🐛 Fix SequenceSet#max(n), cardinality < n <= size

When `SequenceSet#max(n)` is called with `n > cardinality`, it _should_
return a duplicate of the whole set.  But, `#max(n)` is implemented
using `#slice(-n..)`, and (copying the behavior of `Array`), when a
slicing starts from an out-of-range index, it returns `nil`.

It was using `-[count, size].min` to keep the index from going
out-of-range.  Prior to #564, `#size` was the same as `#count`, so it
would give incorrect results when the set contains an endless range.
After #564, this gives incorrect results when the ordered list contains
duplicates.

This change should also give a small performance boost, because it
bypasses the complexity of `#slice(range)` and just calls `#dup` (or
returns `self` when the set is frozen).

This issue was one of the motivations for #563 (and #564), but then I
forgot about the bug, so it wasn't fixed in time for 0.6.0!

Author: nevans

lib/net/imap/sequence_set.rb

@@ -788,7 +788,11 @@ def disjoint?(other)
       # Related: #min, #minmax, #slice
       def max(count = nil, star: :*)
         if count
-          slice(-[count, size].min..) || remain_frozen_empty
+          if cardinality <= count
+            frozen? ? self : dup
+          else
+            slice(-count..) || remain_frozen_empty
+          end
         elsif (val = max_num)
           val == STAR_INT ? star : val
         end

test/net/imap/test_sequence_set.rb

@@ -808,6 +808,15 @@ def obj.to_sequence_set; 192_168.001_255 end
     assert_equal SequenceSet["678"],     SequenceSet["345,678"].max(1)
     assert_equal SequenceSet["345,678"], SequenceSet["345,678"].max(222)
     assert_equal SequenceSet.empty,      SequenceSet.new.max(5)
+    # with different cardinality (150) vs size (200)
+    set = SequenceSet["101:200,51:150"]
+    assert_equal SequenceSet["52:200"], set.max(149)
+    assert_equal SequenceSet["51:200"], set.max(150)
+    assert_equal SequenceSet["51:200"], set.max(200)
+    # with different cardinality (2**32) vs count (2**32 - 1)
+    set = SequenceSet[1..]
+    assert_equal SequenceSet["2:*"], set.max(2**32 - 1)
+    assert_equal SequenceSet["1:*"], set.max(2**32)
   end
 
   test "#minmax" do