// Package childmnemonic derives a mnemonic from another mnemonic.
package childmnemonic

import (
	"errors"
	"fmt"

	"git.eeqj.de/sneak/keyfunc/internal/derive"
	"git.eeqj.de/sneak/secret/pkg/bip85"
	"github.com/btcsuite/btcd/btcutil/hdkeychain"
	bip39 "github.com/tyler-smith/go-bip39"
)

// english is the number BIP-85 gives the English word list.
const english = 0

// The lengths a child mnemonic may have. BIP-85 also allows 15 and 21
// words; these three are the ones this tool offers.
const (
	twelve     = 12
	eighteen   = 18
	twentyFour = 24
)

// DefaultWords is how long a child mnemonic is when the user does not
// say.
const DefaultWords = twelve

// ErrWordCount is returned for a length this tool does not offer.
var ErrWordCount = errors.New("a child mnemonic has 12, 18 or 24 words")

// Derive returns the English child mnemonic of that many words at that
// key index, taken from the master key with BIP-85's own mnemonic
// application, number 39. The words come straight from the BIP-85
// entropy, cut to the length the word count needs, rather than from
// the generator the other key types read their bytes from.
func Derive(
	master *hdkeychain.ExtendedKey,
	words, index uint32,
) (string, error) {
	switch words {
	case twelve, eighteen, twentyFour:
	default:
		return "", fmt.Errorf("%w, not %d", ErrWordCount, words)
	}

	err := derive.CheckIndex(index)
	if err != nil {
		return "", err
	}

	entropy, err := bip85.DeriveBIP39Entropy(master, english, words, index)
	if err != nil {
		return "", fmt.Errorf("deriving entropy: %w", err)
	}

	child, err := bip39.NewMnemonic(entropy)
	if err != nil {
		return "", fmt.Errorf("turning the entropy into words: %w", err)
	}

	return child, nil
}