don't allocate ngrams on the heap (lol)

src/main.rs

@@ -8,7 +8,7 @@ fn main() {
     let mut src = Vec::new();
     io::stdin().read_to_end(&mut src).unwrap();
 
-    let mut table: HashMap<Box<[u8]>, u64> = HashMap::new();
+    let mut table: HashMap<&[u8], u64> = HashMap::new();
     for n in 1..=20 {
         let (ngrams, freqs) = ngram_freqs(n, &src, &mut table);
         let bits = entropy(freqs);
@@ -19,17 +19,17 @@ fn main() {
     }
 }
 
-fn ngram_freqs(
+fn ngram_freqs<'a>(
     n: usize,
-    src: &[u8],
+    src: &'a [u8],
-    counts: &mut HashMap<Box<[u8]>, u64>,
+    counts: &mut HashMap<&'a [u8], u64>,
 ) -> (usize, impl Iterator<Item = f64>) {
     assert!(n > 0);
 
     counts.clear();
 
     for ngram in src.windows(n) {
-        *counts.entry(Box::from(ngram)).or_default() += 1;
+        *counts.entry(ngram).or_default() += 1;
     }
 
     let total_count: u64 = counts.values().sum();