// deriveCoinTypeKey derives the cointype key which can be used to derive the
// extended key for an account according to the hierarchy described by BIP0044
// given the coin type key.
//
// In particular this is the hierarchical deterministic extended key path:
// m/44'/<coin type>'
func deriveCoinTypeKey(masterNode *hdkeychain.ExtendedKey,
	coinType uint32) (*hdkeychain.ExtendedKey, error) {
	// Enforce maximum coin type.
	if coinType > maxCoinType {
		return nil, fmt.Errorf("bad coin type")
	}

	// The hierarchy described by BIP0043 is:
	//  m/<purpose>'/*
	// This is further extended by BIP0044 to:
	//  m/44'/<coin type>'/<account>'/<branch>/<address index>
	//
	// The branch is 0 for external addresses and 1 for internal addresses.

	// Derive the purpose key as a child of the master node.
	purpose, err := masterNode.Child(44 + hdkeychain.HardenedKeyStart)
	if err != nil {
		return nil, err
	}

	// Derive the coin type key as a child of the purpose key.
	coinTypeKey, err := purpose.Child(coinType + hdkeychain.HardenedKeyStart)
	if err != nil {
		return nil, err
	}

	return coinTypeKey, nil
}

// checkBranchKeys ensures deriving the extended keys for the internal and
// external branches given an account key does not result in an invalid child
// error which means the chosen seed is not usable.  This conforms to the
// hierarchy described by BIP0044 so long as the account key is already derived
// accordingly.
//
// In particular this is the hierarchical deterministic extended key path:
//   m/44'/<coin type>'/<account>'/<branch>
//
// The branch is 0 for external addresses and 1 for internal addresses.
func checkBranchKeys(acctKey *hdkeychain.ExtendedKey) error {
	// Derive the external branch as the first child of the account key.
	if _, err := acctKey.Child(ExternalBranch); err != nil {
		return err
	}

	// Derive the external branch as the second child of the account key.
	_, err := acctKey.Child(InternalBranch)
	return err
}

// deriveAccountKey derives the extended key for an account according to the
// hierarchy described by BIP0044 given the master node.
//
// In particular this is the hierarchical deterministic extended key path:
//   m/44'/<coin type>'/<account>'
func deriveAccountKey(coinTypeKey *hdkeychain.ExtendedKey,
	account uint32) (*hdkeychain.ExtendedKey, error) {
	// Enforce maximum account number.
	if account > MaxAccountNum {
		return nil, fmt.Errorf("account num too high")
	}

	// Derive the account key as a child of the coin type key.
	return coinTypeKey.Child(account + hdkeychain.HardenedKeyStart)
}