yaotp/app/src/main/java/com/chameth/yaotp/algos/Hotp.kt

package com.chameth.yaotp.algos

import java.nio.ByteBuffer
import kotlin.experimental.and
import kotlin.math.pow

fun hotp(key: ByteArray, counter: ByteArray, length: Int = 6) = hmacToHotp(hmacSha1(key, counter), length)

internal fun offsetValue(bytes: ByteArray) = (bytes[19] and 0x0F).toInt()

internal fun offset(bytes: ByteArray) = offsetValue(bytes).let { bytes.sliceArray(IntRange(it, it + 3)) }

internal fun mask(bytes: ByteArray) = ByteArray(bytes.size) { pos -> if (pos == 0) bytes[pos] and 0x7f else bytes[pos] }

internal fun hmacToHotp(bytes: ByteArray, length: Int = 6) = (ByteBuffer.wrap(mask(offset(bytes))).int % 10.0.pow(length)).toInt()