[ruby/json] Adjust fpconv to add ".0" to integers

Adds a test case

fix

https://github.com/ruby/json/commit/fa5bdf87cb

ext/json/ext/vendor/fpconv/src/fpconv.c

@@ -222,7 +222,11 @@ static int emit_digits(char* digits, int ndigits, char* dest, int K, bool neg)
         memcpy(dest, digits, ndigits);
         memset(dest + ndigits, '0', K);
 
-        return ndigits + K;
+        /* add a .0 to mark this as a float. */
+        dest[ndigits + K] = '.';
+        dest[ndigits + K + 1] = '0';
+
+        return ndigits + K + 2;
     }
 
     /* write decimal w/o scientific notation */
@@ -290,7 +294,9 @@ static int filter_special(double fp, char* dest)
 {
     if(fp == 0.0) {
         dest[0] = '0';
-        return 1;
+        dest[1] = '.';
+        dest[2] = '0';
+        return 3;
     }
 
     uint64_t bits = get_dbits(fp);

ext/json/generator/generator.c

@@ -1084,18 +1084,9 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
     char* d = buffer->ptr + buffer->len;
     int len = fpconv_dtoa(value, d);
 
-    /* fpconv_dtoa converts a float to its shorted string representation. When
-     * converting a float that is exactly an integer (e.g. `Float(2)`) this
-     * returns in a string that looks like an integer. This is correct, since
-     * JSON treats ints and floats the same. However, to not break integrations
-     * that expect a string representation looking like a float, we append a
-     * "." in that case.
+    /* fpconv_dtoa converts a float to its shortest string representation,
+     * but it adds a ".0" if this is a plain integer.
      */
-    if(!memchr(d, '.', len) && !memchr(d, 'e', len)) {
-        d[len] = '.';
-        d[len+1] = '0';
-        len += 2;
-    }
     buffer->len += len;
 }
 

test/json/json_generator_test.rb

@@ -698,4 +698,13 @@ class JSONGeneratorTest < Test::Unit::TestCase
     object = Object.new
     assert_equal object.object_id.to_json, JSON.generate(object, strict: true, as_json: :object_id)
   end
+
+  def test_json_generate_float
+      values = [-1.0, 1.0, 0.0, 12.2, 7.5 / 3.2, 12.0, 100.0, 1000.0]
+      expecteds = ["-1.0", "1.0", "0.0", "12.2", "2.34375", "12.0", "100.0", "1000.0"]
+
+      values.zip(expecteds).each do |value, expected|
+        assert_equal expected, value.to_json
+      end
+  end
 end