Combination for zero sized array

[leeopop]

Fixes #361

1. Add corner case handling for combinations
2. Add test cases for combinations, combinations_with_replacement, and permutations.

Zero-sized array is property handled in other combinatorics operators.

[andersk]

This looks wrong. The empty vector is a valid combination of zero elements in 0..0, so (0..0).combinations_with_replacement(0) should be vec![vec![]] (like (0..0).combinations(0) and (0..0).permutations(0)), not <Vec<Vec<_>>>::new() == vec![].

[leeopop]

@andersk

It seems that the original test cases of combinations_with_replacement has the same test cases, and I just made a copy of it.
Should fixing combinations_with_replacement and its test cases be included in this PR or should I make a separate PR?

[leeopop]

I found that python itertools implementation shows value 1 (single empty set) for following inputs.

>>> len(list(itertools.combinations_with_replacement([], 0)))
1
>>> len(list(itertools.combinations([], 0)))
1
>>>

[leeopop]

@andersk I just added a commit that fixes combinations_with_replacement dealing with zeros.

[andersk]

Now this is wrong. vec![] is only a valid combination of length 0; it’s not a valid combination of length 2.

[leeopop]

@andersk Fixed here!

[andersk]

Here’s a simpler patch (against master) that achieves the correct results by removing special cases instead of by adding them:

--- a/src/combinations.rs
+++ b/src/combinations.rs
@@ -51,13 +51,11 @@ impl<I> Iterator for Combinations<I>
     type Item = Vec<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
         let mut pool_len = self.pool.len();
-        if self.pool.is_done() {
-            if pool_len == 0 || self.k > pool_len {
-                return None;
-            }
-        }
 
         if self.first {
+            if self.pool.is_done() && self.k > pool_len {
+                return None;
+            }
             self.first = false;
         } else if self.k == 0 {
             return None;
--- a/src/combinations_with_replacement.rs
+++ b/src/combinations_with_replacement.rs
@@ -66,7 +66,7 @@ where
         // 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() {
+            return if self.pool.is_done() && self.k != 0 {
                 None
             // Otherwise, yield the initial state
             } else {

Actually, now that I’m looking at the code harder, I think there are other existing bugs…

[andersk]

I opened #383 which fixes this while simplifying the code even further.

[leeopop]

@andersk Yep, my PR was very rough as I'm not used to the internal implementation.
If you don't mind, you may close this PR and move to the new PR.

[andersk]

Thanks for inspiring me to take a closer look. (But I’m just a contributor like you; I don’t have any special permission to close PRs.)

[leeopop]

moved to #383