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TimWollaTimWolla
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random: Fix γ-section implementation for Randomizer::getFloat() (#12402)
The reference implementation of the "Drawing Random Floating-Point Numbers from an Interval" paper contains two mistakes that will result in erroneous values being returned under certain circumstances: - For large values of `g` the multiplication of `k * g` might overflow to infinity. - The value of `ceilint()` might exceed 2^53, possibly leading to a rounding error when promoting `k` to double within the multiplication of `k * g`. This commit updates the implementation based on Prof. Goualard suggestions after reaching out to him. It will correctly handle inputs larger than 2^-1020 in absolute values. This limitation will be documented and those inputs possibly be rejected in a follow-up commit depending on performance concerns.
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‎NEWS

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Original file line numberDiff line numberDiff line change
@@ -13,6 +13,10 @@ PHP NEWS
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. Fixed bug GH-8143 (Crashes in zend_accel_inheritance_cache_find since
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upgrading to 8.1.3 due to corrupt on-disk file cache). (turchanov)
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- Random:
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. Fix Randomizer::getFloat() returning incorrect results under
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certain circumstances. (timwolla)
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- SOAP:
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. Fixed bug GH-12392 (Segmentation fault on SoapClient::__getTypes).
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(nielsdos)

‎ext/random/gammasection.c

Lines changed: 57 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -13,6 +13,9 @@
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| Authors: Tim Düsterhus <timwolla@php.net> |
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| |
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| Based on code from: Frédéric Goualard |
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| Corrected to handled appropriately random integers larger than 2^53 |
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| converted to double precision floats, and to avoid overflows for |
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| large gammas. |
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+----------------------------------------------------------------------+
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*/
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@@ -46,6 +49,12 @@ static double gamma_max(double x, double y)
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return (fabs(x) > fabs(y)) ? gamma_high(x) : gamma_low(y);
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}
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52+
static void splitint64(uint64_t v, double *vhi, double *vlo)
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{
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*vhi = v >> 2;
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*vlo = v & UINT64_C(0x3);
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}
57+
4958
static uint64_t ceilint(double a, double b, double g)
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{
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double s = b / g - a / g;
@@ -74,9 +83,19 @@ PHPAPI double php_random_gammasection_closed_open(const php_random_algo *algo, p
7483
uint64_t k = 1 + php_random_range64(algo, status, hi - 1); /* [1, hi] */
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7685
if (fabs(min) <= fabs(max)) {
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return k == hi ? min : max - k * g;
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if (k == hi) {
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return min;
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} else {
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (max * 0x1p-2 - k_hi * g) - k_lo * g;
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}
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} else {
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return min + (k - 1) * g;
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double k_hi, k_lo;
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splitint64(k - 1, &k_hi, &k_lo);
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return 0x1p+2 * (min * 0x1p-2 + k_hi * g) + k_lo * g;
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}
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}
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@@ -92,9 +111,23 @@ PHPAPI double php_random_gammasection_closed_closed(const php_random_algo *algo,
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uint64_t k = php_random_range64(algo, status, hi); /* [0, hi] */
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94113
if (fabs(min) <= fabs(max)) {
95-
return k == hi ? min : max - k * g;
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if (k == hi) {
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return min;
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} else {
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (max * 0x1p-2 - k_hi * g) - k_lo * g;
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}
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} else {
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return k == hi ? max : min + k * g;
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if (k == hi) {
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return max;
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} else {
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (min * 0x1p-2 + k_hi * g) + k_lo * g;
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}
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}
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}
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@@ -110,9 +143,19 @@ PHPAPI double php_random_gammasection_open_closed(const php_random_algo *algo, p
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uint64_t k = php_random_range64(algo, status, hi - 1); /* [0, hi - 1] */
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if (fabs(min) <= fabs(max)) {
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return max - k * g;
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (max * 0x1p-2 - k_hi * g) - k_lo * g;
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} else {
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return k == (hi - 1) ? max : min + (k + 1) * g;
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if (k == (hi - 1)) {
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return max;
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} else {
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double k_hi, k_lo;
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splitint64(k + 1, &k_hi, &k_lo);
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return 0x1p+2 * (min * 0x1p-2 + k_hi * g) + k_lo * g;
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}
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}
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}
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@@ -128,8 +171,14 @@ PHPAPI double php_random_gammasection_open_open(const php_random_algo *algo, php
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uint64_t k = 1 + php_random_range64(algo, status, hi - 2); /* [1, hi - 1] */
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if (fabs(min) <= fabs(max)) {
131-
return max - k * g;
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (max * 0x1p-2 - k_hi * g) - k_lo * g;
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} else {
133-
return min + k * g;
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double k_hi, k_lo;
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splitint64(k, &k_hi, &k_lo);
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return 0x1p+2 * (min * 0x1p-2 + k_hi * g) + k_lo * g;
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}
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}

‎ext/random/tests/03_randomizer/methods/getFloat_extreme_range.phpt

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--TEST--
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Random: Randomizer: getFloat(): Extreme ranges are handled correctly
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--FILE--
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<?php
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use Random\Engine;
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use Random\Randomizer;
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final class TestEngine implements Engine
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{
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private array $values = [
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"\x64\xe8\x58\x79\x3e\xf6\x38\x00",
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"\x65\xe8\x58\x79\x3e\xf6\x38\x00",
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"\x00\x00\x00\x00\x00\x00\x00\x00",
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];
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public function generate(): string
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{
19+
return \array_shift($this->values);
20+
}
21+
}
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23+
$r = new Randomizer(new TestEngine());
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25+
$min = -1.6e308;
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$max = 1.6e308;
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printf("%.17g\n", $min);
28+
printf("%.17g\n\n", $max);
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printf("%.17g\n", $r->getFloat($min, $max));
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printf("%.17g\n", $r->getFloat($min, $max));
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printf("%.17g\n", $r->getFloat($min, $max));
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34+
?>
35+
--EXPECT--
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-1.6e+308
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1.6e+308
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-1.5999999999999998e+308
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-1.6e+308
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1.5999999999999998e+308

‎ext/random/tests/03_randomizer/methods/getFloat_opposite_signs.phpt

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--TEST--
2+
Random: Randomizer: getFloat(): Opposite signs are handled correctly
3+
--FILE--
4+
<?php
5+
6+
use Random\Engine;
7+
use Random\Randomizer;
8+
9+
final class TestEngine implements Engine
10+
{
11+
private array $values = [
12+
"\x63\xe8\x58\x79\x3e\xf6\x38\x00",
13+
"\x64\xe8\x58\x79\x3e\xf6\x38\x00",
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"\x65\xe8\x58\x79\x3e\xf6\x38\x00",
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"\x66\xe8\x58\x79\x3e\xf6\x38\x00",
16+
"\x67\xe8\x58\x79\x3e\xf6\x38\x00",
17+
];
18+
19+
public function generate(): string
20+
{
21+
return \array_shift($this->values);
22+
}
23+
}
24+
25+
$r = new Randomizer(new TestEngine());
26+
27+
$min = -1;
28+
$max = 1;
29+
printf("%.17g\n", $min);
30+
printf("%.17g\n\n", $max);
31+
32+
$prev = null;
33+
for ($i = 0; $i < 5; $i++) {
34+
$float = $r->getFloat($min, $max);
35+
printf("%.17f", $float);
36+
if ($prev !== null) {
37+
printf(" (%+.17g)", ($float - $prev));
38+
}
39+
printf("\n");
40+
41+
$prev = $float;
42+
}
43+
?>
44+
--EXPECT--
45+
-1
46+
1
47+
48+
-0.78005908680576086
49+
-0.78005908680576097 (-1.1102230246251565e-16)
50+
-0.78005908680576108 (-1.1102230246251565e-16)
51+
-0.78005908680576119 (-1.1102230246251565e-16)
52+
-0.78005908680576130 (-1.1102230246251565e-16)

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