Faster integer formatting

radiospiel · byroot · commit c5fca982b0f5 · 2025-03-24T09:52:15.000+01:00
This commit provides an alternative implementation for a long → decimal conversion. The main difference is that it uses an algorithm pulled from https://github.com/jeaiii/itoa. The source there is C++, it was converted by hand to C for inclusion with this gem. jeaiii's algorithm is covered by the MIT License, see source code. On addition this version now also generates the string directly into the fbuffer, foregoing the need to run a separate memory copy. As a result, I see a speedup of 32% on Apple Silicon M1 for an integer set of benchmarks.
diff --git a/ext/json/ext/fbuffer/fbuffer.h b/ext/json/ext/fbuffer/fbuffer.h
@@ -3,6 +3,7 @@
 
 #include "ruby.h"
 #include "ruby/encoding.h"
+#include "../vendor/jeaiii-ltoa.h"
 
 /* shims */
 /* This is the fallback definition from Ruby 3.4 */
@@ -150,6 +151,13 @@ static void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len)
     }
 }
 
+/* Appends a character into a buffer. The buffer needs to have sufficient capacity, via fbuffer_inc_capa(...). */
+static inline void fbuffer_append_reserved_char(FBuffer *fb, char chr)
+{
+    fb->ptr[fb->len] = chr;
+    fb->len += 1;
+}
+
 static void fbuffer_append_str(FBuffer *fb, VALUE str)
 {
     const char *newstr = StringValuePtr(str);
@@ -167,25 +175,39 @@ static inline void fbuffer_append_char(FBuffer *fb, char newchr)
     fb->len++;
 }
 
-static long fltoa(long number, char *buf)
-{
-    static const char digits[] = "0123456789";
-    long sign = number;
-    char* tmp = buf;
-
-    if (sign < 0) number = -number;
-    do *tmp-- = digits[number % 10]; while (number /= 10);
-    if (sign < 0) *tmp-- = '-';
-    return buf - tmp;
-}
-
-#define LONG_BUFFER_SIZE 20
+/*
+ * Appends the decimal string representation of \a number into the buffer.
+ */
 static void fbuffer_append_long(FBuffer *fb, long number)
 {
-    char buf[LONG_BUFFER_SIZE];
-    char *buffer_end = buf + LONG_BUFFER_SIZE;
-    long len = fltoa(number, buffer_end - 1);
-    fbuffer_append(fb, buffer_end - len, len);
+    /*
+     * The to_text_from_ulong() function produces digits left-to-right,
+     * allowing us to write directly into the buffer, but we don't know
+     * the number of resulting characters.
+     *
+     * We do know, however, that the `number` argument is always in the
+     * range 0xc000000000000000 to 0x3fffffffffffffff, or, in decimal, 
+     * -4611686018427387904 to 4611686018427387903. The max number of chars
+     * generated is therefore 20 (including a potential sign character).
+     */
+
+    static const int MAX_CHARS_FOR_LONG = 20;
+
+    fbuffer_inc_capa(fb, MAX_CHARS_FOR_LONG);
+
+    if (number < 0) {
+        fbuffer_append_reserved_char(fb, '-');
+
+        /* 
+         * Since number is always > LONG_MIN, `-number` will not overflow
+         * and is always the positive abs() value.
+         */
+        number = -number;
+    }
+
+    char* d = fb->ptr + fb->len;
+    char* end = to_text_from_ulong(d, number);
+    fb->len += end - d;
 }
 
 static VALUE fbuffer_finalize(FBuffer *fb)
diff --git a/ext/json/ext/generator/depend b/ext/json/ext/generator/depend
@@ -1,2 +1,3 @@
 generator.o: generator.c $(srcdir)/../fbuffer/fbuffer.h
 generator.o: generator.c $(srcdir)/../vendor/fpconv.c
+generator.o: generator.c $(srcdir)/../vendor/jeaiii-ltoa.h
diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
@@ -1710,6 +1710,7 @@ void Init_generator(void)
     cFragment = rb_const_get(mJSON, rb_intern("Fragment"));
 
     VALUE mExt = rb_define_module_under(mJSON, "Ext");
+
     VALUE mGenerator = rb_define_module_under(mExt, "Generator");
 
     rb_global_variable(&eGeneratorError);
diff --git a/ext/json/ext/vendor/jeaiii-ltoa.h b/ext/json/ext/vendor/jeaiii-ltoa.h
@@ -0,0 +1,280 @@
+/*
+
+Copyright (c) 2024,2025 Enrico Thierbach - https://github.com/radiospiel
+
+This file is released under the terms of the MIT License. It is based on the
+work of James Edward Anhalt III, with the original license listed below.
+
+MIT License
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+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 AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+*/
+
+/*
+MIT License
+
+Copyright (c) 2022 James Edward Anhalt III - https://github.com/jeaiii/itoa
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+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 AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+*/
+
+#ifndef JEAIII_TO_TEXT_H_
+#define JEAIII_TO_TEXT_H_
+
+#include <stdint.h>
+
+typedef uint_fast32_t u32_t;
+typedef uint_fast64_t u64_t;
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wshorten-64-to-32"
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-braces"
+
+#define u32(x) ((u32_t)(x))
+#define u64(x) ((u64_t)(x))
+
+struct pair
+{
+    char dd[2];
+};
+
+#define cast_to_pair_ptr(b) ((struct pair*)(void*)(b))
+
+static struct pair digits_dd[100] =
+{
+    { '0', '0' }, { '0', '1' }, { '0', '2' }, { '0', '3' }, { '0', '4' }, { '0', '5' }, { '0', '6' }, { '0', '7' }, { '0', '8' }, { '0', '9' },
+    { '1', '0' }, { '1', '1' }, { '1', '2' }, { '1', '3' }, { '1', '4' }, { '1', '5' }, { '1', '6' }, { '1', '7' }, { '1', '8' }, { '1', '9' },
+    { '2', '0' }, { '2', '1' }, { '2', '2' }, { '2', '3' }, { '2', '4' }, { '2', '5' }, { '2', '6' }, { '2', '7' }, { '2', '8' }, { '2', '9' },
+    { '3', '0' }, { '3', '1' }, { '3', '2' }, { '3', '3' }, { '3', '4' }, { '3', '5' }, { '3', '6' }, { '3', '7' }, { '3', '8' }, { '3', '9' },
+    { '4', '0' }, { '4', '1' }, { '4', '2' }, { '4', '3' }, { '4', '4' }, { '4', '5' }, { '4', '6' }, { '4', '7' }, { '4', '8' }, { '4', '9' },
+    { '5', '0' }, { '5', '1' }, { '5', '2' }, { '5', '3' }, { '5', '4' }, { '5', '5' }, { '5', '6' }, { '5', '7' }, { '5', '8' }, { '5', '9' },
+    { '6', '0' }, { '6', '1' }, { '6', '2' }, { '6', '3' }, { '6', '4' }, { '6', '5' }, { '6', '6' }, { '6', '7' }, { '6', '8' }, { '6', '9' },
+    { '7', '0' }, { '7', '1' }, { '7', '2' }, { '7', '3' }, { '7', '4' }, { '7', '5' }, { '7', '6' }, { '7', '7' }, { '7', '8' }, { '7', '9' },
+    { '8', '0' }, { '8', '1' }, { '8', '2' }, { '8', '3' }, { '8', '4' }, { '8', '5' }, { '8', '6' }, { '8', '7' }, { '8', '8' }, { '8', '9' },
+    { '9', '0' }, { '9', '1' }, { '9', '2' }, { '9', '3' }, { '9', '4' }, { '9', '5' }, { '9', '6' }, { '9', '7' }, { '9', '8' }, { '9', '9' },
+};
+
+#define NUL 'x'
+
+static struct pair digits_fd[100] =
+{
+    { '0', NUL }, { '1', NUL }, { '2', NUL }, { '3', NUL }, { '4', NUL }, { '5', NUL }, { '6', NUL }, { '7', NUL }, { '8', NUL }, { '9', NUL },
+    { '1', '0' }, { '1', '1' }, { '1', '2' }, { '1', '3' }, { '1', '4' }, { '1', '5' }, { '1', '6' }, { '1', '7' }, { '1', '8' }, { '1', '9' },
+    { '2', '0' }, { '2', '1' }, { '2', '2' }, { '2', '3' }, { '2', '4' }, { '2', '5' }, { '2', '6' }, { '2', '7' }, { '2', '8' }, { '2', '9' },
+    { '3', '0' }, { '3', '1' }, { '3', '2' }, { '3', '3' }, { '3', '4' }, { '3', '5' }, { '3', '6' }, { '3', '7' }, { '3', '8' }, { '3', '9' },
+    { '4', '0' }, { '4', '1' }, { '4', '2' }, { '4', '3' }, { '4', '4' }, { '4', '5' }, { '4', '6' }, { '4', '7' }, { '4', '8' }, { '4', '9' },
+    { '5', '0' }, { '5', '1' }, { '5', '2' }, { '5', '3' }, { '5', '4' }, { '5', '5' }, { '5', '6' }, { '5', '7' }, { '5', '8' }, { '5', '9' },
+    { '6', '0' }, { '6', '1' }, { '6', '2' }, { '6', '3' }, { '6', '4' }, { '6', '5' }, { '6', '6' }, { '6', '7' }, { '6', '8' }, { '6', '9' },
+    { '7', '0' }, { '7', '1' }, { '7', '2' }, { '7', '3' }, { '7', '4' }, { '7', '5' }, { '7', '6' }, { '7', '7' }, { '7', '8' }, { '7', '9' },
+    { '8', '0' }, { '8', '1' }, { '8', '2' }, { '8', '3' }, { '8', '4' }, { '8', '5' }, { '8', '6' }, { '8', '7' }, { '8', '8' }, { '8', '9' },
+    { '9', '0' }, { '9', '1' }, { '9', '2' }, { '9', '3' }, { '9', '4' }, { '9', '5' }, { '9', '6' }, { '9', '7' }, { '9', '8' }, { '9', '9' },
+};
+
+#undef NUL
+
+static u64_t mask24 = (u64(1) << 24) - 1;
+static u64_t mask32 = (u64(1) << 32) - 1;
+static u64_t mask57 = (u64(1) << 57) - 1;
+
+static 
+char* to_text_from_ulong(char* b, u64_t n) {
+    if (n < u32(1e2))
+    {
+        *cast_to_pair_ptr(b) = digits_fd[n];
+        return n < 10 ? b + 1 : b + 2;
+    }
+    if (n < u32(1e6))
+    {
+        if (n < u32(1e4))
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * n;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= n < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            return b + 4;
+        }
+        u64_t f0 = u64(10 * (1ull << 32ull)/ 1e5 + 1) * n;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+        b -= n < u32(1e5);
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        return b + 6;
+    }
+    if (n < u64(1ull << 32ull))
+    {
+        if (n < u32(1e8))
+        {
+            u64_t f0 = u64(10 * (1ull << 48ull) / 1e7 + 1) * n >> 16;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+            b -= n < u32(1e7);
+            u64_t f2 = (f0 & mask32) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+            u64_t f4 = (f2 & mask32) * 100;
+            *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+            u64_t f6 = (f4 & mask32) * 100;
+            *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+            return b + 8;
+        }
+        u64_t f0 = u64(10 * (1ull << 57ull) / 1e9 + 1) * n;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 57];
+        b -= n < u32(1e9);
+        u64_t f2 = (f0 & mask57) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 57];
+        u64_t f4 = (f2 & mask57) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 57];
+        u64_t f6 = (f4 & mask57) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 57];
+        u64_t f8 = (f6 & mask57) * 100;
+        *cast_to_pair_ptr(b + 8) = digits_dd[f8 >> 57];
+        return b + 10;
+    }
+
+    // if we get here U must be u64 but some compilers don't know that, so reassign n to a u64 to avoid warnings
+    u32_t z = n % u32(1e8);
+    u64_t u = n / u32(1e8);
+
+    if (u < u32(1e2))
+    {
+        // u can't be 1 digit (if u < 10 it would have been handled above as a 9 digit 32bit number)
+        *cast_to_pair_ptr(b) = digits_dd[u];
+        b += 2;
+    }
+    else if (u < u32(1e6))
+    {
+        if (u < u32(1e4))
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= u < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            b += 4;
+        }
+        else
+        {
+            u64_t f0 = u64(10 * (1ull << 32ull) / 1e5 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+            b -= u < u32(1e5);
+            u64_t f2 = (f0 & mask32) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+            u64_t f4 = (f2 & mask32) * 100;
+            *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+            b += 6;
+        }
+    }
+    else if (u < u32(1e8))
+    {
+        u64_t f0 = u64(10 * (1ull << 48ull) / 1e7 + 1) * u >> 16;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 32];
+        b -= u < u32(1e7);
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        u64_t f6 = (f4 & mask32) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+        b += 8;
+    }
+    else if (u < u64(1ull << 32ull))
+    {
+        u64_t f0 = u64(10 * (1ull << 57ull) / 1e9 + 1) * u;
+        *cast_to_pair_ptr(b) = digits_fd[f0 >> 57];
+        b -= u < u32(1e9);
+        u64_t f2 = (f0 & mask57) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 57];
+        u64_t f4 = (f2 & mask57) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 57];
+        u64_t f6 = (f4 & mask57) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 57];
+        u64_t f8 = (f6 & mask57) * 100;
+        *cast_to_pair_ptr(b + 8) = digits_dd[f8 >> 57];
+        b += 10;
+    }
+    else
+    {
+        u32_t y = u % u32(1e8);
+        u /= u32(1e8);
+
+        // u is 2, 3, or 4 digits (if u < 10 it would have been handled above)
+        if (u < u32(1e2))
+        {
+            *cast_to_pair_ptr(b) = digits_dd[u];
+            b += 2;
+        }
+        else
+        {
+            u32_t f0 = u32(10 * (1 << 24) / 1e3 + 1) * u;
+            *cast_to_pair_ptr(b) = digits_fd[f0 >> 24];
+            b -= u < u32(1e3);
+            u32_t f2 = (f0 & mask24) * 100;
+            *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 24];
+            b += 4;
+        }
+        // do 8 digits
+        u64_t f0 = (u64((1ull << 48ull) / 1e6 + 1) * y >> 16) + 1;
+        *cast_to_pair_ptr(b) = digits_dd[f0 >> 32];
+        u64_t f2 = (f0 & mask32) * 100;
+        *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+        u64_t f4 = (f2 & mask32) * 100;
+        *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+        u64_t f6 = (f4 & mask32) * 100;
+        *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+        b += 8;
+    }
+    // do 8 digits
+    u64_t f0 = (u64((1ull << 48ull) / 1e6 + 1) * z >> 16) + 1;
+    *cast_to_pair_ptr(b) = digits_dd[f0 >> 32];
+    u64_t f2 = (f0 & mask32) * 100;
+    *cast_to_pair_ptr(b + 2) = digits_dd[f2 >> 32];
+    u64_t f4 = (f2 & mask32) * 100;
+    *cast_to_pair_ptr(b + 4) = digits_dd[f4 >> 32];
+    u64_t f6 = (f4 & mask32) * 100;
+    *cast_to_pair_ptr(b + 6) = digits_dd[f6 >> 32];
+    return b + 8;
+}
+
+#undef u32
+#undef u64
+
+#pragma clang diagnostic pop
+#pragma GCC diagnostic pop
+
+#endif // JEAIII_TO_TEXT_H_
+
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
@@ -707,4 +707,16 @@ def test_json_generate_float
         assert_equal expected, value.to_json
       end
   end
+  
+  def test_numbers_of_various_sizes
+    numbers = [
+      0, 1, -1, 9, -9, 13, -13, 91, -91, 513, -513, 7513, -7513,
+      17591, -17591, -4611686018427387904, 4611686018427387903,
+      2**62, 2**63, 2**64, -(2**62), -(2**63), -(2**64)
+    ]
+
+    numbers.each do |number|
+      assert_equal "[#{number}]", JSON.generate([number])
+    end
+  end
 end

Original file line number	Diff line number	Diff line change
`@@ -1,2 +1,3 @@`
`1`	`1`	`generator.o: generator.c $(srcdir)/../fbuffer/fbuffer.h`
`2`	`2`	`generator.o: generator.c $(srcdir)/../vendor/fpconv.c`
	`3`	`+generator.o: generator.c $(srcdir)/../vendor/jeaiii-ltoa.h`