Refactor index algorithm lookup so that it only builds the available …

…options once

This way the available options are only built for actually fetching the
algorithm in case the option is given.

The options are going to be necessary a second time only in case the option
is given but does not exist, which is supposed to be due to a typo or
something like that, so no problem.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -774,10 +774,10 @@ def add_index_options(table_name, column_name, options = {})
             index_name = options[:name].to_s if options.key?(:name)
             max_index_length = options.fetch(:internal, false) ? index_name_length : allowed_index_name_length
 
-            if index_algorithms.key?(options[:algorithm])
-              algorithm = index_algorithms[options[:algorithm]]
-            elsif options[:algorithm].present?
-              raise ArgumentError.new("Algorithm must be one of the following: #{index_algorithms.keys.map(&:inspect).join(', ')}")
+            if options.key?(:algorithm)
+              algorithm = index_algorithms.fetch(options[:algorithm]) {
+                raise ArgumentError.new("Algorithm must be one of the following: #{index_algorithms.keys.map(&:inspect).join(', ')}")
+              }
             end
 
             using = "USING #{options[:using]}" if options[:using].present?