.ExtendedKey, error) {
	if !bip39.IsMnemonicValid(mnemonic) {
		return nil, fmt.Errorf("wallet: invalid BIP39 mnemonic (checksum/wordlist)")
	}
	seed := bip39.NewSeed(mnemonic, passphrase)
	master, err := hdkeychain.NewMaster(seed, &chaincfg.MainNetParams)
	if err != nil {
		return nil, fmt.Errorf("wallet: master key: %w", err)
	}
	for _, step := range []uint32{
		hardenedOffset + purposeBIP44,
		hardenedOffset + coinTypeTRON,
		hardenedOffset + account,
	} {
		master, err = master.Derive(step)
		if err != nil {
			return nil, fmt.Errorf("wallet: derive account path: %w", err)
		}
	}
	return master, nil
}

// AccountXpubFromMnemonic returns the account-level xpub string to hand to the
// watcher. OFFLINE ONLY — run this once on the air-gapped machine, copy only the
// returned xpub to the hot service.
func AccountXpubFromMnemonic(mnemonic, passphrase string, account uint32) (string, error) {
	k, err := AccountKeyFromMnemonic(mnemonic, passphrase, account)
	if err != nil {
		return "", err
	}
	pub, err := k.Neuter() // strip the private key -> xpub
	if err != nil {
		return "", fmt.Errorf("wallet: neuter: %w", err)
	}
	return pub.String(), nil
}

// PrivKeyHexFromMnemonic derives the raw secp256k1 private key (hex) for the
// address at m/44'/195'/<account>'/<change>/<index>, for offline sweep signing.
// OFFLINE ONLY.
func PrivKeyHexFromMnemonic(mnemonic, passphrase string, account, change, index uint32) (string, error) {
	acct, err := AccountKeyFromMnemonic(mnemonic, passphrase, account)
	if err != nil {
		return "", err
	}
	chainKey, err := acct.Derive(change)
	if err != nil {
		return "", fmt.Errorf("wallet: derive change: %w", err)
	}
	addrKey, err := chainKey.Derive(index)
	if err != nil {
		return "", fmt.Errorf("wallet: derive index: %w", err)
	}
	priv, err := addrKey.ECPrivKey()
	if err != nil {
		return "", fmt.Errorf("wallet: ec privkey: %w", err)
	}
	return fmt.Sprintf("%x", priv.Serialize()), nil
}

// AddressFromMnemonic derives the TRON address at m/44'/195'/<account>'/<change>/<index>
// straight from the mnemonic. OFFLINE ONLY — used by the sweep signer to verify
// that a derived key matches the address it is about to sign for.
func AddressFromMnemonic(mnemonic, passphrase string, account, change, index uint32) (string, error) {
	acct, err := AccountKeyFromMnemonic(mnemonic, passphrase, account)
	if err != nil {
		return "", err
	}
	chainKey, err := acct.Derive(change)
	if err != nil {
		return "", fmt.Errorf("wallet: derive change: %w", err)
	}
	addrKey, err := chainKey.Derive(index)
	if err != nil {
		return "", fmt.Errorf("wallet: derive index: %w", err)
	}
	pub, err := addrKey.ECPubKey()
	if err != nil {
		return "", fmt.Errorf("wallet: ec pubkey: %w", err)
	}
	return PubKeyToTronAddress(pub.SerializeUncompressed()), nil
}