Add test for permutation and fixed it

src/permutation.rs

@@ -42,42 +42,82 @@ pub fn nth_lex<T: Clone + Ord>(mut digits: Vec<T>, nth: usize) -> Result<Vec<T>,
 
 #[derive(Clone)]
 pub struct Permutator<T: Clone + Ord> {
-    pub current: Vec<T>,
+    pub elements: Vec<T>,
     idx: usize,
 }
 
 impl<T: Clone + Ord> Permutator<T> {
     pub fn new(elements: Vec<T>) -> Self {
-        Self {
-            current: elements,
-            idx: 0,
-        }
+        Self { elements, idx: 0 }
     }
 }
 
-impl<T: Clone + Ord> Iterator for Permutator<T> {
+impl<T: Clone + Ord + std::fmt::Display> Iterator for Permutator<T> {
     type Item = Vec<T>;
     /// Returns the next permutation and changes the current permutation to it
     /// This operation wraps around
     fn next(&mut self) -> Option<Self::Item> {
-        if self.current.is_empty() {
+        if self.elements.is_empty() {
             return None;
         }
-        let mut digits = self.current.clone();
-        if self.idx == factorial(digits.len()) {
+        if self.idx == 0 {
+            self.idx += 1;
+            return Some(self.elements.clone());
+        }
+        let num_unique_digits = self.elements.len();
+        if self.idx == factorial(num_unique_digits) {
             return None;
         }
+        let mut digits = self.elements.clone();
         let mut perm = Vec::new();
-        let num_unique_digits = digits.len();
-        let mut remainder = 1;
-        for idx in 1..=digits.len() {
+        let mut remainder = self.idx;
+        for idx in 1..=num_unique_digits {
             let permutations = remainder / factorial(num_unique_digits - idx);
             remainder %= factorial(num_unique_digits - idx);
-            perm.push(digits[permutations].clone());
-            digits.remove(permutations);
+            perm.push(digits.remove(permutations));
         }
         self.idx += 1;
-        self.current = digits;
-        Some(self.current.clone())
+        Some(perm)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::permutation::{nth_lex, Permutator};
+
+    const SMALL: [i8; 3] = [1, 2, 3];
+
+    // Nth Lex
+
+    #[test]
+    fn nth_lex_zero_small() {
+        let small = SMALL.to_vec();
+        assert!(nth_lex(small, 0).is_err());
+    }
+
+    #[test]
+    fn nth_lex_small() {
+        let small = SMALL.to_vec();
+        assert_eq!(nth_lex(small.clone(), 1).unwrap(), vec![1, 2, 3]);
+        assert_eq!(nth_lex(small.clone(), 2).unwrap(), vec![1, 3, 2]);
+        assert_eq!(nth_lex(small.clone(), 3).unwrap(), vec![2, 1, 3]);
+        assert_eq!(nth_lex(small.clone(), 4).unwrap(), vec![2, 3, 1]);
+        assert_eq!(nth_lex(small.clone(), 5).unwrap(), vec![3, 1, 2]);
+        assert_eq!(nth_lex(small.clone(), 6).unwrap(), vec![3, 2, 1]);
+        assert!(nth_lex(small.clone(), 7).is_err());
+    }
+
+    // Permutator
+    #[test]
+    fn perm_small() {
+        let small = SMALL.to_vec();
+        let mut comb = Permutator::new(small);
+        assert_eq!(comb.next(), Some(vec![1, 2, 3]));
+        assert_eq!(comb.next(), Some(vec![1, 3, 2]));
+        assert_eq!(comb.next(), Some(vec![2, 1, 3]));
+        assert_eq!(comb.next(), Some(vec![2, 3, 1]));
+        assert_eq!(comb.next(), Some(vec![3, 1, 2]));
+        assert_eq!(comb.next(), Some(vec![3, 2, 1]));
+        assert_eq!(comb.next(), None);
     }
 }