Faster float formatting

ext/json/ext/generator/depend

@@ -1 +1,2 @@
 generator.o: generator.c $(srcdir)/../fbuffer/fbuffer.h
+generator.o: generator.c $(srcdir)/../vendor/fpconv.c

ext/json/ext/generator/generator.c

@@ -1,5 +1,6 @@
 #include "ruby.h"
 #include "../fbuffer/fbuffer.h"
+#include "../vendor/fpconv.c"
 
 #include <math.h>
 #include <ctype.h>
@@ -1054,8 +1055,9 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
 {
     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 +1069,24 @@ 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;
     }
-    fbuffer_append_str(buffer, rb_funcall(obj, i_to_s, 0));
+
+    /* 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 shortest string representation,
+     * but it adds a ".0" if this is a plain integer.
+     */
+    buffer->len += len;
 }
 
 static void generate_json_fragment(FBuffer *buffer, struct generate_json_data *data, JSON_Generator_State *state, VALUE obj)