Increase speed of Table#to_csv

If we set the encoding when we call Table#to_csv we do not need
to go row by row to determine the encoding to use. This allows
the use of CSV#generate_lines as a faster exporter.

Add a single_encoding option to Table#to_csv to use the encoding
of the first row of the table as the encoding.

Author: wishdev

lib/csv/table.rb

@@ -987,6 +987,8 @@ def to_a
     # :call-seq:
     #   table.to_csv(**options) -> csv_string
     #
+    # - Argument +single_encoding+, if true, encoding will be determined from the first row.
+    #
     # Returns the table as \CSV string.
     # See {Options for Generating}[../CSV.html#class-CSV-label-Options+for+Generating].
     #
@@ -1001,15 +1003,32 @@ def to_a
     #
     # Limit rows if option +limit+ is given like +2+:
     #   table.to_csv(limit: 2) # => "Name,Value\nfoo,0\nbar,1\n"
-    def to_csv(write_headers: true, limit: nil, **options)
+    def to_csv(write_headers: true, limit: nil, single_encoding: false, **options)
       array = write_headers ? [headers.to_csv(**options)] : []
       limit ||= @table.size
       limit = @table.size + 1 + limit if limit < 0
       limit = 0 if limit < 0
-      @table.first(limit).each do |row|
-        array.push(row.fields.to_csv(**options)) unless row.header_row?
-      end
 
+      if single_encoding || options[:encoding]
+        rows = @table.first(limit).select { |row| !row.header_row? }
+        unless options[:encoding]
+          fallback_encoding = nil
+          output_encoding = nil
+          rows.first.fields.each do |field|
+            next unless field.is_a?(String)
+            fallback_encoding ||= field.encoding
+            next if field.ascii_only?
+            output_encoding = field.encoding
+            break
+          end
+          options[:encoding] = output_encoding || fallback_encoding
+        end
+        array.push(CSV.generate_lines(rows, **options))
+      else
+        @table.first(limit).each do |row|
+          array.push(row.fields.to_csv(**options)) unless row.header_row?
+        end
+      end
       array.join("")
     end
     alias_method :to_s, :to_csv

test/csv/test_table.rb

@@ -373,6 +373,16 @@ def test_to_csv_limit_negative_over
     CSV
   end
 
+  def test_to_csv_single_encoding
+    rows  = [ CSV::Row.new(%w{A}, ["\x00\xac".force_encoding("ASCII-8BIT")]),
+              CSV::Row.new(%w{A}, ["\x00\xac"]) ]
+    table = CSV::Table.new(rows)
+
+    assert_equal('ASCII-8BIT', table.to_csv(single_encoding: true).encoding.to_s)
+    assert_equal('UTF-8', table.to_csv(encoding: 'UTF-8').encoding.to_s)
+    assert_raises(Encoding::CompatibilityError) {table.to_csv}
+  end
+
   def test_append
     # verify that we can chain the call
     assert_equal(@table, @table << [10, 11, 12])