Update gem typo logic and tests

Use downloads count as threshold instead of static list.
Limit validation to new gems.
Update distance threshold as mentioned in segiddins PR.

Existing gem we may consider blocking:
https://gist.github.com/sonalkr132/af05b030af793ce17a69245152d5aa5f
total: 4859 (2.98%)

app/models/gem_typo.rb

@@ -1,46 +1,41 @@
-require 'rubygems/text'
+require "rubygems/text"
 
 class GemTypo
-  PROTECTED_GEMS = [
-    'rspec-core',
-    'diff-lcs',
-    'rspec-expectations',
-    'rspec-mocks',
-    'rspec',
-    'bundler',
-    'rspec-support',
-    'multi_json',
-    'rack',
-    'rake'
-  ].freeze
-
-  DISTANCE_THRESHOLD = 1
-
-  GEM_EXCEPTIONS = [
-    'rspec-coreZ'
-    # Add exceptions here to manage gems which share a close distance,
-    # but are manually reviewed and accepted by rubygems team
-  ].freeze
+  attr_reader :protected_gem
 
   include Gem::Text
 
-  def initialize(rubygem_name, opts = {})
-    @rubygem_name = rubygem_name
-    @protected_gems = opts[:protected_gems] || GemTypo::PROTECTED_GEMS
-    @distance_threshold = opts[:distance_threshold] || GemTypo::DISTANCE_THRESHOLD
-    @gem_exceptions = opts[:gem_exceptions] || GemTypo::GEM_EXCEPTIONS
+  DOWNLOADS_THRESHOLD = 10_000_000
+  SIZE_THRESHOLD = 4
+
+  def initialize(rubygem_name)
+    @rubygem_name = rubygem_name.downcase
   end
 
   def protected_typo?
-    @protected_gems.each do |protected_gem|
-      return false if @rubygem_name == protected_gem
+    return false if @rubygem_name.size < GemTypo::SIZE_THRESHOLD
+
+    protected_gems.each do |protected_gem|
       distance = levenshtein_distance(@rubygem_name, protected_gem)
-      if distance <= @distance_threshold &&
-          !@gem_exceptions.include?(@rubygem_name)
+      if distance <= distance_threshold
+        @protected_gem = protected_gem
         return true
       end
     end
 
     false
   end
+
+  private
+
+  def distance_threshold
+    @rubygem_name.size == GemTypo::SIZE_THRESHOLD ? 1 : 2
+  end
+
+  def protected_gems
+    Rubygem.by_downloads
+      .where("count > ?", GemTypo::DOWNLOADS_THRESHOLD)
+      .pluck(:name)
+      .reject { |gem_name| gem_name == @rubygem_name }
+  end
 end

app/models/rubygem.rb

@@ -18,7 +18,7 @@ class Rubygem < ApplicationRecord
     uniqueness: { case_sensitive: false },
     if: :needs_name_validation?
   validate :blacklist_names_exclusion
-  validate :protected_gem_typo_protection
+  validate :protected_gem_typo, on: :create
 
   after_create :update_unresolved
   before_destroy :mark_unresolved
@@ -309,10 +309,10 @@ def blacklist_names_exclusion
     errors.add :name, "'#{name}' is a reserved gem name."
   end
 
-  def protected_gem_typo_protection
+  def protected_gem_typo
     gem_typo = GemTypo.new(name)
     return unless gem_typo.protected_typo?
-    errors.add :name, "'#{name}' is too close to a typo-protected gem."
+    errors.add :name, "'#{name}' is too close to typo-protected gem: #{gem_typo.protected_gem} "
   end
 
   def update_unresolved

test/unit/gem_typo_test.rb

@@ -1,78 +1,44 @@
-require 'test_helper'
-require 'gem_typo'
+require "test_helper"
 
 class GemTypoTest < ActiveSupport::TestCase
-  teardown do
-    Rails.cache.clear
-  end
-
-  should 'return false for exact match' do
-    gem_typo = GemTypo.new('rspec-core')
-    assert_equal false, gem_typo.protected_typo?
-  end
-
-  should 'return true for 1 char distance match' do
-    gem_typo = GemTypo.new('rspec-core2')
-    assert_equal true, gem_typo.protected_typo?
-  end
-
-  should 'return false for 2 char distance match' do
-    gem_typo = GemTypo.new('rspec-core12')
-    assert_equal false, gem_typo.protected_typo?
-  end
-
-  should 'return false for 3 char distance match' do
-    gem_typo = GemTypo.new('rspec-core123')
-    assert_equal false, gem_typo.protected_typo?
-  end
-
-  should 'return false for 1 char distance match on the exception list' do
-    gem_typo = GemTypo.new('rspec-coreZ')
-    assert_equal false, gem_typo.protected_typo?
-  end
-
-  should 'allow customized protected_gems' do
-    opts = {
-      protected_gems: ["hello"]
-    }
-
-    gem_typo = GemTypo.new('hello', opts)
-    assert_equal false, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('hello1', opts)
-    assert_equal true, gem_typo.protected_typo?
-  end
-
-  should 'allow customized distance_threshold' do
-    opts = {
-      distance_threshold: 3
-    }
-
-    gem_typo = GemTypo.new('rack', opts)
-    assert_equal false, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('rack1', opts)
-    assert_equal true, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('rack12', opts)
-    assert_equal true, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('rack123', opts)
-    assert_equal true, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('rack1234', opts)
-    assert_equal false, gem_typo.protected_typo?
-  end
-
-  should 'allow customized protected_gem_exceptions' do
-    opts = {
-      gem_exceptions: ["rake1"]
-    }
-
-    gem_typo = GemTypo.new('rake', opts)
-    assert_equal false, gem_typo.protected_typo?
-
-    gem_typo = GemTypo.new('rake1', opts)
-    assert_equal false, gem_typo.protected_typo?
+  context "with above downloads threshold gem" do
+    setup do
+      above_downloads_thres = GemTypo::DOWNLOADS_THRESHOLD + 1
+      create(:rubygem, name: "four", downloads: above_downloads_thres)
+    end
+
+    should "return false for exact match" do
+      gem_typo = GemTypo.new("four")
+      assert_equal false, gem_typo.protected_typo?
+    end
+
+    should "return false for gem name size below protected threshold" do
+      gem_typo = GemTypo.new("fou")
+      assert_equal false, gem_typo.protected_typo?
+    end
+
+    context "size equals protected threshold" do
+      should "return true for one character distance" do
+        gem_typo = GemTypo.new("fous")
+        assert_equal true, gem_typo.protected_typo?
+      end
+
+      should "return false for two character distance" do
+        gem_typo = GemTypo.new("foss")
+        assert_equal false, gem_typo.protected_typo?
+      end
+    end
+
+    context "size above protected threshold" do
+      should "return true for two character distance" do
+        gem_typo = GemTypo.new("fourss")
+        assert_equal true, gem_typo.protected_typo?
+      end
+
+      should "return false for three characher distance" do
+        gem_typo = GemTypo.new("foursss")
+        assert_equal false, gem_typo.protected_typo?
+      end
+    end
   end
 end