decodeBinaryFormat.elm

-- This is a naive brainstorm draft for a binary parser for an imaginary binary image format.
-- The image format is assumed to be:
-- Little endian
-- 4 bytes: magic identifier 0x23 0x23 0x23 0x23
-- 2 bytes: width of image in pixels
-- 2 bytes: height of image in pixels
-- (width * height * 3) bytes: R G B values stored for each pixel (unsigned int)

import Binary.DataView.UInt8 exposing (uint8ViewLittleEndian, getBytes, getUInt16)

-- signatures of library functions:
uint8ViewLittleEndian : ArrayBuffer -> UInt8DataView
getBytes : Int -> Int -> UInt8DataView -> List Int
getUInt16 : Int -> UInt8DataView -> Int


ArrayBufferHelpers.areEqual : List Int -> List Int -> Bool

readPixelAt : Int -> Int -> ArrayBuffer -> Maybe Color
readPixelAt x y arrayBuffer =
  let 
    uint8View = uint8ViewLittleEndian arrayBuffer

    first4Bytes = getBytes 0 4 uint8View

    identifierCorrect = 
      Maybe.withDefault false (Maybe.map (\bytes -> ArrayBufferHelpers.areEqual bytes [0x23, 0x23, 0x23, 0x23])) first4Bytes

    maybeWidth = getUInt16 4 uint8View
    maybeHeight = getUInt16 6 uint8View

    extractPixel isValid width height =
      if isValid then
        let
          maybePixelBytes = getBytes (8 + (y * width + x) * 3) 3 uint8View
          maybeColor = 
            maybePixelBytes
              `Maybe.andThen`
              (\pixelBytes ->
                case pixeBytes of
                  [] -> Nothing
                  _ :: [] -> Nothing
                  _ :: _ :: [] -> Nothing
                  r :: g :: b :: [] -> Just <| rgb r g b 
                  _ -> Nothing
              )
        in
          maybeColor
      else
        Nothing

    maybeColor = Maybe.map3 extractPixel identifierCorrect maybeWidth maybeHeight
  in
    maybeColor