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16. Optimizing stable sort

The measure of a good programmer

With algorithms and components you have to know the bunch. It’s like a toolchest. A good woodworker doesn’t just have one kind of tool. You have a bunch of tools, whether making a door or whatever, and then you find the right tools. The goodness, or best measure of a programmer is the number of tools, or algorithms, he is fluent with. There is an erroneous view that good programmer is someone who can invent new algorithms. This is good, but most people don’t need to. A carpenter doesn’t invent new tools. You can be an exceptionally good, or world class carpenter, without ever introducing a new woodworking tool. One of the things you need to know is existing tools.

I constantly hear, “I never needed __ algorithm”. This might be true. A carpenter doesn’t use every tool in a given day. Nevertheless, they have a large tool chest for those cases when they need it. Larger the tool chest, better off you are, easier it is going to be to solve problems. Moreover, if the tools are known to everyone, they can understand what you do.

This is why I try to come up with a set of systematic components. I think programming is a wonderful thing if you know many algorithms and data structures and just use them.

How good is our stable sort?

The merge sort algorithm we wrote last week is in many respects good code. It’s a good example of a stable sort. It’s a good basis on which we could get fast results and figure out how to make it very fast. We will do measurements.

The current algorithm has log(n) recursive levels. At every level we have a merge which is O(n log(n)). so the overall complexity is O(n log^2(n)). It’s better than bubble sort and shell sort. So how bad is it? I made a test harness1:

Sorting double from 8 up to 2097152 elements generated with iota at: Sun Jun 27 20:29:54 2021
    size    stable  inplace 
       8        12      18  
      16         8      25  
      32         8      35  
      64         9      40  
     128         9      49  
     256        12      66  
     512        14      94  
    1024        23     124  
    2048        34     150  
    4096        46     175  
    8192        54     197  
   16384        63     218  
   32768        68     234  
   65536        73     255  
  131072        77     276  
  262144        87     355  
  524288        92     321  
 1048576       100     343  
 2097152       102     364

Stable here is std::stable_sort which is highly optimized library code. So it tells us we can do better. The numbers are nano-seconds per element. Our merge sort is called inplace. We start off ok, and then gradually get slower and slower in comparison. Obviously, as the size increases you have to do more work. It’s not linear. But, ours is clearly 3-4x slower than that.

A plan for improvement

We are going to write a faster one. I am going to work with the class but I will not always tell you the truth. Let us derive together and have a conversation but if you go on a wrong path I might not necessarily stop you. In real life I’m not going to be there to stop you when you write code. So let us try together.


First we will decide our requirements. These might be more strict than possible, so we can remove them later if needed.

  1. Maintain stability.

    Otherwise we would just write quicksort.

  2. Requires only ForwardIterator.

    InputIterator is the only weaker thing, and it’s not enough for sorting.

  3. Use limited extra memory.

    The requirement of no extra memory at all is bogus. With modern systems we often have more memory than we need. But, we often have to parallelize stuff, so, we cannot put too much data on the processor. We want data to be able to fit in the cache. One way to do that is by exposing control.

  4. Fast.

    What would be good? Within 5-10% of the std::stable_sort. What would be great? Observe STL also has std::sort because it’s faster. If it’s as fast as sort, that would be great.2.

Ideas to explore

So we are going to try to make it fast, using all kinds of brilliant algorithm ideas. Let me tell you one great secret. When designing algorithms there is a huge space of possible inputs. You might have really big structure. Or very expensive comparisons. Should you concentrate on those? No. First, make your algorithm work really fast for what I call a common case, such as double. You don’t know how big integers are, but the size of double is fixed for centuries to come, by IEEE standard. But, integers, doubles, and pointers are all good. Always make your algorithms as fast as possible for the most common case. Then, you can look at bigger things and make other tradeoffs.

What ideas can we come up with?

Using insertion sort for small cases

In insertion sort you build up a sorted list by inserting elements one at a time. One part of the list is always sorted. We take an element which is not inserted, and linear search to find where it belongs.

[  sorted       | not sorted ]

What is the complexity of insertion sort? It’s O(n^2). What is the complexity of merge sort, in terms of comparisons? O(n log(n)). What is the specific coefficient for insertion sort? The sorted portion is on average half the length of the original input. In addition, on average when we insert an element we only have to go half the length before finding its location. Therefore the complexity is roughly: n^2/4.

Now we will solve for when it’s better to do one than the other.

n^2/4 < n log(n)
    n < 4 log(n)
  2^n < n^4

So the cutoff is n = 16:

2^16 = 65536 = 16^4

That’s what theory tells us. What should we really do? We should measure. But, the theory tells us around which numbers to try (around 16).

Binary insertion sort

We can also binary search to find where to insert the element. That will also make the cutoff point larger. Is it going to be dramatically better? In insertion sort, we already have to do a linear complexity shift to make room.

Get rid of recursion

Perhaps we can use binary_counter.

Write an excellent merge

Basically we need an algorithm that will say, “do this version if we have enough memory, otherwise do that”. The term is adaptive.

You might think everybody can write a good merge. If you Google “std merge” it shows you this: Since you’re a normal programmer you might say, “oh it’s on the Web, therefore I can copy and paste and use it in my code.”

template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator merge (InputIterator1 first1, InputIterator1 last1,
    InputIterator2 first2, InputIterator2 last2,
    OutputIterator result)
  while (true) {
    *result++ = (*first2<*first1)? *first2++ : *first1++;
    if (first1==last1) return std::copy(first2,last2,result);
    if (first2==last2) return std::copy(first1,last1,result);

This is a broken piece of code. An empty range will melt the computer.

Faster rotate

What is the complexity of std::rotate? It’s tricky, because it depends on the kind of iterators which you have. With RandomAccessIterator the best theoretical algorithm does n + gcd(n_1, n_2) assignments, not swaps. On average GCD is small, but larger than one. So we can get almost to n assignments, which is a lot better than n swaps. For ForwardIterator it happens to be n - gcd(n_1, n_2) swaps. It is roughly n swaps for BidirectionalIterator (3n assignments).

As we will observe, we can use a faster rotate than the rotate in STL because we have this additional storage. If you want to rotate and you have enough storage, then you only need n + n_1 assignments which is for sure less than 3n.

First steps

There’s lots of things to do. How should we go about it? The problem with programming, specifically designing components and composing the system, is that you do not know what is right in isolation. You never know what the correct interface is until you see it in other algorithms, and you see how those are used. This is why you just have to try things and ideas start emerging.

You might think it’s an infinite process. No it’s not infinite, that’s the wonderful thing about life. It sort of terminates (I cannot prove it of course). In practice if you start fitting things together you sort of discover what you need to return, what you need to pass, what is the right thing to do, and that’s what I am trying to teach.

When should we try insertion sort? As a rule we want to fix the asymptotic complexity. Doing insertion sort at the bottom won’t help that. Right now we have a problem with our asymptotic complexity. It’s O(n log^2(n)). We want to get rid of that square as fast as possible.

I’m very lazy. So, we saw how fast we can get using no memory. What about if we had infinite memory? How much extra memory do I really need? I think we can do it in n/2.

  1. AMD Ryzen 5 2400G (8 core, 3.6 GHz). GCC 9.3.0
  2. Alex: It has been my dream for decades to make my stable sort as fast as my sort, at which point I could throw away the other and just have one sort. But, I make progress with stable sort, then I make progress with sort.
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