How to create a TinyVec of a specific size (not capacity!), initialized to zero (or user-defined value) #210

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1. dEajL3kA

   changed the title [-]Create a TinyVec of a specific size (not capacity!), initialized to zero (or user-defined value)[/-] [+]How to create a TinyVec of a specific size (not capacity!), initialized to zero (or user-defined value)[/+] on Dec 5, 2025
2. lordmulder commented on Dec 17, 2025

   

   lordmulder

   on Dec 17, 2025

   Last edited by lordmulder

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   I did some performance tests:

   1. #[inline(always)]
      pub fn mk_tinyvec_1<const N: usize>(length: usize) -> TinyVec<[u8; N]> {
          TinyVec::from_iter(iter::repeat_n(0u8, length))
      }

   2. #[inline(always)]
      pub fn mk_tinyvec_2<const N: usize>(length: usize) -> TinyVec<[u8; N]> {
          let mut vec = TinyVec::with_capacity(length);
          vec.resize(length, 0u8);
          vec
      }

   3. #[inline(always)]
      pub fn mk_tinyvec_3<const N: usize>(length: usize) -> TinyVec<[u8; N]> {
          if length > N {
              let mut vec = TinyVec::with_capacity(length);
              vec.resize(length, 0u8);
              vec
          } else {
              TinyVec::from_array_len([0u8; N], length)
          }
      }

   4. #[inline(always)]
      pub fn mk_tinyvec_4<const N: usize, T: Copy + Default>(length: usize) -> TinyVec<[T; N]> {
          if length <= N {
              TinyVec::Inline(ArrayVec::from_array_len([T::default(); N], length))
          } else {
              TinyVec::Heap(vec![T::default(); length])
          }
      }

   Test bench:

   const LOOPS: usize = 999999999usize;
   type TinyVecType = TinyVec<[u8; 256usize]>;
   
   macro_rules! do_test_helper {
       ($func:ident, $n:literal) => {{
           {
               let start_time = Instant::now();
               for _ in 0usize..LOOPS {
                   let instance: TinyVecType = $func($n);
                   assert_eq!(instance.len(), $n);
                   assert_eq!(instance[0usize], 0u8);
                   assert_eq!(instance[$n - 1usize], 0u8);
               }
               let elapsed = start_time.elapsed();
               println!("{} - {} - {:.3}", stringify!($func), $n, elapsed.as_secs_f64());
           }
       }};
   }
   
   macro_rules! do_test {
       ($func:ident) => {{
           do_test_helper!($func, 128usize);
           do_test_helper!($func, 256usize);
           do_test_helper!($func, 512usize);
           println!();
       }};
   }
   
   fn main() {
       for i in 0..2 {
           if i > 0 {
               println!("----");
               println!();
           }
           do_test!(mk_tinyvec_1);
           do_test!(mk_tinyvec_2);
           do_test!(mk_tinyvec_3);
           do_test!(mk_tinyvec_4);
       }
   }

   Results:

   mk_tinyvec_1 - 128 - 25.646
   mk_tinyvec_1 - 256 - 36.296
   mk_tinyvec_1 - 512 - 59.758
   
   mk_tinyvec_2 - 128 - 36.814
   mk_tinyvec_2 - 256 - 26.966
   mk_tinyvec_2 - 512 - 39.912
   
   mk_tinyvec_3 - 128 - 0.000
   mk_tinyvec_3 - 256 - 0.000
   mk_tinyvec_3 - 512 - 40.039
   
   mk_tinyvec_4 - 128 - 0.000
   mk_tinyvec_4 - 256 - 0.000
   mk_tinyvec_4 - 512 - 5.359
   

   Version #4 wins easily 🥳

   👍React with 👍2Reacted by Eric Rothstein and Daniel Trick
3. Lokathor commented on Aug 19, 2026

   

   Lokathor

   on Aug 19, 2026

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   Based on the benchmarks, looks like a solid answer and we can close this.
4. Lokathor

   closed this as completedon Aug 19, 2026
5. lordmulder commented on Aug 19, 2026

   

   lordmulder

   on Aug 19, 2026

   Last edited by lordmulder

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   Too bad nobody will find this information.

   Wouldn't it make sense to have a built-in function that creates a TinyVec of specific size? (pre-filled with the given value)
6. Lokathor commented on Aug 19, 2026

   

   Lokathor

   on Aug 19, 2026

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   Yeah make a PR. You'll probably have to feature gate it behind latest_stable_rust to pass CI.
7. lordmulder commented on Aug 20, 2026

   

   lordmulder

   on Aug 20, 2026

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   Here is a first suggestion:
   #224