// Package wallet derives TRON (TRC20) receiving addresses from a BIP32 account
// extended public key (xpub) — watch-only, no private keys involved. The
// pangolin-pay watcher uses AddressFromAccountXpub to assign a unique receiving
// address per order (m/44'/195'/0'/0/i). Private-key material (seed.go) is for
// OFFLINE use only (sweep signing / vector generation), never on the hot service.
package wallet

import (
	"crypto/sha256"
	"fmt"

	"github.com/btcsuite/btcd/btcutil/base58"
	"github.com/btcsuite/btcd/btcutil/hdkeychain"
	"golang.org/x/crypto/sha3"
)

// tronAddrPrefix is the TRON mainnet address version byte (0x41). It is prepended
// to the 20-byte address body before Base58Check encoding, yielding the familiar
// "T..." addresses.
const tronAddrPrefix = 0x41

// AddressFromAccountXpub derives the TRON address at m/…/<change>/<index> from an
// account-level extended public key (e.g. the xpub of m/44'/195'/0'). It is
// watch-only: an xpub can derive child addresses/public keys but never private
// keys, so this is safe to run on an internet-facing service.
//
// change is 0 for the external (receiving) chain; index is the per-order address
// index. Both are non-hardened, which is exactly why the account-level xpub can
// derive them.
func AddressFromAccountXpub(xpub string, change, index uint32) (string, error) {
	acct, err := hdkeychain.NewKeyFromString(xpub)
	if err != nil {
		return "", fmt.Errorf("wallet: parse xpub: %w", err)
	}
	if acct.IsPrivate() {
		return "", fmt.Errorf("wallet: expected an xpub (public extended key), got a private one")
	}
	chainKey, err := acct.Derive(change)
	if err != nil {
		return "", fmt.Errorf("wallet: derive change %d: %w", change, err)
	}
	addrKey, err := chainKey.Derive(index)
	if err != nil {
		return "", fmt.Errorf("wallet: derive index %d: %w", index, err)
	}
	pub, err := addrKey.ECPubKey()
	if err != nil {
		return "", fmt.Errorf("wallet: ec pubkey: %w", err)
	}
	return PubKeyToTronAddress(pub.SerializeUncompressed()), nil
}

// PubKeyToTronAddress converts a 65-byte uncompressed secp256k1 public key
// (0x04 || X || Y) to a TRON Base58Check address:
//
//	body     = 0x41 || keccak256(X||Y)[12:]   // last 20 bytes of the Keccak hash
//	address  = Base58( body || dsha256(body)[:4] )
//
// Note: TRON/Ethereum use *legacy* Keccak-256 (not the finalized SHA3-256).
func PubKeyToTronAddress(uncompressed []byte) string {
	h := sha3.NewLegacyKeccak256()
	h.Write(uncompressed[1:]) // drop the 0x04 prefix; hash the 64-byte X||Y
	sum := h.Sum(nil)
	body := append([]byte{tronAddrPrefix}, sum[12:]...) // 0x41 + last 20 bytes
	return base58CheckEncode(body)
}

// base58CheckEncode appends a 4-byte double-SHA256 checksum and Base58-encodes.
// (TRON's version byte 0x41 is already inside input, so this is a plain
// checksum-append, not btcutil's version-byte CheckEncode.)
func base58CheckEncode(input []byte) string {
	first := sha256.Sum256(input)
	second := sha256.Sum256(first[:])
	full := make([]byte, 0, len(input)+4)
	full = append(full, input...)
	full = append(full, second[:4]...)
	return base58.Encode(full)
}