Specifies the API for using a custom Cache.

Caches are keyed by the path the disk cache would store the HGT file at, such as ./srtm_cache/N36W117.hgt. Path.basename(identifier, ".hgt") recovers the name that SRTM.DataCell.new/2 needs to parse one, and SRTM.DataCell.to_binary/1 serializes one back.

The default caches are SRTM.Cache.PersistentTerm and SRTM.Cache.File.

Example

defmodule MyApp.ETSCache do
  @behaviour SRTM.Cache

  @impl true
  def fetch(identifier) do
    hgt_name = Path.basename(identifier, ".hgt")

    with [{^identifier, hgt_data}] <- :ets.lookup(:srtm_tiles, identifier),
         {:ok, data_cell} <- SRTM.DataCell.new(hgt_name, hgt_data) do
      {:ok, data_cell}
    else
      _otherwise -> :error
    end
  end

  @impl true
  def store(identifier, data_cell) do
    :ets.insert(:srtm_tiles, {identifier, SRTM.DataCell.to_binary(data_cell)})
    :ok
  end
end

Pass it to SRTM.get_elevation/3:

SRTM.get_elevation(36.455556, -116.866667, in_memory_cache_module: MyApp.ETSCache)