Combination for zero sized array

src/combinations.rs

@@ -50,6 +50,11 @@ impl<I> Iterator for Combinations<I>
 {
     type Item = Vec<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
+        if self.first && self.k == 0 {
+            self.first = false;
+            return Some(Vec::new());
+        }
+
         let mut pool_len = self.pool.len();
         if self.pool.is_done() {
             if pool_len == 0 || self.k > pool_len {

src/combinations_with_replacement.rs

@@ -65,13 +65,12 @@ where
     fn next(&mut self) -> Option<Self::Item> {
         // If this is the first iteration, return early
         if self.first {
-            // In empty edge cases, stop iterating immediately
-            return if self.k == 0 || self.pool.is_done() {
-                None
-            // Otherwise, yield the initial state
-            } else {
-                self.first = false;
-                Some(self.current())
+            self.first = false;
+            // Handle corner cases of ((N,0)), ((0,N)), and ((0,0))
+            return match (self.pool.is_done(), self.k == 0) {
+                (_, true) => Some(Vec::new()), // ((0/N, 0)) = 1
+                (true, false) => None,         // ((0, N)) = 0
+                _ => Some(self.current())      // Otherwise, yield the initial state
             };
         }
 

tests/test_std.rs

@@ -599,6 +599,13 @@ fn combinations_of_too_short() {
 #[test]
 fn combinations_zero() {
     it::assert_equal((1..3).combinations(0), vec![vec![]]);
+    it::assert_equal((0..0).combinations(0), vec![vec![]]);
+}
+
+#[test]
+fn permutations_zero() {
+    it::assert_equal((1..3).permutations(0), vec![vec![]]);
+    it::assert_equal((0..0).permutations(0), vec![vec![]]);
 }
 
 #[test]
@@ -620,7 +627,12 @@ fn combinations_with_replacement() {
     // Zero size
     it::assert_equal(
         (0..3).combinations_with_replacement(0),
-        <Vec<Vec<_>>>::new(),
+        vec![vec![]],
+    );
+    // Zero size on empty pool
+    it::assert_equal(
+        (0..0).combinations_with_replacement(0),
+        vec![vec![]],
     );
     // Empty pool
     it::assert_equal(