let you specify min and max ngram length

src/main.rs

@@ -1,15 +1,45 @@
 #![allow(clippy::cast_precision_loss)]
 use std::{
     collections::HashMap,
+    env,
     io::{self, Read},
+    process,
 };
 
+const USAGE: &str = "\
+usage: (ngram-entropy) | (ngram-entropy N) | (ngram-entropy MIN MAX)\
+";
+
 fn main() {
+    let arg1 = env::args().nth(1);
+    let arg2 = env::args().nth(2);
+
+    let (min, max) = match (arg1, arg2) {
+        (Some(n), None) => {
+            if let Ok(n) = n.parse::<usize>() {
+                (n, n)
+            } else {
+                eprintln!("{USAGE}");
+                process::exit(1);
+            }
+        }
+        (Some(min), Some(max)) => {
+            if let (Ok(min), Ok(max)) = (min.parse::<usize>(), max.parse::<usize>()) {
+                (min, max)
+            } else {
+                eprintln!("{USAGE}");
+                process::exit(1);
+            }
+        }
+        _ => (1, 20),
+    };
+
     let mut src = Vec::new();
     io::stdin().read_to_end(&mut src).unwrap();
 
     let mut table: HashMap<&[u8], u64> = HashMap::new();
-    for n in 1..=20 {
+
+    for n in min..=max {
         let (ngrams, freqs) = ngram_freqs(n, &src, &mut table);
         let bits = entropy(freqs);
         println!(