Fix case with kana surface and ruby that aren't the same length.

It's a weird case that didn't occur to me that it could even happen, but
I guess there are dictionary entries like that.

src/main.rs

@@ -65,7 +65,7 @@ fn add_html_furigana(text: &str, tokenizer: &Tokenizer) -> String {
         let surface = t.surface();
         let kana = t.feature().split(",").nth(1).unwrap();
 
-        let (start_bytes, end_bytes) = matching_ends(surface, kana);
+        let (start_bytes, end_bytes) = matching_kana_ends(surface, kana);
 
         if kana.is_empty()
             || start_bytes == surface.len()
@@ -94,7 +94,12 @@ fn add_html_furigana(text: &str, tokenizer: &Tokenizer) -> String {
 }
 
 /// Returns (matching_start_bytes, matching_end_bytes).
-fn matching_ends(a: &str, b: &str) -> (usize, usize) {
+///
+/// Note that the bytes are in terms of `a`'s bytes.
+///
+/// If `matching_start_bytes == a.len()` you can assume that strings are kana
+/// equivalents, and thus no ruby is needed.
+fn matching_kana_ends(a: &str, b: &str) -> (usize, usize) {
     let mut start_bytes = 0;
     for (ca, cb) in a.chars().zip(b.chars()) {
         if ca == cb || is_equivalent_kana(ca, cb) {
@@ -113,8 +118,8 @@ fn matching_ends(a: &str, b: &str) -> (usize, usize) {
         }
     }
 
-    if start_bytes == a.len() {
-        (start_bytes, 0)
+    if (start_bytes + end_bytes) >= a.len() || (start_bytes + end_bytes) >= b.len() {
+        (a.len(), 0)
     } else {
         (start_bytes, end_bytes)
     }
@@ -184,3 +189,28 @@ pub fn katakana_to_hiragana(c: char) -> Option<char> {
         None
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn matching_kana_ends_01() {
+        let surface = "へぇ";
+        let kana = "ヘー";
+        let (start_bytes, end_bytes) = matching_kana_ends(surface, kana);
+
+        assert_eq!(6, start_bytes);
+        assert_eq!(0, end_bytes);
+    }
+
+    #[test]
+    fn matching_kana_ends_02() {
+        let surface = "へぇー";
+        let kana = "ヘー";
+        let (start_bytes, end_bytes) = matching_kana_ends(surface, kana);
+
+        assert_eq!(9, start_bytes);
+        assert_eq!(0, end_bytes);
+    }
+}