package com.infendro.otp

import com.infendro.bytearray.toByteArray
import com.infendro.bytearray.toInt
import com.infendro.hash.HashFunction
import com.infendro.hash.SHA1
import com.infendro.mac.HMAC
import kotlin.experimental.and
import kotlin.math.pow

class HOTP(
    val function: HashFunction = SHA1,
    val length: Int = 6,
) {
    fun verify(
        secret: ByteArray,
        counter: Long,
        otp: String,
    ): Boolean {
        return generate(secret, counter) == otp
    }

    fun generate(
        secret: ByteArray,
        counter: Long,
    ): String {
        if (counter < 0) throw Exception()

        val hash = generateHash(
            secret,
            counter.toByteArray()
        )
        return otp(hash)
    }

    private fun generateHash(
        secret: ByteArray,
        value: ByteArray,
    ): ByteArray {
        return HMAC(function).hash(secret, value)
    }

    private fun otp(
        hash: ByteArray,
    ): String {
        val offset = (hash.last() and 0xF).toInt()

        var truncatedHash = hash
            .drop(offset)
            .take(4)
            .toInt() and 0x7FFFFFFF
        truncatedHash %= 10.0.pow(length).toInt()

        return truncatedHash.toString()
            .padStart(length, '0')
    }
}