module NSWTopo::DEM

def blur_dem(dem_path, blur_path)
  half = (3 * @smooth / @mm_per_px).ceil
  coeffs = (-half..half).map do |n|
    n * @mm_per_px / @smooth
  end.map do |x|
    Math::exp(-x**2)
  end
  vrt = OS.gdalbuildvrt "/vsistdout/", dem_path
  xml = REXML::Document.new vrt
  xml.elements.each("//ComplexSource|//SimpleSource") do |source|
    kernel_filtered_source = source.parent.add_element("KernelFilteredSource")
    source.elements.each("SourceFilename|OpenOptions|SourceBand|SourceProperties|SrcRect|DstRect") do |element|
      kernel_filtered_source.add_element element
    end
    kernel = kernel_filtered_source.add_element("Kernel", "normalized" => 1)
    kernel.add_element("Size").text = coeffs.size
    kernel.add_element("Coefs").text = coeffs.join ?\s
    source.parent.delete source
  end
  log_update "%s: smoothing DEM raster" % @name
  OS.gdal_translate "/vsistdin/", blur_path do |stdin|
    stdin.write xml
  end
end

def get_dem(temp_dir, dem_path)
  txt_path = temp_dir / "dem.txt"
  vrt_path = temp_dir / "dem.vrt"
  cutline = @map.cutline(**margin)
  Dir.chdir(@source ? @source.parent : Pathname.pwd) do
    log_update "%s: examining DEM" % @name
    gdal_rasters @path do |index, total|
      log_update "%s: examining DEM file %i of %i" % [@name, index, total]
    end
  end.tap do |rasters|
    raise "no elevation data found at specified path" if rasters.none?
    log_update "%s: extracting DEM raster" % @name
  end.group_by do |path, info|
    @epsg ? Projection.epsg(@epsg) : Projection.new(info.dig("coordinateSystem", "wkt"))
  end.map.with_index do |(projection, rasters), index|
    raise "DEM data not in planar projection with metre units" unless projection.metres?
    paths, resolutions = rasters.map do |path, info|
      rx, ry = info["geoTransform"].values_at(1, 2)
      next path, Vector[rx, ry].norm
    end.sort_by(&:last).transpose
    txt_path.write paths.reverse.join(?\n)
    @mm_per_px ||= @map.to_mm(resolutions.first)
    indexed_dem_path = temp_dir / "dem.#{index}.tif"
    OS.gdalbuildvrt "-overwrite", "-allow_projection_difference", "-a_srs", projection, "-input_file_list", txt_path, vrt_path
    OS.gdalwarp "-t_srs", @map.projection, "-tr", @mm_per_px, @mm_per_px, "-r", "bilinear", "-cutline", "GeoJSON:/vsistdin/", "-crop_to_cutline", vrt_path, indexed_dem_path do |stdin|
      stdin.puts cutline.to_json
    end
    indexed_dem_path
  end.tap do |dem_paths|
    txt_path.write dem_paths.join(?\n)
    OS.gdalbuildvrt "-overwrite", "-input_file_list", txt_path, vrt_path
    OS.gdal_translate vrt_path, dem_path
  end
end