module HmacSha256 exposing (hmac, packedStringOfIntArray, intArrayOfPackedString, presentableIntArray, presentationToIntArray)

import Array exposing (Array)
import Bitwise
import Char
import Debug exposing (log)
import ParseInt as I
{-- Note: relies on a modified version of billstclair's sha256 module with utf-8 handling disabled --}
import Sha256 exposing (sha256)

hmacBlockSize : Int
hmacBlockSize = 64

hextuples : String -> Array String
hextuples s =
    Array.initialize ((String.length s) // 2) identity
    |> Array.map (\i -> String.slice (i * 2) (i * 2 + 2) s)

presentationToIntArray : String -> Array Int
presentationToIntArray =
    hextuples >> Array.map (I.parseIntHex >> Result.toMaybe >> Maybe.withDefault 0)

presentableIntArray : Array Int -> String
presentableIntArray =
    Array.map (I.toHex >> String.pad 2 '0') >> Array.toList >> String.concat

packedStringOfIntArray : Array Int -> String
packedStringOfIntArray =
    Array.map (Char.fromCode >> String.fromChar) >> Array.toList >> String.concat

intArrayOfPackedString : String -> Array Int
intArrayOfPackedString =
    String.toList >> Array.fromList >> Array.map Char.toCode

sha256Ints : Array Int -> Array Int
sha256Ints =
    packedStringOfIntArray >> Sha256.sha256 >> presentationToIntArray

arrayPad : Int -> Int -> Array Int -> Array Int
arrayPad n v a =
    if Array.length a >= n then
        a
    else
        Array.append a (Array.repeat (n - (Array.length a)) v)

xor : Int -> Array Int -> Array Int
xor v = Array.map (Bitwise.xor v)

hmac : Array Int -> Array Int -> Array Int
hmac key msg =
    if Array.length key > hmacBlockSize then
        hmac (sha256Ints key) msg
    else if Array.length key < hmacBlockSize then
        hmac (arrayPad hmacBlockSize 0 key) msg
    else
        let o_key_pad = xor 0x5c key in
        let i_key_pad = xor 0x36 key in
        sha256Ints (Array.append o_key_pad (sha256Ints (Array.append i_key_pad msg)))