Add ActiveSupport::Multibyte::Chars#limit.

The limit method limits the number of bytes in a string. Useful when the
storage space of the string is limited, for instance in a database column
definition.

Sharpen up the implementation of translate offset.

[#3192 state:committed]

activesupport/lib/active_support/multibyte/chars.rb

@@ -363,6 +363,16 @@ def slice!(*args)
         slice
       end
 
+      # Limit the byte size of the string to a number of bytes without breaking characters. Usable
+      # when the storage for a string is limited for some reason.
+      #
+      # Example:
+      #   s = 'こんにちは'
+      #   s.mb_chars.limit(7) #=> "こに"
+      def limit(limit)
+        slice(0...translate_offset(limit))
+      end
+
       # Returns the codepoint of the first character in the string.
       #
       # Example:
@@ -651,24 +661,15 @@ def tidy_bytes(string)
       end
 
       protected
-
+        
         def translate_offset(byte_offset) #:nodoc:
           return nil if byte_offset.nil?
           return 0   if @wrapped_string == ''
-          chunk = @wrapped_string[0..byte_offset]
           begin
-            begin
-              chunk.unpack('U*').length - 1
-            rescue ArgumentError => e
-              chunk = @wrapped_string[0..(byte_offset+=1)]
-              # Stop retrying at the end of the string
-              raise e unless byte_offset < chunk.length 
-              # We damaged a character, retry
-              retry
-            end
-          # Catch the ArgumentError so we can throw our own
-          rescue ArgumentError 
-            raise EncodingError, 'malformed UTF-8 character'
+            @wrapped_string[0...byte_offset].unpack('U*').length
+          rescue ArgumentError => e
+            byte_offset -= 1
+            retry
           end
         end
 

activesupport/test/multibyte_chars_test.rb

@@ -169,6 +169,7 @@ def test_string_methods_are_chainable
     assert chars('').strip.kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars('').reverse.kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars(' ').slice(0).kind_of?(ActiveSupport::Multibyte.proxy_class)
+    assert chars('').limit(0).kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars('').upcase.kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars('').downcase.kind_of?(ActiveSupport::Multibyte.proxy_class)
     assert chars('').capitalize.kind_of?(ActiveSupport::Multibyte.proxy_class)
@@ -196,7 +197,9 @@ def test_sortability
   def test_should_return_character_offset_for_regexp_matches
     assert_nil(@chars =~ /wrong/u)
     assert_equal 0, (@chars =~ /こ/u)
+    assert_equal 0, (@chars =~ /こに/u)
     assert_equal 1, (@chars =~ /に/u)
+    assert_equal 2, (@chars =~ /ち/u)
     assert_equal 3, (@chars =~ /わ/u)
   end
 
@@ -493,6 +496,44 @@ def test_capitalize_should_be_unicode_aware
     end
   end
 
+  def test_limit_should_not_break_on_blank_strings
+    chars = ''.mb_chars
+
+    assert_equal '', chars.limit(0)
+    assert_equal '', chars.limit(1)
+  end
+
+  def test_limit_should_work_on_a_multibyte_string
+    chars = UNICODE_STRING.mb_chars
+
+    assert_equal UNICODE_STRING, chars.limit(UNICODE_STRING.length)
+    assert_equal '', chars.limit(0)
+    assert_equal '', chars.limit(1)
+    assert_equal 'こ', chars.limit(3)
+    assert_equal 'こに', chars.limit(6)
+    assert_equal 'こに', chars.limit(8)
+    assert_equal 'こにち', chars.limit(9)
+    assert_equal 'こにちわ', chars.limit(50)
+  end
+
+  def test_limit_should_work_on_an_ascii_string
+    ascii = ASCII_STRING.mb_chars
+
+    assert_equal ASCII_STRING, ascii.limit(ASCII_STRING.length)
+    assert_equal '', ascii.limit(0)
+    assert_equal 'o', ascii.limit(1)
+    assert_equal 'oh', ascii.limit(2)
+    assert_equal 'ohay', ascii.limit(4)
+    assert_equal 'ohayo', ascii.limit(50)
+  end
+
+  def test_limit_should_keep_under_the_specified_byte_limit
+    chars = UNICODE_STRING.mb_chars
+    (1..UNICODE_STRING.length).each do |limit|
+      assert chars.limit(limit).to_s.length <= limit
+    end
+  end
+
   def test_composition_exclusion_is_set_up_properly
     # Normalization of DEVANAGARI LETTER QA breaks when composition exclusion isn't used correctly
     qa = [0x915, 0x93c].pack('U*')
@@ -603,3 +644,21 @@ def string_from_classes(classes)
     end.pack('U*')
   end
 end
+
+class MultibyteInternalsTest < ActiveSupport::TestCase
+  include MultibyteTestHelpers
+
+  test "Chars translates a character offset to a byte offset" do
+    chars = "Puisque c'était son erreur, il m'a aidé".mb_chars
+    [
+      [0, 0],
+      [3, 3],
+      [12, 11],
+      [14, 13],
+      [41, 39]
+    ].each do |byte_offset, character_offset|
+      assert_equal character_offset, chars.send(:translate_offset, byte_offset),
+        "Expected byte offset #{byte_offset} to translate to #{character_offset}"
+    end
+  end
+end