yaotp/app/src/main/java/com/chameth/yaotp/util/Base32.kt

package com.chameth.yaotp.util

private val alphabet = ('A'..'Z') + ('2'..'7') + '='

fun base32Decode(input: String): ByteArray {
    val values = padToEights(input).toUpperCase().map(alphabet::indexOf)
    val bytes = ArrayList<Byte>()

    values.chunked(8).forEach { chunk ->
        // Each chunk of 8 values is mapped on (up to) 5 return bytes:
        //      [A1   A2   A3   A4   A5] [B1   B2   B3
        //       B4   B5] [C1   C2   C3   C4   C5] [D1   -- only if C is present
        //       D2   D3   D4   D5] [E1   E2   E3   E4   -- only if E is present
        //       E5] [F1   F2   F3   F4   F5] [G1   G2   -- only if F is present
        //       G3   G4   G5] [H1   H2   H3   H4   H5]  -- only if H is present

        if (chunk[0] != 32) bytes.add(((chunk[0] shl 3) or (chunk[1] ushr 2)).toByte())
        if (chunk[2] != 32) bytes.add((((chunk[1] and 3) shl 6) or (chunk[2] shl 1) or (chunk[3] ushr 4)).toByte())
        if (chunk[4] != 32) bytes.add((((chunk[3] and 15) shl 4) or (chunk[4] ushr 1)).toByte())
        if (chunk[5] != 32) bytes.add((((chunk[4] and 1) shl 7) or (chunk[5] shl 2) or (chunk[6] ushr 3)).toByte())
        if (chunk[7] != 32) bytes.add((((chunk[6] and 7) shl 5) or chunk[7]).toByte())
    }

    return bytes.toByteArray()
}

internal fun padToEights(input: String) = input + "=".repeat((8 - input.length % 8) % 8)