Graham McIntire
5a9359191f
Eliminates the external wgrib2 C tool dependency that blocked HRRR processing. Implements Lambert Conformal projection, simple packing (Template 5.0), complex packing with spatial differencing (Template 5.3), and GRIB2 section parsing — enough to extract point values from HRRR grid data using only Elixir.
83 lines
2.6 KiB
Elixir
83 lines
2.6 KiB
Elixir
defmodule Microwaveprop.Weather.Grib2.Extractor do
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@moduledoc false
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alias Microwaveprop.Weather.Grib2.ComplexPacking
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alias Microwaveprop.Weather.Grib2.LambertConformal
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alias Microwaveprop.Weather.Grib2.Section
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alias Microwaveprop.Weather.Grib2.SimplePacking
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@doc """
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Extract weather values from a GRIB2 binary blob at the given lat/lon.
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Returns `{:ok, %{"VAR:LEVEL" => float}}` or `{:error, term}`.
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"""
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def extract_points(binary, lat, lon) do
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messages = split_messages(binary)
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results =
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Enum.reduce_while(messages, {:ok, %{}}, fn msg, {:ok, acc} ->
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case extract_single(msg, lat, lon) do
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{:ok, key, value} -> {:cont, {:ok, Map.put(acc, key, value)}}
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{:error, :outside_grid} -> {:halt, {:error, :outside_grid}}
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{:error, reason} -> {:halt, {:error, reason}}
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end
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end)
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results
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end
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@doc """
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Split a binary blob into individual GRIB2 messages by scanning for "GRIB" magic bytes
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and reading the total length from the indicator section.
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"""
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def split_messages(binary), do: split_messages(binary, [])
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@doc """
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Compute linear index from grid (i, j) and scan mode.
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For HRRR scan_mode=64 (j positive, i positive, i consecutive): index = j * nx + i
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"""
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def linear_index({i, j}, nx, _scan_mode) do
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j * nx + i
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end
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# --- Private ---
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defp split_messages(<<>>, acc), do: Enum.reverse(acc)
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defp split_messages(<<"GRIB", _::32, total_length::64-big, _rest::binary>> = binary, acc) do
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if total_length > byte_size(binary) do
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Enum.reverse(acc)
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else
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msg = binary_part(binary, 0, total_length)
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remaining = binary_part(binary, total_length, byte_size(binary) - total_length)
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split_messages(remaining, [msg | acc])
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end
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end
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defp split_messages(<<_::8, rest::binary>>, acc) do
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# Skip non-GRIB bytes (padding between messages)
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split_messages(rest, acc)
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end
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defp extract_single(msg, lat, lon) do
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with {:ok, parsed} <- Section.parse_message(msg),
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%{grid_params: grid, product: prod, packing_params: packing, data: data} = parsed,
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key = "#{prod.var}:#{prod.level}",
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{:ok, {i, j}} <- LambertConformal.to_grid_index(grid, lat, lon),
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index = linear_index({i, j}, grid.nx, grid.scan_mode),
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{:ok, value} <- unpack_value(packing, data, index) do
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{:ok, key, value}
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end
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rescue
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e -> {:error, "GRIB2 extraction failed: #{inspect(e)}"}
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end
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defp unpack_value(%{template: 3} = params, data, index) do
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ComplexPacking.extract_value(params, data, index)
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end
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defp unpack_value(params, data, index) do
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SimplePacking.extract_value(params, data, index)
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end
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end
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