Add TestRankAfterDel test case.

Author: werbenhu

ranklist.go

@@ -179,7 +179,8 @@ func (sl *RankList[K, V]) Set(key K, value V) {
 		} else {
 			newNode.span[i] = rank[0] - rank[i] + 1
 			if newNode.forward[i] != nil {
-				newNode.forward[i].span[i] = newNode.span[i] + 1
+				// 后面节点的span，被插入的节点切割了
+				newNode.forward[i].span[i] = newNode.forward[i].span[i] - newNode.span[i] + 1
 			}
 		}
 	}
@@ -242,11 +243,20 @@ func (sl *RankList[K, V]) del(key K) bool {
 	// Update forward pointers and spans
 	for i := 0; i < sl.level; i++ {
 		curr = prev[i].forward[i]
+
 		if curr != nil && curr.data.Key == key && curr.data.Value == value {
-			next := curr.forward[i]
-			if next != nil {
-				next.span[i] += curr.span[i] - 1
+			// 如果这一层找到了删除的节点，那么将删除节点清除，并将删除节点的 span 甩给后面的节点
+			// If the node to be deleted is found at this level, remove the node and pass its span to the next node
+			prev[i].forward[i] = curr.forward[i]
+			if curr.forward[i] != nil {
+				curr.forward[i].span[i] += (curr.span[i] - 1)
 			}
+
+		} else if curr != nil {
+			// 如果没有找到节点，说明这些层级比删除的节点的层级高，这些比删除节点高的节点，span 要 -1
+			// If the node is not found, it means these levels are higher than the level of the node to be deleted,
+			// so the span of these higher-level nodes needs to be decremented by 1
+			curr.span[i]--
 		}
 	}
 

ranklist_test.go

@@ -99,6 +99,7 @@ func TestRank(t *testing.T) {
 		{"c", 3, 3},
 		{"d", 3, 4},
 		{"e", 4, 5},
+		{"f", 5, 6},
 	}
 
 	for _, data := range testData {
@@ -124,6 +125,42 @@ func TestRank(t *testing.T) {
 	}
 }
 
+func TestRankAfterDel(t *testing.T) {
+	sl := New[string, int]()
+
+	testData := []struct {
+		key   string
+		value int
+		rank  int
+	}{
+		{"a", 1, 1},
+		{"b", 2, 2},
+		{"d", 3, 3},
+		{"e", 4, 4},
+		{"g", 6, 5},
+		{"h", 7, 6},
+	}
+
+	for _, data := range testData {
+		sl.Set(data.key, data.value)
+	}
+
+	sl.Set("c", 3)
+	sl.Del("c")
+	sl.Set("f", 5)
+	sl.Del("f")
+
+	for _, data := range testData {
+		rank, exists := sl.Rank(data.key)
+		if !exists {
+			t.Fatalf("Key %s should exist", data.key)
+		}
+		if rank != data.rank {
+			t.Errorf("Key %s: expected rank %d, got %d", data.key, data.rank, rank)
+		}
+	}
+}
+
 func TestMassiveRank(t *testing.T) {
 	sl := New[string, int64]()
 	for k := 0; k < 10000; k++ {