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.
265 lines
7.1 KiB
Elixir
265 lines
7.1 KiB
Elixir
defmodule Microwaveprop.Weather.Grib2.SectionTest do
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use ExUnit.Case, async: true
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alias Microwaveprop.Weather.Grib2.Section
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describe "parse_message/1" do
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test "parses a synthetic GRIB2 message" do
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msg = build_synthetic_grib2()
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assert {:ok, parsed} = Section.parse_message(msg)
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# Grid params from Section 3
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assert parsed.grid_params.nx == 1799
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assert parsed.grid_params.ny == 1059
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assert_in_delta parsed.grid_params.la1, 21.138123, 0.000001
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assert_in_delta parsed.grid_params.lo1, 237.280472, 0.000001
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assert_in_delta parsed.grid_params.dx, 3000.0, 0.1
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assert_in_delta parsed.grid_params.dy, 3000.0, 0.1
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assert_in_delta parsed.grid_params.latin1, 38.5, 0.000001
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assert_in_delta parsed.grid_params.latin2, 38.5, 0.000001
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assert parsed.grid_params.scan_mode == 64
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# Product from Section 4
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assert parsed.product.var == "TMP"
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assert parsed.product.level == "2 m above ground"
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# Packing params from Section 5
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assert parsed.packing_params.num_data_points == 100
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assert parsed.packing_params.bits_per_value == 16
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# Data from Section 7
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assert is_binary(parsed.data)
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# Bitmap indicator
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assert parsed.bitmap == :none
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end
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test "rejects non-GRIB2 binary" do
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assert {:error, _} = Section.parse_message(<<"NOT_GRIB", 0::64-big>>)
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end
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test "rejects GRIB edition 1" do
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indicator = <<"GRIB", 0::16, 0::8, 1::8, 100::64-big>>
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assert {:error, _} = Section.parse_message(indicator)
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end
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end
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describe "product identification" do
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test "identifies all supported variables" do
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# discipline 0 (meteorological)
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assert Section.identify_var(0, 0, 0) == "TMP"
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assert Section.identify_var(0, 0, 6) == "DPT"
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assert Section.identify_var(0, 3, 0) == "PRES"
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assert Section.identify_var(0, 3, 5) == "HGT"
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assert Section.identify_var(0, 3, 18) == "HPBL"
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assert Section.identify_var(0, 1, 3) == "PWAT"
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end
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test "returns category:number for unknown variables" do
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assert Section.identify_var(0, 99, 99) == "99:99"
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end
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end
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describe "level identification" do
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test "surface" do
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assert Section.identify_level(1, 0) == "surface"
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end
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test "pressure level in mb" do
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# Stored in Pa, displayed in mb
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assert Section.identify_level(100, 100_000) == "1000 mb"
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assert Section.identify_level(100, 85_000) == "850 mb"
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end
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test "meters above ground" do
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assert Section.identify_level(103, 2) == "2 m above ground"
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assert Section.identify_level(103, 10) == "10 m above ground"
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end
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test "entire atmosphere" do
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assert Section.identify_level(200, 0) ==
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"entire atmosphere (considered as a single layer)"
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end
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test "unknown level type" do
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assert Section.identify_level(255, 0) == "unknown:255:0"
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end
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end
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describe "sign_magnitude_16/1" do
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test "positive value" do
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assert Section.sign_magnitude_16(<<0::1, 5::15>>) == 5
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end
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test "negative value" do
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assert Section.sign_magnitude_16(<<1::1, 5::15>>) == -5
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end
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test "zero" do
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assert Section.sign_magnitude_16(<<0::16>>) == 0
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end
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end
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# --- Helpers ---
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defp build_synthetic_grib2 do
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# Section 1 (Identification) - minimal
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sec1_body = <<
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# Originating center, subcenter, etc.
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0::16-big,
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0::16-big,
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# Master/local tables
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2::8,
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1::8,
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# Significance of reference time
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1::8,
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# Year, month, day, hour, minute, second
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2026::16-big,
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3::8,
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28::8,
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18::8,
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0::8,
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0::8,
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# Production status, type
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0::8,
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1::8
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>>
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sec1 = section_wrap(1, sec1_body)
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# Section 3 (Grid Definition) - Template 3.30 (Lambert Conformal)
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sec3_body = <<
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# Source of grid definition
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0::8,
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# Number of data points
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1_905_141::32-big,
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# Number of octets for optional list
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0::8,
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# Interpretation
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0::8,
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# Grid Definition Template Number = 30 (Lambert Conformal)
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30::16-big,
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# Shape of earth (6 = spherical, R=6371229)
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6::8,
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# Scale factor/value of radius (not used for shape 6)
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0::8,
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0::32-big,
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# Scale factor/value of major axis
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0::8,
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0::32-big,
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# Scale factor/value of minor axis
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0::8,
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0::32-big,
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# Nx, Ny
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1799::32-big,
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1059::32-big,
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# La1 in microdegrees
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21_138_123::32-big,
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# Lo1 in microdegrees
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237_280_472::32-big,
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# Resolution and component flags
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0b00111000::8,
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# LaD in microdegrees
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38_500_000::32-big,
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# LoV in microdegrees
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262_500_000::32-big,
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# Dx in millimetres
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3_000_000::32-big,
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# Dy in millimetres
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3_000_000::32-big,
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# Projection centre flag
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0::8,
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# Scanning mode
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64::8,
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# Latin1 in microdegrees
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38_500_000::32-big,
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# Latin2 in microdegrees
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38_500_000::32-big,
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# Latitude/longitude of southern pole
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0::32-big,
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0::32-big
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>>
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sec3 = section_wrap(3, sec3_body)
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# Section 4 (Product Definition) - Template 4.0
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sec4_body = <<
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# Number of coordinate values
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0::16-big,
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# Product Definition Template Number = 0
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0::16-big,
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# Parameter category (0 = Temperature)
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0::8,
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# Parameter number (0 = Temperature)
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0::8,
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# Type of generating process
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2::8,
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# Background + forecast generating process
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0::8,
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0::8,
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# Hours/minutes of observational data cutoff
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0::16-big,
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0::8,
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# Indicator of unit of time range (1=hour)
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1::8,
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# Forecast time
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0::32-big,
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# Type of first fixed surface (103 = above ground)
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103::8,
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# Scale factor and scaled value of first fixed surface
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0::8,
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2::32-big,
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# Type of second fixed surface (255 = missing)
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255::8,
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0::8,
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0::32-big
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>>
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sec4 = section_wrap(4, sec4_body)
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# Section 5 (Data Representation) - Template 5.0 (Simple packing)
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# Reference value as IEEE 754 float32
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ref_val = <<0.0::float-32-big>>
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sec5_body = <<
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# Number of data points
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100::32-big,
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# Data Representation Template Number = 0
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0::16-big,
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# Reference value (IEEE 754)
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ref_val::binary,
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# Binary scale factor (sign-magnitude 16-bit)
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0::16-big,
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# Decimal scale factor (sign-magnitude 16-bit)
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0::16-big,
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# Number of bits per value
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16::8
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>>
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sec5 = section_wrap(5, sec5_body)
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# Section 6 (Bitmap) - no bitmap
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sec6_body = <<255::8>>
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sec6 = section_wrap(6, sec6_body)
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# Section 7 (Data) - 100 values of 16 bits = 200 bytes
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data_bytes = :binary.copy(<<42::16-big>>, 100)
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sec7 = section_wrap(7, data_bytes)
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# Section 8 (End)
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sec8 = "7777"
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body = sec1 <> sec3 <> sec4 <> sec5 <> sec6 <> sec7 <> sec8
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total_length = 16 + byte_size(body)
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# Section 0 (Indicator)
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sec0 = <<"GRIB", 0::16, 0::8, 2::8, total_length::64-big>>
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sec0 <> body
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end
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defp section_wrap(section_number, body) do
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length = 5 + byte_size(body)
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<<length::32-big, section_number::8, body::binary>>
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end
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end
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