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.
44 lines
1.2 KiB
Elixir
44 lines
1.2 KiB
Elixir
defmodule Microwaveprop.Weather.Grib2.SimplePacking do
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@moduledoc false
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@doc """
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Extract a single value from GRIB2 simple-packed data at the given grid index.
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Formula: value = (R + X * 2^E) * 10^(-D)
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Where R = reference_value, E = binary_scale, D = decimal_scale,
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X = raw packed integer at the given index.
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"""
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def extract_value(%{bits_per_value: 0} = params, _data, _index) do
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{:ok, params.reference_value * :math.pow(10, -params.decimal_scale)}
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end
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def extract_value(params, data, index) do
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%{
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reference_value: r,
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binary_scale: e,
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decimal_scale: d,
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bits_per_value: n,
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num_data_points: num
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} = params
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if index < 0 or index >= num do
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{:error, :index_out_of_range}
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else
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x = extract_bits(data, index, n)
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value = (r + x * :math.pow(2, e)) * :math.pow(10, -d)
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{:ok, value}
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end
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end
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defp extract_bits(data, index, bits_per_value) do
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bit_offset = index * bits_per_value
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byte_offset = div(bit_offset, 8)
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remainder = rem(bit_offset, 8)
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<<_skip::binary-size(byte_offset), _pad::size(remainder), x::size(bits_per_value)-unsigned-big, _rest::bitstring>> =
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data
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x
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end
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end
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