core: add char_at_reverse

src/libcore/str.rs

@@ -1769,7 +1769,7 @@ impl Equiv<~str> for &'self str {
     return CharRange {ch: val as char, next: i};
 }
 
-/// Pluck a character out of a string
+/// Plucks the `n`th character from the beginning of a string
 pub pure fn char_at(s: &str, i: uint) -> char {
     return char_range_at(s, i).ch;
 }
@@ -1799,6 +1799,11 @@ pub struct CharRange {
     return CharRange {ch:ch, next:prev};
 }
 
+/// Plucks the `n`th character from the end of a string
+pub pure fn char_at_reverse(s: &str, i: uint) -> char {
+    char_range_at_reverse(s, i).ch
+}
+
 /**
  * Loop through a substring, char by char
  *
@@ -2274,6 +2279,7 @@ pub trait StrSlice {
     pure fn to_owned(&self) -> ~str;
     pure fn to_managed(&self) -> @str;
     pure fn char_at(&self, i: uint) -> char;
+    pure fn char_at_reverse(&self, i: uint) -> char;
     fn to_bytes(&self) -> ~[u8];
 }
 
@@ -2419,6 +2425,11 @@ impl StrSlice for &'self str {
     #[inline]
     pure fn char_at(&self, i: uint) -> char { char_at(*self, i) }
 
+    #[inline]
+    pure fn char_at_reverse(&self, i: uint) -> char {
+        char_at_reverse(*self, i)
+    }
+
     fn to_bytes(&self) -> ~[u8] { to_bytes(*self) }
 }
 
@@ -3426,6 +3437,28 @@ fn test_utf16() {
         }
     }
 
+    #[test]
+    fn test_char_at() {
+        let s = ~"ศไทย中华Việt Nam";
+        let v = ~['ศ','ไ','ท','ย','中','华','V','i','ệ','t',' ','N','a','m'];
+        let mut pos = 0;
+        for v.each |ch| {
+            fail_unless!(s.char_at(pos) == *ch);
+            pos += from_char(*ch).len();
+        }
+    }
+
+    #[test]
+    fn test_char_at_reverse() {
+        let s = ~"ศไทย中华Việt Nam";
+        let v = ~['ศ','ไ','ท','ย','中','华','V','i','ệ','t',' ','N','a','m'];
+        let mut pos = s.len();
+        for v.each_reverse |ch| {
+            fail_unless!(s.char_at_reverse(pos) == *ch);
+            pos -= from_char(*ch).len();
+        }
+    }
+
     #[test]
     fn test_each_char() {
         let s = ~"abc";