import gleam/int
import gleam/list
import gleam/result

pub type EncodingError {
  InvalidComponentsSize(message: String)
  InvalidImageSize(message: String)
  InvalidPixelArraySize(message: String)
  InvalidPixelValue(message: String)
}

/// Data representation of an image - width and height cannot be zero or negeative.
/// Pixels is a list of values from 0 to 255 inclusive. Each pixel is represented by 3 values in this order: [r, g, b].
///
/// For example - an image that's 320x240 pixels should contain 320x240x3 = 230400 values.
pub type ImageData {
  ImageData(width: Int, height: Int, pixels: List(Int))
}

/// Components size - both x and y should be an integer between 1 and 9 inclusive.
/// Higher values produce "better" hash image, but the resulting blurhash string is longer.
/// A good default would be something like 4x4.
pub type Components {
  Components(x: Int, y: Int)
}

/// Encodes the provided image into a BlurHash - that can be later decoded into thumbnail image or used with other BlurHash-compatible libraries.
pub fn encode(
  image: ImageData,
  components: Components,
) -> Result(String, EncodingError) {
  use _ <- result.try(validate_components(components))
  use _ <- result.try(validate_image_data(image))

  let blurhash =
    encode_blurhash_via_ffi(
      image.pixels,
      image.width,
      image.height,
      components.x,
      components.y,
    )

  Ok(blurhash)
}

@external(erlang, "Elixir.BlurHash", "encode")
@external(javascript, "./blush_ffi.mjs", "encode")
fn encode_blurhash_via_ffi(
  pixels: List(Int),
  width: Int,
  height: Int,
  components_x: Int,
  components_y: Int,
) -> String

fn validate_components(components: Components) -> Result(Nil, EncodingError) {
  use _ <- result.try(validate_component_size(components.x, "x"))
  use _ <- result.try(validate_component_size(components.y, "y"))

  Ok(Nil)
}

fn validate_component_size(
  value: Int,
  label: String,
) -> Result(Nil, EncodingError) {
  case value {
    valid if valid > 0 && valid < 10 -> Ok(Nil)
    _invalid -> {
      let message =
        "Invalid component size: "
        <> label
        <> ". Expected an integer value that's between 1 and 9 inclusive."

      Error(InvalidComponentsSize(message))
    }
  }
}

fn validate_image_data(image: ImageData) -> Result(Nil, EncodingError) {
  use _ <- result.try(validate_image_size(image.width, "width"))
  use _ <- result.try(validate_image_size(image.height, "height"))
  use _ <- result.try(validate_pixels(image))

  [
    validate_image_size(image.width, "width"),
    validate_image_size(image.height, "height"),
  ]
  |> result.all()
  |> result.replace(Nil)
}

fn validate_image_size(value: Int, label: String) -> Result(Nil, EncodingError) {
  case value {
    valid if valid > 0 -> Ok(Nil)
    _invalid -> {
      let message =
        "Invalid image size. Expected " <> label <> " to be greater than zero."

      Error(InvalidImageSize(message))
    }
  }
}

fn validate_pixels(image: ImageData) -> Result(Nil, EncodingError) {
  let expected_items = image.width * image.height * 3
  let actual_items = list.length(image.pixels)

  case actual_items == expected_items {
    False -> {
      let message =
        "Expected "
        <> int.to_string(image.width)
        <> " * "
        <> int.to_string(image.height)
        <> " * 3 (r, g, b) = "
        <> int.to_string(expected_items)
        <> " items in the pixel array, but the pixel array size contains "
        <> int.to_string(actual_items)
        <> " items instead."

      Error(InvalidPixelArraySize(message))
    }
    True -> validate_pixel_values(image)
  }
}

fn validate_pixel_values(image: ImageData) -> Result(Nil, EncodingError) {
  let offending_pixel_value =
    list.find(image.pixels, fn(value) {
      case value {
        valid if valid >= 0 && valid <= 255 -> False
        _invalid -> True
      }
    })

  case offending_pixel_value {
    Ok(pixel) -> {
      let message =
        "All pixel values must be integers between 0 and 255 inclusive. Encountered pixel with value "
        <> int.to_string(pixel)
        <> " which is not valid."

      Error(InvalidPixelValue(message))
    }
    _ -> Ok(Nil)
  }
}