reuse hashmap

Cargo.toml

@@ -4,3 +4,6 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+
+[profile.release]
+debug = true

src/main.rs

@@ -8,8 +8,9 @@ fn main() {
     let mut src = Vec::new();
     io::stdin().read_to_end(&mut src).unwrap();
 
+    let mut table: HashMap<Box<[u8]>, u64> = HashMap::new();
     for n in 1..=20 {
-        let (ngrams, freqs) = ngram_freqs(n, &src);
+        let (ngrams, freqs) = ngram_freqs(n, &src, &mut table);
         let bits = entropy(freqs);
         println!(
             "{n:2}: {bits:6.3} bits, {:.3} bits/letter ({ngrams:7} unique ngrams)",
@@ -18,10 +19,14 @@ fn main() {
     }
 }
 
-fn ngram_freqs(n: usize, src: &[u8]) -> (usize, impl Iterator<Item = f64>) {
+fn ngram_freqs(
+    n: usize,
+    src: &[u8],
+    counts: &mut HashMap<Box<[u8]>, u64>,
+) -> (usize, impl Iterator<Item = f64>) {
     assert!(n > 0);
 
-    let mut counts: HashMap<Box<[u8]>, u64> = HashMap::new();
+    counts.clear();
 
     for ngram in src.windows(n) {
         *counts.entry(Box::from(ngram)).or_default() += 1;
@@ -31,9 +36,7 @@ fn ngram_freqs(n: usize, src: &[u8]) -> (usize, impl Iterator<Item = f64>) {
 
     (
         counts.len(),
-        counts
-            .into_values()
-            .map(move |p| p as f64 / total_count as f64),
+        counts.values().map(move |p| *p as f64 / total_count as f64),
     )
 }