From fdc2a31c77cf2c0a428339bdb0766349ea5c15f7 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Mon, 30 Mar 2026 09:54:10 -0500
Subject: [PATCH] Fix complex packing bit-padding bug corrupting decoded values
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extract_n_values_array used Enum.take(count) on a reversed list
before reversing it, which included padding values from byte
alignment and dropped actual values. When group count * bits
wasn't a multiple of 8, the extra padding bits produced a
phantom value that shifted the entire array by one position.

This caused cascading errors in spatial differencing — values
started correct but diverged exponentially (DPT decoded as
38 billion K instead of 275 K).

Fix: reverse the list first, then take count, so padding values
at the end are discarded instead of actual values at the start.
---
 .../weather/grib2/complex_packing.ex          |  4 ++--
 .../weather/grib2/extractor_test.exs          | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/lib/microwaveprop/weather/grib2/complex_packing.ex b/lib/microwaveprop/weather/grib2/complex_packing.ex
index abc5d909..978aadfc 100644
--- a/lib/microwaveprop/weather/grib2/complex_packing.ex
+++ b/lib/microwaveprop/weather/grib2/complex_packing.ex
@@ -132,8 +132,8 @@ defmodule Microwaveprop.Weather.Grib2.ComplexPacking do
     <<chunk::binary-size(total_bytes), rest::binary>> = data
 
     vals = consume_bits_simple(<<chunk::binary>>, nbits, [])
-    # consume_bits_simple returns reversed, take only count values
-    arr = vals |> Enum.take(count) |> Enum.reverse() |> :array.from_list()
+    # consume_bits_simple prepends (reversed); reverse first, then take count to discard padding
+    arr = vals |> Enum.reverse() |> Enum.take(count) |> :array.from_list()
 
     {arr, rest}
   end
diff --git a/test/microwaveprop/weather/grib2/extractor_test.exs b/test/microwaveprop/weather/grib2/extractor_test.exs
index 02ad9f97..6acf016e 100644
--- a/test/microwaveprop/weather/grib2/extractor_test.exs
+++ b/test/microwaveprop/weather/grib2/extractor_test.exs
@@ -118,6 +118,25 @@ defmodule Microwaveprop.Weather.Grib2.ExtractorTest do
         assert map_size(result) == 2
       end
     end
+
+    @tag :fixture
+    @tag timeout: 300_000
+    test "DPT values are meteorologically reasonable (not corrupted by bit padding)" do
+      if File.exists?(@multi_fixture_path) do
+        fixture = File.read!(@multi_fixture_path)
+
+        assert {:ok, result} = Extractor.extract_points(fixture, 32.78, -96.8)
+
+        tmp_k = result["TMP:2 m above ground"]
+        dpt_k = result["DPT:2 m above ground"]
+
+        assert tmp_k > 200.0 and tmp_k < 340.0,
+               "TMP #{tmp_k}K unreasonable"
+
+        assert dpt_k > 200.0 and dpt_k < 340.0,
+               "DPT #{dpt_k}K unreasonable — likely bit-padding corruption"
+      end
+    end
   end
 
   # --- Synthetic GRIB2 builder ---