Add Alphabetic distribution (#1587)

CHANGELOG.md

@@ -10,6 +10,7 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 
 ## [Unreleased]
 - Fix feature `simd_support` for recent nightly rust (#1586)
+- Add `Alphabetic` distribution. (#1587)
 
 ## [0.9.0] - 2025-01-27
 ### Security and unsafe

benches/benches/standard.rs

@@ -9,7 +9,7 @@
 use core::time::Duration;
 use criterion::measurement::WallTime;
 use criterion::{criterion_group, criterion_main, BenchmarkGroup, Criterion};
-use rand::distr::{Alphanumeric, Open01, OpenClosed01, StandardUniform};
+use rand::distr::{Alphabetic, Alphanumeric, Open01, OpenClosed01, StandardUniform};
 use rand::prelude::*;
 use rand_pcg::Pcg64Mcg;
 
@@ -52,6 +52,7 @@ pub fn bench(c: &mut Criterion) {
     do_ty!(f32, f64);
     do_ty!(char);
 
+    bench_ty::<u8, Alphabetic>(&mut g, "Alphabetic");
     bench_ty::<u8, Alphanumeric>(&mut g, "Alphanumeric");
 
     bench_ty::<f32, Open01>(&mut g, "Open01/f32");

src/distr/distribution.rs

@@ -250,7 +250,7 @@ mod tests {
     #[test]
     #[cfg(feature = "alloc")]
     fn test_dist_string() {
-        use crate::distr::{Alphanumeric, SampleString, StandardUniform};
+        use crate::distr::{Alphabetic, Alphanumeric, SampleString, StandardUniform};
         use core::str;
         let mut rng = crate::test::rng(213);
 
@@ -261,5 +261,9 @@ mod tests {
         let s2 = StandardUniform.sample_string(&mut rng, 20);
         assert_eq!(s2.chars().count(), 20);
         assert_eq!(str::from_utf8(s2.as_bytes()), Ok(s2.as_str()));
+
+        let s3 = Alphabetic.sample_string(&mut rng, 20);
+        assert_eq!(s3.len(), 20);
+        assert_eq!(str::from_utf8(s3.as_bytes()), Ok(s3.as_str()));
     }
 }

src/distr/mod.rs

@@ -46,6 +46,9 @@
 //! numbers of the `char` type; in contrast [`StandardUniform`] may sample any valid
 //! `char`.
 //!
+//! There's also an [`Alphabetic`] distribution which acts similarly to [`Alphanumeric`] but
+//! doesn't include digits.
+//!
 //! For floats (`f32`, `f64`), [`StandardUniform`] samples from `[0, 1)`. Also
 //! provided are [`Open01`] (samples from `(0, 1)`) and [`OpenClosed01`]
 //! (samples from `(0, 1]`). No option is provided to sample from `[0, 1]`; it
@@ -104,7 +107,7 @@ pub use self::bernoulli::{Bernoulli, BernoulliError};
 pub use self::distribution::SampleString;
 pub use self::distribution::{Distribution, Iter, Map};
 pub use self::float::{Open01, OpenClosed01};
-pub use self::other::Alphanumeric;
+pub use self::other::{Alphabetic, Alphanumeric};
 #[doc(inline)]
 pub use self::uniform::Uniform;
 
@@ -126,7 +129,8 @@ use crate::Rng;
 ///   code points in the range `0...0x10_FFFF`, except for the range
 ///   `0xD800...0xDFFF` (the surrogate code points). This includes
 ///   unassigned/reserved code points.
-///   For some uses, the [`Alphanumeric`] distribution will be more appropriate.
+///   For some uses, the [`Alphanumeric`] or [`Alphabetic`] distribution will be more
+///   appropriate.
 /// * `bool` samples `false` or `true`, each with probability 0.5.
 /// * Floating point types (`f32` and `f64`) are uniformly distributed in the
 ///   half-open range `[0, 1)`. See also the [notes below](#floating-point-implementation).

src/distr/other.rs

@@ -70,6 +70,35 @@ use serde::{Deserialize, Serialize};
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct Alphanumeric;
 
+/// Sample a [`u8`], uniformly distributed over letters:
+/// a-z and A-Z.
+///
+/// # Example
+///
+/// You're able to generate random Alphabetic characters via mapping or via the
+/// [`SampleString::sample_string`] method like so:
+///
+/// ```
+/// use rand::Rng;
+/// use rand::distr::{Alphabetic, SampleString};
+///
+/// // Manual mapping
+/// let mut rng = rand::rng();
+/// let chars: String = (0..7).map(|_| rng.sample(Alphabetic) as char).collect();
+/// println!("Random chars: {}", chars);
+///
+/// // Using [`SampleString::sample_string`]
+/// let string = Alphabetic.sample_string(&mut rand::rng(), 16);
+/// println!("Random string: {}", string);
+/// ```
+///
+/// # Passwords
+///
+/// Refer to [`Alphanumeric#Passwords`].
+#[derive(Debug, Clone, Copy, Default)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct Alphabetic;
+
 // ----- Implementations of distributions -----
 
 impl Distribution<char> for StandardUniform {
@@ -123,6 +152,17 @@ impl Distribution<u8> for Alphanumeric {
     }
 }
 
+impl Distribution<u8> for Alphabetic {
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> u8 {
+        const RANGE: u8 = 26 + 26;
+
+        let offset = rng.random_range(0..RANGE) + b'A';
+
+        // Account for upper-cases
+        offset + (offset > b'Z') as u8 * (b'a' - b'Z' - 1)
+    }
+}
+
 #[cfg(feature = "alloc")]
 impl SampleString for Alphanumeric {
     fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, string: &mut String, len: usize) {
@@ -133,6 +173,20 @@ impl SampleString for Alphanumeric {
     }
 }
 
+#[cfg(feature = "alloc")]
+impl SampleString for Alphabetic {
+    fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, string: &mut String, len: usize) {
+        // SAFETY: With this distribution we guarantee that we're working with valid ASCII
+        // characters.
+        // See [#1590](https://github.com/rust-random/rand/issues/1590).
+        unsafe {
+            let v = string.as_mut_vec();
+            v.reserve_exact(len);
+            v.extend(self.sample_iter(rng).take(len));
+        }
+    }
+}
+
 impl Distribution<bool> for StandardUniform {
     #[inline]
     fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> bool {
@@ -294,6 +348,20 @@ mod tests {
         assert!(!incorrect);
     }
 
+    #[test]
+    fn test_alphabetic() {
+        let mut rng = crate::test::rng(806);
+
+        // Test by generating a relatively large number of chars, so we also
+        // take the rejection sampling path.
+        let mut incorrect = false;
+        for _ in 0..100 {
+            let c: char = rng.sample(Alphabetic).into();
+            incorrect |= !c.is_ascii_alphabetic();
+        }
+        assert!(!incorrect);
+    }
+
     #[test]
     fn value_stability() {
         fn test_samples<T: Copy + core::fmt::Debug + PartialEq, D: Distribution<T>>(
@@ -321,6 +389,7 @@ mod tests {
             ],
         );
         test_samples(&Alphanumeric, 0, &[104, 109, 101, 51, 77]);
+        test_samples(&Alphabetic, 0, &[97, 102, 89, 116, 75]);
         test_samples(&StandardUniform, false, &[true, true, false, true, false]);
         test_samples(
             &StandardUniform,