Fix and add tests for negative slices #52

src/mango/util/strslice/slice.rs

@@ -5,25 +5,28 @@ pub fn charslice<S: Into<String>>(text: S, start: isize, end: isize) -> String {
     let stext = text.into();
     let from: usize;
     let length: usize;
+    let charcount = stext.chars().count();
     if start < 0 {
         // LATER: may remove this check and just default to 0 in the future.
         assert!(
-            -start as usize <= stext.len(),
+            -start as usize <= charcount,
             "charslice: if 'start' is negative, the magnitude may not exceed the length"
         );
-        // TODO: off by one?
-        from = (stext.len() as isize + start) as usize;
+        println!(
+            ">> charcount as isize + start = {} + {}",
+            charcount as isize, start
+        );
+        from = (charcount as isize + start) as usize;
     } else {
         from = start as usize;
     }
     if end < 0 {
         // LATER: may remove this check and just default to 0 in the future.
         assert!(
-            -end as usize <= stext.len(),
+            -end as usize <= charcount,
             "charslice: if 'end' is negative, the magnitude may not exceed the length"
         );
-        // TODO: off by one?
-        let new_end = (stext.len() as isize + end) as usize;
+        let new_end = (charcount as isize + end) as usize;
         assert!(
             new_end >= from,
             "charslice: 'start' may not be before 'end' (end was positive)"
@@ -36,12 +39,13 @@ pub fn charslice<S: Into<String>>(text: S, start: isize, end: isize) -> String {
         );
         length = end as usize - from;
     }
+    println!("from: {}, length: {}", from, length);
     stext.chars().skip(from).take(length).collect()
 }
 
 pub fn charslicefrom<S: Into<String>>(text: S, start: isize) -> String {
     let stext = text.into();
-    let len = stext.len() as isize;
+    let len = stext.chars().count() as isize;
     charslice(stext, start, len)
 }
 
@@ -59,6 +63,9 @@ mod tests {
         assert_eq!("你好", charslice("你好!", 0, 2));
         assert_eq!("!", charslicefrom("你好!", 2));
         assert_eq!("你好", charsliceto("你好!", 2));
-        // TODO: test negative values
+        // Negative indices should match Python 3 behaviour:
+        assert_eq!("你好", charslice("你好!", -3, -1));
+        assert_eq!("!", charslicefrom("你好!", -1));
+        assert_eq!("你好", charsliceto("你好!", -1));
     }
 }