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Avoid surrogates when generating char using Standard distribution #519

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merged 2 commits into from Jun 21, 2018
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Avoid surrogates when generating char using Standard distribution #519

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Avoid surrogates when generating `char` using Standard distribution
Pazzaz Jun 20, 2018
02a7a11
Mention reason for interval choise
Pazzaz Jun 20, 2018
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22 changes: 14 additions & 8 deletions src/distributions/other.rs
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Expand Up @@ -44,15 +44,21 @@ pub struct Alphanumeric;
impl Distribution<char> for Standard {
#[inline]
fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> char {
let range = Uniform::new(0u32, 0x11_0000);
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Could we make this new(0u32, char::MAX as u32)? More explicit about what we're doing

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This line is being removed. But I don't think this is a good idea; you're off by 1 (should be inclusive) and if char::MAX were ever to change we don't know now whether we should use the whole range (minus the existing gap). So better just to use local constants as in the current implementation.

loop {
match char::from_u32(range.sample(rng)) {
Some(c) => return c,
// About 0.2% of numbers in the range 0..0x110000 are invalid
// codepoints (surrogates).
None => {}
}
// A valid `char` is either in the interval `[0, 0xD800)` or
// `(0xDFFF, 0x11_0000)`. All `char`s must therefore be in
// `[0, 0x11_0000)` but not in the "gap" `[0xD800, 0xDFFF]` which is
// reserved for surrogates. This is the size of that gap.
const GAP_SIZE: u32 = 0xDFFF - 0xD800 + 1;

// Uniform::new(0, 0x11_0000 - GAP_SIZE) can also be used but it
// seemed slower.
let range = Uniform::new(GAP_SIZE, 0x11_0000);

let mut n = range.sample(rng);
if n <= 0xDFFF {
n -= GAP_SIZE;
}
unsafe { char::from_u32_unchecked(n) }
}
}

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