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Merge pull request #11641 from erikarvstedt/fixup_rand_float_improvement

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Minor fixes for random float generation
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Andrew Kelley 2022-05-13 16:50:18 -04:00 committed by GitHub
commit f32928c50d
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2 changed files with 14 additions and 11 deletions
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19
lib/std/rand.zig
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@ -247,19 +247,21 @@ pub const Random = struct {
/// Return a floating point value evenly distributed in the range [0, 1).
pub fn float(r: Random, comptime T: type) T {
// Generate a uniformly random value between for the mantissa.
// Generate a uniformly random value for the mantissa.
// Then generate an exponentially biased random value for the exponent.
// Over the previous method, this has the advantage of being able to
// represent every possible value in the available range.
// This covers every possible value in the range.
switch (T) {
f32 => {
// Use 23 random bits for the mantissa, and the rest for the exponent.
// If all 41 bits are zero, generate additional random bits, until a
// set bit is found, or 126 bits have been generated.
const rand = r.int(u64);
var rand_lz = @clz(u64, rand | 0x7FFFFF);
if (rand_lz == 41) {
rand_lz += @clz(u64, r.int(u64));
var rand_lz = @clz(u64, rand);
if (rand_lz >= 41) {
// TODO: when #5177 or #489 is implemented,
// tell the compiler it is unlikely (1/2^41) to reach this point.
// (Same for the if branch and the f64 calculations below.)
rand_lz = 41 + @clz(u64, r.int(u64));
if (rand_lz == 41 + 64) {
// It is astronomically unlikely to reach this point.
rand_lz += @clz(u32, r.int(u32) | 0x7FF);
@ -274,8 +276,9 @@ pub const Random = struct {
// If all 12 bits are zero, generate additional random bits, until a
// set bit is found, or 1022 bits have been generated.
const rand = r.int(u64);
var rand_lz: u64 = @clz(u64, rand | 0xFFFFFFFFFFFFF);
if (rand_lz == 12) {
var rand_lz: u64 = @clz(u64, rand);
if (rand_lz >= 12) {
rand_lz = 12;
while (true) {
// It is astronomically unlikely for this loop to execute more than once.
const addl_rand_lz = @clz(u64, r.int(u64));

6
lib/std/rand/test.zig
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@ -336,13 +336,13 @@ test "Random float chi-square goodness of fit" {
if (f32_put.found_existing) {
f32_put.value_ptr.* += 1;
} else {
f32_put.value_ptr.* = 0;
f32_put.value_ptr.* = 1;
}
var f64_put = try f64_hist.getOrPut(@floatToInt(u32, rand_f64 * @intToFloat(f64, num_buckets)));
if (f64_put.found_existing) {
f64_put.value_ptr.* += 1;
} else {
f64_put.value_ptr.* = 0;
f64_put.value_ptr.* = 1;
}
}
@ -371,7 +371,7 @@ test "Random float chi-square goodness of fit" {
}
}
// Corresponds to a p-value > 0.05.
// Accept p-values >= 0.05.
// Critical value is calculated by opening a Python interpreter and running:
// scipy.stats.chi2.isf(0.05, num_buckets - 1)
const critical_value = 1073.6426506574246;