2021 day8/rust: use ArrayVec

This avoids an allocation and, more importantly, turns `digits` into a
fixed size array, allowing the final calculations to be unrolled
completely.

2021/day8/day8_rs/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+arrayvec = "0.7.2"
 
 [dev-dependencies]
 criterion = "0.3.5"

2021/day8/day8_rs/src/v2.rs

@@ -1,4 +1,5 @@
 use crate::LineResult;
+use arrayvec::ArrayVec;
 
 const FREQ_TABLE: [usize; 50] = {
     let mut tab = [0; 50];
@@ -28,14 +29,16 @@ pub fn unscramble(line: &str) -> LineResult {
         }
     }
 
-    let digits: Vec<_> = parts
+    let digits = parts
         .next()
         .unwrap()
         .trim()
         .split(' ')
         .map(|s| s.bytes().map(|c| counts[c as usize - b'a' as usize]).sum())
         .map(|n: usize| FREQ_TABLE[n])
-        .collect();
+        .collect::<ArrayVec<_, 4>>()
+        .into_inner()
+        .unwrap();
 
     LineResult {
         unique_digits: digits.iter().filter(|d| [1, 4, 7, 8].contains(d)).count(),