Faster float formatting

This commit provides an alternative implementation for a float → decimal conversion.

It integrates a C implementation of Fabian Loitsch's Grisu-algorithm [[pdf]](http://florian.loitsch.com/publications/dtoa-pldi2010.pdf), extracted from https://github.com/night-shift/fpconv. The relevant files are added in this PR, they are, as is all of https://github.com/night-shift/fpconv, available under a MIT License.

As a result, I see a speedup of 900% on Apple Silicon M1 for a float set of benchmarks.

floats don't have a single correct  string representation: a float like `1000.0` can be represented as "1000", "1e3", "1000.0" (and more).

The Grisu algorithm converts floating point numbers to an optimal decimal string representation without loss of precision. As a result, a float that is exactly an integer (like `Float(10)`) will be converted by that algorithm into `"10"`. While technically correct – the JSON format treats floats and integers identically –, this differs from the current behaviour of the `"json"` gem. To address this, the integration checks for that case, and explicitely adds a ".0" suffix in those cases.

This is sufficient to meet all existing tests; there is, however, a chance that the current implementation and this implementation occasionally encode floats differently.

```
== Encoding floats (4179311 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
        json (local)     4.000 i/100ms
Calculating -------------------------------------
        json (local)     46.046 (± 2.2%) i/s   (21.72 ms/i) -    232.000 in   5.039611s

Normalize to 2090234 byte
== Encoding floats (4179242 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
       json (2.10.2)     1.000 i/100ms
Calculating -------------------------------------
       json (2.10.2)      4.614 (± 0.0%) i/s  (216.74 ms/i) -     24.000 in   5.201871s
```

These benchmarks are run via a script ([link](https://gist.github.com/radiospiel/04019402726a28b31616df3d0c17bd1c)) which is based on the gem's `benchmark/encoder.rb` file. There are probably better ways to run benchmarks :) My version allows to combine multiple test cases into a single one.

The `dumps` benchmark, which covers the JSON files in `benchmark/data/*.json` – with the exception of `canada.json` – , reported a minor speedup within statistical uncertainty.

Author: radiospiel

ext/json/ext/generator/generator.c

@@ -1050,12 +1050,15 @@ static void generate_json_integer(FBuffer *buffer, struct generate_json_data *da
 }
 #endif
 
+#include "../vendor/fpconv/src/fpconv.c"
+
 static void generate_json_float(FBuffer *buffer, struct generate_json_data *data, JSON_Generator_State *state, VALUE obj)
 {
     double value = RFLOAT_VALUE(obj);
     char allow_nan = state->allow_nan;
-    if (!allow_nan) {
-        if (isinf(value) || isnan(value)) {
+    if (isinf(value) || isnan(value)) {
+        /* for NaN and Infinity values we either raise an error or rely on Float#to_s. */
+        if (!allow_nan) {
             if (state->strict && state->as_json) {
                 VALUE casted_obj = rb_proc_call_with_block(state->as_json, 1, &obj, Qnil);
                 if (casted_obj != obj) {
@@ -1067,8 +1070,33 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
             }
             raise_generator_error(obj, "%"PRIsVALUE" not allowed in JSON", rb_funcall(obj, i_to_s, 0));
         }
+
+        VALUE tmp = rb_funcall(obj, i_to_s, 0);
+        fbuffer_append_str(buffer, tmp);
+        return;
+    }
+
+    /* This implementation writes directly into the buffer. We reserve
+     * the 24 characters that fpconv_dtoa states as its maximum, plus
+     * 2 more characters for the potential ".0" suffix. 
+     */
+    fbuffer_inc_capa(buffer, 26);
+    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.
+     */
+    if(!memchr(d, '.', len) && !memchr(d, 'e', len)) {
+        d[len] = '.';
+        d[len+1] = '0';
+        len += 2;
     }
-    fbuffer_append_str(buffer, rb_funcall(obj, i_to_s, 0));
+    buffer->len += len;
 }
 
 static void generate_json_fragment(FBuffer *buffer, struct generate_json_data *data, JSON_Generator_State *state, VALUE obj)

ext/json/ext/vendor/fpconv/README.md

@@ -0,0 +1,3 @@
+The contents of this directory is extracted from https://github.com/night-shift/fpconv
+
+It is licensed under the provisions of the Boost Software License - Version 1.0 - August 17th, 2003. See the ./license file for details.

ext/json/ext/vendor/fpconv/license

@@ -0,0 +1,23 @@
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.