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switch sample_single int to O'neill's modulo method & update gen_rang… #1154

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switch sample_single int to O'neill's modulo method & update gen_rang… #1154

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e783cb0
switch sample_single int to O'neill's modulo method & update gen_rang…
TheIronBorn Jul 20, 2021
c5f0150
update stability test values
TheIronBorn Jul 28, 2021
a69d6cc
remove reference to old benchmarks
TheIronBorn Aug 6, 2021
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15 changes: 8 additions & 7 deletions benches/distributions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -217,13 +217,14 @@ gen_range_int!(gen_range_i32_low, i32, -1i32, 0);
gen_range_int!(gen_range_i64_low, i64, -1i64, 0);
gen_range_int!(gen_range_i128_low, i128, -1i128, 0);

// These were the initially tested ranges. They are likely to see fewer
// rejections than the low tests.
gen_range_int!(gen_range_i8_high, i8, -20i8, 100);
gen_range_int!(gen_range_i16_high, i16, -500i16, 2000);
gen_range_int!(gen_range_i32_high, i32, -200_000_000i32, 800_000_000);
gen_range_int!(gen_range_i64_high, i64, 3i64, 123_456_789_123);
gen_range_int!(gen_range_i128_high, i128, -12345678901234i128, 123_456_789_123_456_789);
// These are likely to see fewer rejections than the low tests. The starting
// range here is `2^(N - 1) + 1`, the region with the highest rejection chance for
// modulo/bitmask/leading_zeros methods.
gen_range_int!(gen_range_i8_high, i8, i8::min_value(), 1);
gen_range_int!(gen_range_i16_high, i16, i16::min_value(), 1);
gen_range_int!(gen_range_i32_high, i32, i32::min_value(), 1);
gen_range_int!(gen_range_i64_high, i64, i64::min_value(), 1);
gen_range_int!(gen_range_i128_high, i128, i128::min_value(), 1);

// construct and sample from a floating-point range
macro_rules! gen_range_float {
Expand Down
38 changes: 20 additions & 18 deletions src/distributions/uniform.rs
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Expand Up @@ -529,26 +529,28 @@ macro_rules! uniform_int_impl {
return rng.gen();
}

let zone = if ::core::$unsigned::MAX <= ::core::u16::MAX as $unsigned {
// Using a modulus is faster than the approximation for
// i8 and i16. I suppose we trade the cost of one
// modulus for near-perfect branch prediction.
let unsigned_max: $u_large = ::core::$u_large::MAX;
let ints_to_reject = (unsigned_max - range + 1) % range;
unsigned_max - ints_to_reject
} else {
// conservative but fast approximation. `- 1` is necessary to allow the
// same comparison without bias.
(range << range.leading_zeros()).wrapping_sub(1)
};

loop {
let v: $u_large = rng.gen();
let (hi, lo) = v.wmul(range);
if lo <= zone {
return low.wrapping_add(hi as $ty);
// we use the "Debiased Int Mult (t-opt, m-opt)" rejection sampling method
// described here https://www.pcg-random.org/posts/bounded-rands.html
// and here https://github.com/imneme/bounded-rands

let (mut hi, mut lo) = rng.gen::<$u_large>().wmul(range);
// this shortcut works best with small ranges
if lo < range {
let mut threshold = range.wrapping_neg();
// this shortcut works best with large ranges
if threshold >= range {
threshold -= range;
if threshold >= range {
threshold %= range;
}
}
while lo < threshold {
let (new_hi, new_lo) = rng.gen::<$u_large>().wmul(range);
hi = new_hi;
lo = new_lo;
}
}
low.wrapping_add(hi as $ty)
}
}
};
Expand Down
10 changes: 5 additions & 5 deletions src/seq/index.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -662,11 +662,11 @@ mod test {
);
};

do_test(10, 6, &[8, 0, 3, 5, 9, 6]); // floyd
do_test(25, 10, &[18, 15, 14, 9, 0, 13, 5, 24]); // floyd
do_test(300, 8, &[30, 283, 150, 1, 73, 13, 285, 35]); // floyd
do_test(300, 80, &[31, 289, 248, 154, 5, 78, 19, 286]); // inplace
do_test(300, 180, &[31, 289, 248, 154, 5, 78, 19, 286]); // inplace
do_test(10, 6, &[0, 9, 8, 6, 5, 4]); // floyd
do_test(25, 10, &[24, 1, 20, 16, 19, 22, 14, 9]); // floyd
do_test(300, 8, &[30, 283, 243, 150, 218, 240, 1, 189]); // floyd
do_test(300, 80, &[31, 289, 248, 154, 221, 243, 7, 192]); // inplace
do_test(300, 180, &[31, 289, 248, 154, 221, 243, 7, 192]); // inplace

do_test(1_000_000, 8, &[
103717, 963485, 826422, 509101, 736394, 807035, 5327, 632573,
Expand Down
40 changes: 20 additions & 20 deletions src/seq/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -710,29 +710,29 @@ mod test {
let mut nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

assert_eq!(chars.choose(&mut r), Some(&'l'));
assert_eq!(nums.choose_mut(&mut r), Some(&mut 10));
assert_eq!(nums.choose_mut(&mut r), Some(&mut 3));

#[cfg(feature = "alloc")]
assert_eq!(
&chars
.choose_multiple(&mut r, 8)
.cloned()
.collect::<Vec<char>>(),
&['d', 'm', 'b', 'n', 'c', 'k', 'h', 'e']
&['f', 'i', 'd', 'b', 'c', 'm', 'j', 'k']
);

#[cfg(feature = "alloc")]
assert_eq!(chars.choose_weighted(&mut r, |_| 1), Ok(&'f'));
assert_eq!(chars.choose_weighted(&mut r, |_| 1), Ok(&'l'));
#[cfg(feature = "alloc")]
assert_eq!(nums.choose_weighted_mut(&mut r, |_| 1), Ok(&mut 5));
assert_eq!(nums.choose_weighted_mut(&mut r, |_| 1), Ok(&mut 8));

let mut r = crate::test::rng(414);
nums.shuffle(&mut r);
assert_eq!(nums, [9, 5, 3, 10, 7, 12, 8, 11, 6, 4, 0, 2, 1]);
assert_eq!(nums, [10, 11, 8, 7, 4, 6, 12, 5, 3, 0, 9, 2, 1]);
nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
let res = nums.partial_shuffle(&mut r, 6);
assert_eq!(res.0, &mut [7, 4, 8, 6, 9, 3]);
assert_eq!(res.1, &mut [0, 1, 2, 12, 11, 5, 10]);
assert_eq!(res.0, &mut [6, 10, 7, 2, 0, 8]);
assert_eq!(res.1, &mut [11, 1, 12, 3, 4, 5, 9]);
}

#[derive(Clone)]
Expand Down Expand Up @@ -1122,15 +1122,15 @@ mod test {

assert_eq!(choose([].iter().cloned()), None);
assert_eq!(choose(0..100), Some(33));
assert_eq!(choose(UnhintedIterator { iter: 0..100 }), Some(40));
assert_eq!(choose(UnhintedIterator { iter: 0..100 }), Some(54));
assert_eq!(
choose(ChunkHintedIterator {
iter: 0..100,
chunk_size: 32,
chunk_remaining: 32,
hint_total_size: false,
}),
Some(39)
Some(74)
);
assert_eq!(
choose(ChunkHintedIterator {
Expand All @@ -1139,23 +1139,23 @@ mod test {
chunk_remaining: 32,
hint_total_size: true,
}),
Some(39)
Some(74)
);
assert_eq!(
choose(WindowHintedIterator {
iter: 0..100,
window_size: 32,
hint_total_size: false,
}),
Some(90)
Some(34)
);
assert_eq!(
choose(WindowHintedIterator {
iter: 0..100,
window_size: 32,
hint_total_size: true,
}),
Some(90)
Some(34)
);
}

Expand All @@ -1167,16 +1167,16 @@ mod test {
}

assert_eq!(choose([].iter().cloned()), None);
assert_eq!(choose(0..100), Some(40));
assert_eq!(choose(UnhintedIterator { iter: 0..100 }), Some(40));
assert_eq!(choose(0..100), Some(54));
assert_eq!(choose(UnhintedIterator { iter: 0..100 }), Some(54));
assert_eq!(
choose(ChunkHintedIterator {
iter: 0..100,
chunk_size: 32,
chunk_remaining: 32,
hint_total_size: false,
}),
Some(40)
Some(54)
);
assert_eq!(
choose(ChunkHintedIterator {
Expand All @@ -1185,23 +1185,23 @@ mod test {
chunk_remaining: 32,
hint_total_size: true,
}),
Some(40)
Some(54)
);
assert_eq!(
choose(WindowHintedIterator {
iter: 0..100,
window_size: 32,
hint_total_size: false,
}),
Some(40)
Some(54)
);
assert_eq!(
choose(WindowHintedIterator {
iter: 0..100,
window_size: 32,
hint_total_size: true,
}),
Some(40)
Some(54)
);
}

Expand All @@ -1216,7 +1216,7 @@ mod test {

do_test(0..4, &[0, 1, 2, 3]);
do_test(0..8, &[0, 1, 2, 3, 4, 5, 6, 7]);
do_test(0..100, &[58, 78, 80, 92, 43, 8, 96, 7]);
do_test(0..100, &[77, 95, 38, 23, 25, 8, 58, 40]);

#[cfg(feature = "alloc")]
{
Expand All @@ -1227,7 +1227,7 @@ mod test {

do_test(0..4, &[0, 1, 2, 3]);
do_test(0..8, &[0, 1, 2, 3, 4, 5, 6, 7]);
do_test(0..100, &[58, 78, 80, 92, 43, 8, 96, 7]);
do_test(0..100, &[77, 95, 38, 23, 25, 8, 58, 40]);
}
}

Expand Down