Make deep_merge a soft-requirement

The goal here is to remove deep_merge and rely entirely on
ActiveSupport's `Hash#deep_merge` implementation.

This does mean that if you want to keep the existing behavior, or rely
on DeepMerge's specific options, you need to add the following to your
Gemfile:

```ruby
gem 'deep_merge', '~> 1.2', '>= 1.2.1'
```

config.gemspec

@@ -27,7 +27,6 @@ Gem::Specification.new do |s|
   s.require_paths         = ['lib']
   s.required_ruby_version = '>= 2.6.0'
 
-  s.add_dependency 'deep_merge', '~> 1.2', '>= 1.2.1'
   s.add_dependency 'ostruct'
 
   s.add_development_dependency 'dry-validation', *Config::DryValidationRequirements::VERSIONS

gemfiles/sinatra.gemfile

@@ -3,5 +3,6 @@
 source "https://rubygems.org"
 
 gem "sinatra", "2.0.8.1"
+gem "deep_merge", "~> 1.2", ">= 1.2.1"
 
 gemspec path: "../"

lib/config.rb

@@ -6,7 +6,18 @@
 require 'config/sources/hash_source'
 require 'config/sources/env_source'
 require 'config/validation/schema'
-require 'deep_merge/core'
+
+begin
+  gem 'deep_merge', '~> 1.2', '>= 1.2.1'
+  require 'deep_merge/core'
+rescue LoadError
+  warn <<~WARNING
+    The `deep_merge` gem is not available. Trying to use ActiveSupport's Hash#deep_merge instead.
+    If you want to continue using 'deep_merge' gem, please add it to your Gemfile:
+      gem 'deep_merge', '~> 1.2', '>= 1.2.1'
+  WARNING
+  require 'active_support/core_ext/hash/deep_merge'
+end
 
 module Config
   extend Config::Validation::Schema

lib/config/options.rb

@@ -40,15 +40,19 @@ def reload!
         if conf.empty?
           conf = source_conf
         else
-          DeepMerge.deep_merge!(
-                                source_conf,
-                                conf,
-                                preserve_unmergeables: false,
-                                knockout_prefix:       Config.knockout_prefix,
-                                overwrite_arrays:      Config.overwrite_arrays,
-                                merge_nil_values:      Config.merge_nil_values,
-                                merge_hash_arrays:     Config.merge_hash_arrays
-                               )
+          if defined?(DeepMerge)
+            DeepMerge.deep_merge!(
+                                  source_conf,
+                                  conf,
+                                  preserve_unmergeables: false,
+                                  knockout_prefix:       Config.knockout_prefix,
+                                  overwrite_arrays:      Config.overwrite_arrays,
+                                  merge_nil_values:      Config.merge_nil_values,
+                                  merge_hash_arrays:     Config.merge_hash_arrays
+                                 )
+          else
+            conf = conf.deep_merge!(source_conf)
+          end
         end
       end
 
@@ -98,15 +102,19 @@ def as_json(options = nil)
 
     def merge!(hash)
       current = to_hash
-      DeepMerge.deep_merge!(
-                            hash.dup,
-                            current,
-                            preserve_unmergeables: false,
-                            knockout_prefix:       Config.knockout_prefix,
-                            overwrite_arrays:      Config.overwrite_arrays,
-                            merge_nil_values:      Config.merge_nil_values,
-                            merge_hash_arrays:     Config.merge_hash_arrays
-                           )
+      if defined?(DeepMerge)
+        DeepMerge.deep_merge!(
+                              hash.dup,
+                              current,
+                              preserve_unmergeables: false,
+                              knockout_prefix:       Config.knockout_prefix,
+                              overwrite_arrays:      Config.overwrite_arrays,
+                              merge_nil_values:      Config.merge_nil_values,
+                              merge_hash_arrays:     Config.merge_hash_arrays
+                             )
+      else
+        current.deep_merge!(hash)
+      end
       marshal_load(__convert(current).marshal_dump)
       self
     end

spec/config_spec.rb

@@ -331,6 +331,11 @@
       end
 
       it 'should not overwrite values with nil' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         old_value = config.inner.something1
         config.merge!(hash_with_nil)
         expect(config.inner.something1).to eq(old_value)
@@ -418,6 +423,11 @@
         end
 
         it 'should remove elements from settings' do
+          unless defined?(DeepMerge)
+            skip <<~REASON
+              DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+            REASON
+          end
           expect(config.array1).to eq(['item4', 'item5', 'item6'])
           expect(config.array2.inner).to eq(['item4', 'item5', 'item6'])
           expect(config.array3).to eq('')
@@ -474,6 +484,11 @@
         end
 
         it 'should merge arrays from multiple configs' do
+          unless defined?(DeepMerge)
+            skip <<~REASON
+              DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+            REASON
+          end
           expect(config.arraylist1.size).to eq(6)
           expect(config.arraylist2.inner.size).to eq(6)
         end

spec/options_spec.rb

@@ -112,6 +112,11 @@
       end
 
       it 'should overwrite the previous values' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         expect(config['tvrage']['service_url']).to eq('http://url2')
       end
 
@@ -124,6 +129,11 @@
       end
 
       it 'should overwrite the previous values' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         expect(config['tvrage']['service_url']).to eq('http://url3')
       end
     end
@@ -144,6 +154,11 @@
     end
 
     it 'should add keys from the added file' do
+      unless defined?(DeepMerge)
+        skip <<~REASON
+          DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+        REASON
+      end
       expect(config['tvrage']['service_url']).to eq('http://services.tvrage.com')
     end
 
@@ -154,6 +169,11 @@
       end
 
       it 'should overwrite the previous values' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         expect(config['tvrage']['service_url']).to eq('http://services.tvrage.com')
       end
     end
@@ -168,6 +188,11 @@
       end
 
       it 'should be overwritten by the following values' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         expect(config['tvrage']['service_url']).to eq('http://services.tvrage.com')
       end
 
@@ -246,6 +271,11 @@
       } }
 
       it 'should merge the arrays' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         config = Config.load_files("#{fixture_path}/deep_merge3/config1.yml", "#{fixture_path}/deep_merge3/config2.yml")
 
         expect(config.array.length).to eq(1)
@@ -261,6 +291,11 @@
       } }
 
       it 'should merge the arrays' do
+        unless defined?(DeepMerge)
+          skip <<~REASON
+            DeepMerge is not available in the current context. This test only applies when the `deep_merge` gem is available.
+          REASON
+        end
         config = Config.load_files("#{fixture_path}/deep_merge3/config1.yml", "#{fixture_path}/deep_merge3/config2.yml")
 
         expect(config.array.length).to eq(2)