package com.infendro.otp

import com.infendro.bytes.bytearray.toInt
import com.infendro.bytes.long.toByteArray
import com.infendro.hash.HashFunction
import com.infendro.hash.sha1.SHA1
import com.infendro.mac.HMAC
import com.infendro.otp.util.pow
import kotlin.experimental.and

class HOTP(
    val function: HashFunction = SHA1(),
    val length: Int = 6,
) {
    private val hmac = HMAC(function)

    fun generate(secret: ByteArray, counter: Long): String {
        require(counter >= 0)
        return otp(secret, counter)
    }

    fun verify(secret: ByteArray, counter: Long, otp: String): Boolean {
        return generate(secret, counter) == otp
    }

    private fun otp(secret: ByteArray, counter: Long): String {
        val hash = hmac.hash(secret, counter.toByteArray())
        val offset = (hash.last() and 0xF).toInt()
        val otp = (hash.toInt(offset) and 0x7FFFFFFF) % 10.pow(length)
        return "$otp".padStart(length, '0')
    }
}