not understand the code

918-240623/main.cpp

@@ -0,0 +1,14 @@
+class Solution {
+public:
+    int maxSubarraySumCircular(vector<int>& A) {
+        int sum = 0, mn = INT_MAX, mx = INT_MIN, curMax = 0, curMin = 0;
+		for (int num : A) {
+			curMin = min(curMin + num, num);
+            mn = min(mn, curMin);
+            curMax = max(curMax + num, num);
+            mx = max(mx, curMax);
+            sum += num;
+		}
+        return (sum - mn == 0) ? mx : max(mx, sum - mn);
+    }
+};