| author |  Erik Arvstedt <erik.arvstedt@gmail.com> 2022-05-12 14:13:20+02:00 |
| committer | Erik Arvstedt <erik.arvstedt@gmail.com> 2022-05-12 15:39:19+02:00 |
| log | 1d5ea10bee39c6378c5d41597ed477075c3c667f |
| tree | 92fa79b1cbaf0eb4673f7c9f69b848ebb733aa41 |
| parent | 7bedeb9659af84daab33a4804c54b4c8811c0c5d |
std.rand: fixup 'improve random float generation'
- Test: Fix bucket counting. Previously, the first hit was not counted.
This off-by-one error slightly increased the mean of `*_total_variance`,
which decreased the acceptance rate for a particular random seed
from 95% to 92.6%. (Irrelevant for test failure because the seed is fixed.)
- Improve comments2 files changed, 8 insertions(+), 6 deletions(-)
lib/std/rand.zig+5-3
| ... | @@ -247,10 +247,9 @@ pub const Random = struct { | ... | @@ -247,10 +247,9 @@ pub const Random = struct { |
| 247 | 247 | ||
| 248 | /// Return a floating point value evenly distributed in the range [0, 1). | 248 | /// Return a floating point value evenly distributed in the range [0, 1). |
| 249 | pub fn float(r: Random, comptime T: type) T { | 249 | pub fn float(r: Random, comptime T: type) T { |
| 250 | // Generate a uniformly random value between for the mantissa. | 250 | // Generate a uniformly random value for the mantissa. |
| 251 | // Then generate an exponentially biased random value for the exponent. | 251 | // Then generate an exponentially biased random value for the exponent. |
| 252 | // Over the previous method, this has the advantage of being able to | 252 | // This covers every possible value in the range. |
| 253 | // represent every possible value in the available range. | ||
| 254 | switch (T) { | 253 | switch (T) { |
| 255 | f32 => { | 254 | f32 => { |
| 256 | // Use 23 random bits for the mantissa, and the rest for the exponent. | 255 | // Use 23 random bits for the mantissa, and the rest for the exponent. |
| ... | @@ -259,6 +258,9 @@ pub const Random = struct { | ... | @@ -259,6 +258,9 @@ pub const Random = struct { |
| 259 | const rand = r.int(u64); | 258 | const rand = r.int(u64); |
| 260 | var rand_lz = @clz(u64, rand | 0x7FFFFF); | 259 | var rand_lz = @clz(u64, rand | 0x7FFFFF); |
| 261 | if (rand_lz == 41) { | 260 | if (rand_lz == 41) { |
| 261 | // TODO: when #5177 or #489 is implemented, | ||
| 262 | // tell the compiler it is unlikely (1/2^41) to reach this point. | ||
| 263 | // (Same for the if branch and the f64 calculations below.) | ||
| 262 | rand_lz += @clz(u64, r.int(u64)); | 264 | rand_lz += @clz(u64, r.int(u64)); |
| 263 | if (rand_lz == 41 + 64) { | 265 | if (rand_lz == 41 + 64) { |
| 264 | // It is astronomically unlikely to reach this point. | 266 | // It is astronomically unlikely to reach this point. |
lib/std/rand/test.zig+3-3
| ... | @@ -336,13 +336,13 @@ test "Random float chi-square goodness of fit" { | ... | @@ -336,13 +336,13 @@ test "Random float chi-square goodness of fit" { |
| 336 | if (f32_put.found_existing) { | 336 | if (f32_put.found_existing) { |
| 337 | f32_put.value_ptr.* += 1; | 337 | f32_put.value_ptr.* += 1; |
| 338 | } else { | 338 | } else { |
| 339 | f32_put.value_ptr.* = 0; | 339 | f32_put.value_ptr.* = 1; |
| 340 | } | 340 | } |
| 341 | var f64_put = try f64_hist.getOrPut(@floatToInt(u32, rand_f64 * @intToFloat(f64, num_buckets))); | 341 | var f64_put = try f64_hist.getOrPut(@floatToInt(u32, rand_f64 * @intToFloat(f64, num_buckets))); |
| 342 | if (f64_put.found_existing) { | 342 | if (f64_put.found_existing) { |
| 343 | f64_put.value_ptr.* += 1; | 343 | f64_put.value_ptr.* += 1; |
| 344 | } else { | 344 | } else { |
| 345 | f64_put.value_ptr.* = 0; | 345 | f64_put.value_ptr.* = 1; |
| 346 | } | 346 | } |
| 347 | } | 347 | } |
| 348 | 348 | ||
| ... | @@ -371,7 +371,7 @@ test "Random float chi-square goodness of fit" { | ... | @@ -371,7 +371,7 @@ test "Random float chi-square goodness of fit" { |
| 371 | } | 371 | } |
| 372 | } | 372 | } |
| 373 | 373 | ||
| 374 | // Corresponds to a p-value > 0.05. | 374 | // Accept p-values >= 0.05. |
| 375 | // Critical value is calculated by opening a Python interpreter and running: | 375 | // Critical value is calculated by opening a Python interpreter and running: |
| 376 | // scipy.stats.chi2.isf(0.05, num_buckets - 1) | 376 | // scipy.stats.chi2.isf(0.05, num_buckets - 1) |
| 377 | const critical_value = 1073.6426506574246; | 377 | const critical_value = 1073.6426506574246; |