From 4ef755f0cf10d76bebf47717d1a2e0b227847904 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Mon, 30 Mar 2026 09:39:24 -0500
Subject: [PATCH] Download HRRR GRIB ranges individually instead of multi-range

AWS S3 doesn't support multi-range HTTP requests. When given
Range: bytes=0-999, 2000-2999 it ignores the header and returns
the full file (200 instead of 206). Download each range separately.
---
 lib/microwaveprop/weather/hrrr_client.ex      | 25 ++++++-------
 .../weather/hrrr_client_test.exs              | 35 +++++++++++++++++++
 2 files changed, 45 insertions(+), 15 deletions(-)

diff --git a/lib/microwaveprop/weather/hrrr_client.ex b/lib/microwaveprop/weather/hrrr_client.ex
index 45a5240b..21db10ef 100644
--- a/lib/microwaveprop/weather/hrrr_client.ex
+++ b/lib/microwaveprop/weather/hrrr_client.ex
@@ -174,23 +174,18 @@ defmodule Microwaveprop.Weather.HrrrClient do
   defp download_grib_ranges(_url, []), do: {:ok, <<>>}
 
   defp download_grib_ranges(url, ranges) do
-    range_header =
-      Enum.map_join(ranges, ", ", fn {start, stop} -> "#{start}-#{stop}" end)
+    Enum.reduce_while(ranges, {:ok, <<>>}, fn {start, stop}, {:ok, acc} ->
+      case Req.get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
+        {:ok, %{status: 206, body: body}} ->
+          {:cont, {:ok, acc <> body}}
 
-    case Req.get(url, [{:headers, [{"Range", "bytes=#{range_header}"}]} | req_options()]) do
-      {:ok, %{status: 206, body: body}} ->
-        {:ok, body}
+        {:ok, %{status: status}} ->
+          {:halt, {:error, "HRRR grib HTTP #{status}"}}
 
-      {:ok, %{status: 200}} ->
-        Logger.warning("HRRR server ignored Range header, returned full file")
-        {:error, "HRRR range request not supported"}
-
-      {:ok, %{status: status}} ->
-        {:error, "HRRR grib HTTP #{status}"}
-
-      {:error, reason} ->
-        {:error, reason}
-    end
+        {:error, reason} ->
+          {:halt, {:error, reason}}
+      end
+    end)
   end
 
   defp req_options do
diff --git a/test/microwaveprop/weather/hrrr_client_test.exs b/test/microwaveprop/weather/hrrr_client_test.exs
index 0055c6db..49c4fa84 100644
--- a/test/microwaveprop/weather/hrrr_client_test.exs
+++ b/test/microwaveprop/weather/hrrr_client_test.exs
@@ -117,6 +117,41 @@ defmodule Microwaveprop.Weather.HrrrClientTest do
     end
   end
 
+  describe "fetch_profile/3" do
+    test "succeeds when ranges are downloaded individually" do
+      grib_data = File.read!("test/fixtures/grib2/hrrr_tmp_2m.grib2")
+
+      # Idx with multiple surface variables to trigger multi-range download
+      grib_size = byte_size(grib_data)
+
+      idx_text = """
+      1:0:d=2026032818:TMP:2 m above ground:anl:
+      2:#{grib_size}:d=2026032818:DPT:2 m above ground:anl:
+      3:#{grib_size * 2}:d=2026032818:PRES:surface:anl:
+      """
+
+      Req.Test.stub(HrrrClient, fn conn ->
+        if String.ends_with?(conn.request_path, ".idx") do
+          Plug.Conn.send_resp(conn, 200, idx_text)
+        else
+          [range] = Plug.Conn.get_req_header(conn, "range")
+
+          if String.contains?(range, ",") do
+            # Multi-range: S3 ignores Range header, returns full file
+            Plug.Conn.send_resp(conn, 200, "full file")
+          else
+            # Single range: S3 supports this fine
+            Plug.Conn.send_resp(conn, 206, grib_data)
+          end
+        end
+      end)
+
+      assert {:ok, profile} = HrrrClient.fetch_profile(32.90, -97.04, ~U[2026-03-28 18:00:00Z])
+      assert is_float(profile.surface_temp_c)
+      assert is_struct(profile.run_time, DateTime)
+    end
+  end
+
   describe "build_profile/1" do
     test "assembles profile from parsed data" do
       parsed = %{