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Fix underflow in char_range_at_reverse #5611

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Fix underflow in char_range_at_reverse #5611

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17 changes: 15 additions & 2 deletions src/libcore/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1865,8 +1865,10 @@ pub struct CharRange {
* Given a byte position and a str, return the previous char and its position
*
* This function can be used to iterate over a unicode string in reverse.
*
* returns 0 for next index if called on start index 0
*/
fn char_range_at_reverse(ss: &str, start: uint) -> CharRange {
pub fn char_range_at_reverse(ss: &str, start: uint) -> CharRange {
let mut prev = start;

// while there is a previous byte == 10......
Expand All @@ -1875,7 +1877,12 @@ fn char_range_at_reverse(ss: &str, start: uint) -> CharRange {
}

// now refer to the initial byte of previous char
prev -= 1u;
if prev > 0u {
prev -= 1u;
} else {
prev = 0u;
}


let ch = char_at(ss, prev);
return CharRange {ch:ch, next:prev};
Expand Down Expand Up @@ -3761,4 +3768,10 @@ mod tests {
"12345555".cmp(& &"123456") == Less;
"22".cmp(& &"1234") == Greater;
}

#[test]
fn test_char_range_at_reverse_underflow() {
assert!(char_range_at_reverse("abc", 0).next == 0);
}

}