fix merges of pairing heap

immut/priority_queue/moon.pkg.json

@@ -5,6 +5,7 @@
     "moonbitlang/core/immut/list",
     "moonbitlang/core/quickcheck"
   ],
+  "test-import": ["moonbitlang/core/random"],
   "targets": {
     "panic_test.mbt": ["not", "native"]
   }

immut/priority_queue/priority_queue.mbt

@@ -104,7 +104,8 @@ fn meld[A : Compare](l : Cont[A], r : Cont[A]) -> Cont[A] {
 fn merges[A : Compare](x : @list.T[Cont[A]]) -> Cont[A] {
   loop x, Empty {
     Nil, acc => acc
-    Cons(t, rest), acc => continue rest, meld(t, acc)
+    Cons(t, Nil), acc => meld(t, acc)
+    Cons(t1, Cons(t2, rest)), acc => continue rest, meld(meld(t1, t2), acc)
   }
 }
 

immut/priority_queue/priority_queue_test.mbt

@@ -163,3 +163,25 @@ test "equal" {
   let pq5 = @priority_queue.of([1, 2, 3])
   inspect!(pq1 == pq5, content="false")
 }
+
+///|
+test "complex" {
+  fn rand(upper : Int) -> Int {
+    let rng = @random.new()
+    rng.int(limit=upper)
+  }
+
+  let arr = (0).until(1000, inclusive=true).collect()
+  arr.shuffle_in_place(rand~)
+  let mut pq = new()
+  for i in 0..=1000 {
+    pq = pq.push(arr[i])
+  }
+  assert_eq!(pq.length(), 1001)
+  for i in 0..=1000 {
+    assert_eq!(pq.peek(), Some(1000 - i))
+    pq = pq.pop().unwrap()
+  }
+  inspect!(pq.iter(), content="[]")
+  assert_eq!(pq.length(), 0)
+}