spatial_hash

defmodule SpatialHash do
  @moduledoc """
  Documentation for SpatialHash.
  """

  @eps 0.000001

  @doc """
  Returns an array containing the hash elements for the given point for each dimension.

  ## Examples

      iex> SpatialHash.hash([-0.2, -1.3], [{-180, 180, 0.05}, {-90, 90, 0.2}])
      [3596, 443]
      iex> SpatialHash.hash([0.2, -80.2], [{-180, 180, 0.05}, {-90, 90, 0.1}])
      [3604, 98]

  """
  def hash([], []), do: []
  def hash([a | rest_a], [dim | rest_dim]) do
    [ do_hash(a, dim) | hash(rest_a, rest_dim) ]
  end

  defp do_hash(a, {min, _, step}) do
    hash = (a - min) / step
    err = Float.ceil(hash) - hash

    if err < @eps && err > 0 do
      round(Float.floor(hash)) + 1
    else
      round(Float.floor(hash))
    end
  end

  @doc """
  Returns array of hash ranges for a given axis-aligned envelope

  ## Examples

      iex> SpatialHash.hash_range(%Envelope{
      ...>   min_x: -90.082756,
      ...>   min_y: 29.949766,
      ...>   max_x: -90.079484,
      ...>   max_y: 29.952280
      ...> }, [{-180, 180, 0.01}, {-90, 90, 0.01}])
      [8991..8992, 11994..11995]

      iex> SpatialHash.hash_range(
      ...>  %{type: "LineString", coordinates: [
      ...>    { -90.082746, 29.950955},
      ...>    {-90.081453, 29.952280},
      ...>    {-90.079489, 29.949770}
      ...>  ]})
      [89917..89920, 119949..119952]
  """
  def hash_range(shape), do: hash_range(shape, world_grid())
  def hash_range(%Envelope{} = env, [dim_x, dim_y]) do
    min_x_hash = do_hash(env.min_x, dim_x)
    max_x_hash = do_hash(env.max_x, dim_x)
    min_y_hash = do_hash(env.min_y, dim_y)
    max_y_hash = do_hash(env.max_y, dim_y)

    [min_x_hash .. max_x_hash, min_y_hash .. max_y_hash]
  end
  def hash_range(%{coordinates: coords}, dims), do: hash_range(Envelope.from_geo(coords), dims)

  def world_grid(step \\ 0.001) do
    [{-180, 180, step}, {-90, 90, step}]
  end
end