Avoid scientific notation before exponent 15

Fix: #861

It's not incorrect to use scientific notation, but it tend
to throw people off a bit, so it's best to keep it for very large
numbers.

Author: byroot

CHANGES.md

@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+* Tuned the floating point number generator to not use scientific notation as agressively.
+
 ### 2025-09-18 (2.14.1)
 
 * Fix `IndexOutOfBoundsException` in the JRuby extension when encoding shared strings.

ext/json/ext/generator/generator.c

@@ -1345,12 +1345,11 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
     }
 
     /* This implementation writes directly into the buffer. We reserve
-     * the 28 characters that fpconv_dtoa states as its maximum.
+     * the 32 characters that fpconv_dtoa states as its maximum.
      */
-    fbuffer_inc_capa(buffer, 28);
+    fbuffer_inc_capa(buffer, 32);
     char* d = buffer->ptr + buffer->len;
     int len = fpconv_dtoa(value, d);
-
     /* fpconv_dtoa converts a float to its shortest string representation,
      * but it adds a ".0" if this is a plain integer.
      */

ext/json/ext/vendor/fpconv.c

@@ -29,6 +29,10 @@
 #include <string.h>
 #include <stdint.h>
 
+#ifdef JSON_DEBUG
+#include <assert.h>
+#endif
+
 #define npowers     87
 #define steppowers  8
 #define firstpower -348 /* 10 ^ -348 */
@@ -320,15 +324,7 @@ static int emit_digits(char* digits, int ndigits, char* dest, int K, bool neg)
 {
     int exp = absv(K + ndigits - 1);
 
-    int max_trailing_zeros = 7;
-
-    if(neg) {
-        max_trailing_zeros -= 1;
-    }
-
-    /* write plain integer */
-    if(K >= 0 && (exp < (ndigits + max_trailing_zeros))) {
-
+    if(K >= 0 && exp < 15) {
         memcpy(dest, digits, ndigits);
         memset(dest + ndigits, '0', K);
 
@@ -432,10 +428,12 @@ static int filter_special(double fp, char* dest)
  *
  * Input:
  * fp -> the double to convert, dest -> destination buffer.
- * The generated string will never be longer than 28 characters.
- * Make sure to pass a pointer to at least 28 bytes of memory.
+ * The generated string will never be longer than 32 characters.
+ * Make sure to pass a pointer to at least 32 bytes of memory.
  * The emitted string will not be null terminated.
  *
+ *
+ *
  * Output:
  * The number of written characters.
  *
@@ -474,6 +472,9 @@ static int fpconv_dtoa(double d, char dest[28])
     int ndigits = grisu2(d, digits, &K);
 
     str_len += emit_digits(digits, ndigits, dest + str_len, K, neg);
+#ifdef JSON_DEBUG
+    assert(str_len <= 32);
+#endif
 
     return str_len;
 }

test/json/json_generator_test.rb

@@ -825,26 +825,41 @@ def test_json_generate_as_json_convert_to_proc
     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"]
-
-      if RUBY_ENGINE == "jruby"
-        values << 1746861937.7842371
-        expecteds << "1.7468619377842371E9"
-      else
-        values << 1746861937.7842371
-        expecteds << "1746861937.7842371"
-      end
-
-      if RUBY_ENGINE == "ruby"
-        values << -2.2471348024634545e-08 << -2.2471348024634545e-09 << -2.2471348024634545e-10
-        expecteds << "-0.000000022471348024634545" << "-0.0000000022471348024634545" << "-2.2471348024634546e-10"
-      end
+  def assert_float_roundtrip(expected, actual)
+    assert_equal(expected, JSON.generate(actual))
+    assert_equal(actual, JSON.parse(JSON.generate(actual)), "JSON: #{JSON.generate(actual)}")
+  end
 
-      values.zip(expecteds).each do |value, expected|
-        assert_equal expected, value.to_json
-      end
+  def test_json_generate_float
+    assert_float_roundtrip "-1.0", -1.0
+    assert_float_roundtrip "1.0", 1.0
+    assert_float_roundtrip "0.0", 0.0
+    assert_float_roundtrip "12.2", 12.2
+    assert_float_roundtrip "2.34375", 7.5 / 3.2
+    assert_float_roundtrip "12.0", 12.0
+    assert_float_roundtrip "100.0", 100.0
+    assert_float_roundtrip "1000.0", 1000.0
+
+    if RUBY_ENGINE == "jruby"
+      assert_float_roundtrip "1.7468619377842371E9", 1746861937.7842371
+    else
+      assert_float_roundtrip "1746861937.7842371", 1746861937.7842371
+    end
+
+    if RUBY_ENGINE == "ruby"
+      assert_float_roundtrip "100000000000000.0", 100000000000000.0
+      assert_float_roundtrip "1e+15", 1e+15
+      assert_float_roundtrip "-100000000000000.0", -100000000000000.0
+      assert_float_roundtrip "-1e+15", -1e+15
+      assert_float_roundtrip "1111111111111111.1", 1111111111111111.1
+      assert_float_roundtrip "1.1111111111111112e+16", 11111111111111111.1
+      assert_float_roundtrip "-1111111111111111.1", -1111111111111111.1
+      assert_float_roundtrip "-1.1111111111111112e+16", -11111111111111111.1
+
+      assert_float_roundtrip "-0.000000022471348024634545", -2.2471348024634545e-08
+      assert_float_roundtrip "-0.0000000022471348024634545", -2.2471348024634545e-09
+      assert_float_roundtrip "-2.2471348024634546e-10", -2.2471348024634545e-10
+    end
   end
 
   def test_numbers_of_various_sizes