Optimize String#to_json (#5456)

larubujo · RX14 · commit be9d9c7c7970 · 2018-01-02T16:09:26.000Z
diff --git a/spec/std/json/serialization_spec.cr b/spec/std/json/serialization_spec.cr
@@ -215,6 +215,19 @@ describe "JSON serialization" do
       "\u{19}".to_json.should eq("\"\\u0019\"")
     end
 
+    it "does for String with control codes in a few places" do
+      "\fab".to_json.should eq(%q("\fab"))
+      "ab\f".to_json.should eq(%q("ab\f"))
+      "ab\fcd".to_json.should eq(%q("ab\fcd"))
+      "ab\fcd\f".to_json.should eq(%q("ab\fcd\f"))
+      "ab\fcd\fe".to_json.should eq(%q("ab\fcd\fe"))
+      "\u{19}ab".to_json.should eq(%q("\u0019ab"))
+      "ab\u{19}".to_json.should eq(%q("ab\u0019"))
+      "ab\u{19}cd".to_json.should eq(%q("ab\u0019cd"))
+      "ab\u{19}cd\u{19}".to_json.should eq(%q("ab\u0019cd\u0019"))
+      "ab\u{19}cd\u{19}e".to_json.should eq(%q("ab\u0019cd\u0019e"))
+    end
+
     it "does for Array" do
       [1, 2, 3].to_json.should eq("[1,2,3]")
     end
diff --git a/src/json/builder.cr b/src/json/builder.cr
@@ -94,35 +94,53 @@ class JSON::Builder
   # This method can also be used to write the name of an object field.
   def string(value)
     string = value.to_s
+
     scalar(string: true) do
       io << '"'
-      string.each_char do |char|
-        case char
+
+      start_pos = 0
+      reader = Char::Reader.new(string)
+
+      while reader.has_next?
+        case char = reader.current_char
         when '\\'
-          io << "\\\\"
+          escape = "\\\\"
         when '"'
-          io << "\\\""
+          escape = "\\\""
         when '\b'
-          io << "\\b"
+          escape = "\\b"
         when '\f'
-          io << "\\f"
+          escape = "\\f"
         when '\n'
-          io << "\\n"
+          escape = "\\n"
         when '\r'
-          io << "\\r"
+          escape = "\\r"
         when '\t'
-          io << "\\t"
+          escape = "\\t"
         when .ascii_control?
+          io.write string.to_slice[start_pos, reader.pos - start_pos]
           io << "\\u"
           ord = char.ord
           io << '0' if ord < 0x1000
           io << '0' if ord < 0x100
           io << '0' if ord < 0x10
           ord.to_s(16, io)
+          reader.next_char
+          start_pos = reader.pos
+          next
         else
-          io << char
+          reader.next_char
+          next
         end
+
+        io.write string.to_slice[start_pos, reader.pos - start_pos]
+        io << escape
+        reader.next_char
+        start_pos = reader.pos
       end
+
+      io.write string.to_slice[start_pos, reader.pos - start_pos]
+
       io << '"'
     end
   end
