algorithms – How to analysis the complexity of this LIS program?

I found a pseudocode online and I don’t know why the complexity of it is $O(2^N)$, according to the site.

Given a fixed curr, the for-loop in lis_ending_here(...) will recursively call curr times itself will new curr range from 0 to curr-1, then I stuck here and don’t know what’s the next step to get the conclusion $O(2^N)$. You can also provide me resource (i.e. book name) which is about the technique to analysis the complexity of recursion, since I don’t know what are the beginner friendly ones.

int lis_ending_here(int arr(), int curr)
    // Only one subsequence ends at first index, the number itself
    if(curr == 0)
        return 1
    int ans = 1
    for(i = curr-1 to 0, decrement of -1)
        if(arr(i) < arr(curr))
            ans = max(ans, 1 + lis_ending_here(arr, i))
    return ans
int longest_increasing_subsequence(int arr(), int N)
    // Because a single number can be a subsequence too
    int max_ans = 1
    for(i = 0 to N-1)
        max_ans = max(max_ans, lis_ending_here(arr, i))
    return max_ans