Use choices struct field at creation time instead of using unsafe at …

…call time

Signed-off-by: Justus Fluegel <justusfluegel@gmail.com>

src/distributions/slice.rs

@@ -9,8 +9,7 @@
 use core::num::NonZeroUsize;
 
 use crate::distributions::{Distribution, Uniform};
-#[cfg(feature = "alloc")]
-use alloc::string::String;
+#[cfg(feature = "alloc")] use alloc::string::String;
 
 /// A distribution to sample items uniformly from a slice.
 ///
@@ -69,26 +68,28 @@ use alloc::string::String;
 pub struct Slice<'a, T> {
     slice: &'a [T],
     range: Uniform<usize>,
+    choices: NonZeroUsize,
 }
 
 impl<'a, T> Slice<'a, T> {
     /// Create a new `Slice` instance which samples uniformly from the slice.
     /// Returns `Err` if the slice is empty.
     pub fn new(slice: &'a [T]) -> Result<Self, EmptySlice> {
-        match slice.len() {
-            0 => Err(EmptySlice),
-            len => Ok(Self {
-                slice,
-                range: Uniform::new(0, len).unwrap(),
-            }),
-        }
+        let len = match NonZeroUsize::new(slice.len()) {
+            None => return Err(EmptySlice),
+            Some(len) => len,
+        };
+
+        Ok(Self {
+            slice,
+            range: Uniform::new(0, len).unwrap(),
+            choices: len,
+        })
     }
 
     /// Returns the count of choices in this distribution
     pub fn num_choices(&self) -> NonZeroUsize {
-        // Safety: at construction time, it was ensured that the slice was
-        // non-empty, as such the length can never be 0.
-        unsafe { NonZeroUsize::new_unchecked(self.slice.len()) }
+        self.choices
     }
 }
 
@@ -148,7 +149,11 @@ impl<'a> super::DistString for Slice<'a, char> {
 
         // Split the extension of string to reuse the unused capacities.
         // Skip the split for small length or only ascii slice.
-        let mut extend_len = if max_char_len == 1 || len < 100 { len } else { len / 4 };
+        let mut extend_len = if max_char_len == 1 || len < 100 {
+            len
+        } else {
+            len / 4
+        };
         let mut remain_len = len;
         while extend_len > 0 {
             string.reserve(max_char_len * extend_len);