initial slow version

src/main.rs

@@ -1,3 +1,4 @@
+#![allow(clippy::cast_precision_loss)]
 use std::{
     collections::HashMap,
     io::{self, Read},
@@ -7,9 +8,10 @@ fn main() {
     let mut src = Vec::new();
     io::stdin().read_to_end(&mut src).unwrap();
 
-    for n in 1..2 {
+    for n in 1..=20 {
         let freqs = ngram_freqs(n, &src);
-        println!("{}", freqs.values().sum::<f64>());
+        let bits = entropy(freqs.values().copied());
+        println!("{n:2}: {bits:6.3} bits, {:.3} bits/letter", bits / n as f64);
     }
 }
 
@@ -29,3 +31,7 @@ fn ngram_freqs(n: usize, src: &[u8]) -> HashMap<Box<[u8]>, f64> {
         .map(|(k, v)| (k, v as f64 / total_count as f64))
         .collect()
 }
+
+fn entropy(probs: impl Iterator<Item = f64>) -> f64 {
+    -probs.map(|prob| prob * prob.log2()).sum::<f64>()
+}