Don't go through hex.pm API server to install Hex

Signed-off-by: Eric Meadows-Jönsson <eric.meadows.jonsson@gmail.com>

Author: ericmj

lib/mix/lib/mix/tasks/local.hex.ex

@@ -1,13 +1,15 @@
 defmodule Mix.Tasks.Local.Hex do
   use Mix.Task
 
-  @hex_url "https://hex.pm/installs/hex.ez"
-  @hex_requirement ">= 0.5.0"
+  @hex_s3           "https://s3.amazonaws.com/s3.hex.pm"
+  @hex_list_url     @hex_s3 <> "/installs/list.csv"
+  @hex_archive_url  @hex_s3 <> "/installs/[VERSION]/hex.ez"
+  @hex_requirement  ">= 0.5.0"
 
   @shortdoc "Install hex locally"
 
   @moduledoc """
-  Install hex locally from #{@hex_url}.
+  Install Hex locally.
 
       mix local.hex
 
@@ -18,7 +20,9 @@ defmodule Mix.Tasks.Local.Hex do
   """
   @spec run(OptionParser.argv) :: boolean
   def run(args) do
-    url = @hex_url <> "?elixir=" <> System.version
+    version = get_matching_version()
+    url = String.replace(@hex_archive_url, "[VERSION]", version)
+
     Mix.Tasks.Archive.Install.run [url, "--shell" | args]
   end
 
@@ -75,4 +79,25 @@ defmodule Mix.Tasks.Local.Hex do
         :erlang.raise(kind, reason, stacktrace)
     end
   end
+
+  defp get_matching_version do
+    Mix.Utils.read_path!(@hex_list_url)
+    |> parse_csv
+    |> all_eligibile_versions
+    |> List.last
+  end
+
+  defp parse_csv(body) do
+    :binary.split(body, "\n", [:global, :trim])
+    |> Enum.flat_map(fn line ->
+         [_hex|elixirs] = :binary.split(line, ",", [:global, :trim])
+         elixirs
+       end)
+    |> Enum.uniq
+  end
+
+  defp all_eligibile_versions(versions) do
+    {:ok, current_version} = Version.parse(System.version)
+    Enum.filter(versions, &Version.compare(&1, current_version) != :gt)
+  end
 end