package main

import (
	"crypto/hmac"
	"crypto/sha1"
	"crypto/sha256"
	"encoding/base32"
	"encoding/base64"
	"encoding/binary"
	"errors"
	"fmt"
	"hash"
	"math"
	"math/rand"
	"strings"
	"time"
)

func NewSalt() string {
	var letterRunes = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
	b := make([]rune, 4)
	for i := range b {
		b[i] = letterRunes[rand.Intn(len(letterRunes))]
	}
	return string(b)
}

func ComputeHmac256(message string, secret string) []byte {
	h := hmac.New(sha256.New, []byte(secret))
	h.Write([]byte(message))
	return h.Sum(nil)
}

func encode64(secret []byte) string {
	return strings.TrimRight(base64.StdEncoding.EncodeToString(secret), "=")
}

func encodeSecret(secret []byte) string {
	return strings.TrimRight(base32.StdEncoding.EncodeToString(secret), "=")
}

func GenericTotpCode(secret string, t time.Time, algo string, digits int, period int) (string, error) {
	var mac hash.Hash
	var format string

	now := [8]byte{}

	binary.BigEndian.PutUint64(now[:], uint64(math.Floor(float64(t.Unix())/float64(period))))
	binsecret, _ := base32.StdEncoding.DecodeString(secret)

	switch algo {
	case "sha1":
		mac = hmac.New(sha1.New, binsecret)
	case "sha256":
		mac = hmac.New(sha256.New, binsecret)
	default:
		return "", errors.New("unsupported algorithm")
	}

	switch digits {
	case 6:
		format = "%06d"
	case 8:
		format = "%08d"
	default:
		return "", errors.New("unsupported code size")
	}

	mac.Write(now[:])
	sum := mac.Sum(nil)
	return fmt.Sprintf(format, (binary.BigEndian.Uint32(sum[sum[len(sum)-1]&0xf:])&0x7fffffff)%1000000), nil
}