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-- HELPERS


isHex : String -> String
isHex str =
    case String.startsWith "0x" str of
        True ->
            hexToString (String.dropLeft 2 str)

        False ->
            toSingleByte str


hexToString : String -> String
hexToString hex =
    -- Take chunks of 2 hex digits and apply formula: (a * 16^1) + (b * 16^0)
    -- Example (d_16 = 13_10): 0xd3 => (d * 16^1) + (3 * 16^0) = 208 + 3 = 211
    let
        zero =
            hex |> String.all (\c -> c == '0')

        asciiToNumber x =
            let
                y =
                    x |> Char.toCode
            in
                case y > 96 of
                    -- ASCII codes for [a-f] => [97-102]
                    True ->
                        y - 97 + 10

                    -- ASCII codes for [0-9] => [48-57]
                    False ->
                        y - 48

        rec xs acc =
            case xs == "" of
                False ->
                    let
                        ( y, ys ) =
                            case String.uncons xs of
                                Nothing ->
                                    ( Nothing, "" )

                                Just ( a, rest ) ->
                                    ( Just a, rest )

                        ( z, zs ) =
                            case String.uncons ys of
                                Nothing ->
                                    ( Nothing, "" )

                                Just ( b, rest ) ->
                                    ( Just b, rest )

                        keyCode =
                            case ( y, z ) of
                                ( Just a, Just b ) ->
                                    ((asciiToNumber a) * 16)
                                        + (asciiToNumber b)

                                ( Just a, _ ) ->
                                    (asciiToNumber a) * 16

                                ( _, _ ) ->
                                    0
                    in
                        rec zs (acc ++ (keyCode |> stringify))

                True ->
                    acc
    in
        case zero of
            True ->
                ""

            False ->
                rec (hex |> String.toLower) ""

References:

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