add single digit parser

src/parsing.rs

@@ -9,18 +9,43 @@ pub fn newline(tokens: &mut impl Tokens<Item = char>) -> bool {
 }
 
 pub fn parse_number<T: Num>(tokens: &mut impl Tokens<Item = char>) -> Option<T> {
-    parse_number_with_radix(tokens, 10)
+    parse_number_with_radix(tokens, 10, None)
+}
+
+pub fn parse_digit<T: Num>(tokens: &mut impl Tokens<Item = char>) -> Option<T> {
+    parse_number_with_radix(tokens, 10, Some(1))
 }
 
 pub fn parse_number_with_radix<T: Num>(
     tokens: &mut impl Tokens<Item = char>,
     base: u32,
+    limit: Option<usize>,
 ) -> Option<T> {
     tokens.skip_tokens_while(|t| *t == ' ');
-    let digits: String = tokens.tokens_while(|c| c.is_digit(base)).collect();
+    let mut remaining = limit.map(|x| x as isize).unwrap_or(isize::MAX);
+    let digits: String = tokens
+        .tokens_while(|c| {
+            remaining -= 1;
+            c.is_digit(base) && remaining >= 0
+        })
+        .collect();
     if !digits.is_empty() {
         T::from_str_radix(&digits, base).ok()
     } else {
         None
     }
 }
+
+#[cfg(test)]
+mod test {
+    use yap::IntoTokens;
+
+    #[test]
+    fn parse_digit() {
+        let mut tokens = "1234".into_tokens();
+        assert_eq!(Some(1), super::parse_digit(&mut tokens));
+        assert_eq!(Some(2), super::parse_digit(&mut tokens));
+        assert_eq!(Some(3), super::parse_digit(&mut tokens));
+        assert_eq!(Some(4), super::parse_digit(&mut tokens));
+    }
+}