Timsort

I wrote before about the inherent complexity of the real world and how software that behaves well in the real world must necessarily take on some complexity (Time and Complexity). A lot of the software engineering best practices try to reduce or eliminate the accidental complexity of large systems (making things more complicated than they should be). But we don’t live in a perfect world, so modeling it using software requires some inherent complexity in the software, to reflect reality. One of the algorithms which perfectly illustrates this is the Timsort sorting algorithm.

Timsort is an algorithm developed by Tim Peters in 2002 to replace Python’s previous sorting algorithm. It has since been adopted inJava’s OpenJDK, the V8 JavaScript engine, and the Swift and Rust languages. This is a testament that Timsort is a performant sort.

Timsort is a stable sorting algorithm, which means it will never changes the relative order of equal elements. This is an important property in certain situations. This is not important when sorting numbers, but becomes important when sorting objects with custom comparisons.

But in 2002 we already had plenty of well known sorting algorithms which were quite efficient. How did Timsort manage to outperform these?

Merging Runs

The key insight of Timsort is that in the real world, many lists of elements that require sorting contain subsequences of elements that are already sorted. These are called runs and tend to appear naturally. For example, in the list [5, 2, 3, 4, 9, 1, 6, 8, 10, 7] we have two runs: [2, 3, 4] and [1, 6, 8, 10].

If we know runs will show up more often than not in our input, how can we best leverage this to our advantage, and avoid extraneous comparisons and data movement?

Timsort starts by finding the minimum “accepted” run length for a given input. This doesn’t have anything to do with the content of the input, rather it is a function of the size of the input. More on this later.

Then we do a single pass over the array and identify consecutive runs. If the next minimum accepted run length elements are not already sorted (they don’t form a run), we sort them using insertion sort (so they do end up as a run). We push these runs on a stack, then we then merge pairs of them until we end up with a single run, which is our sorted list.

A Simple Implementation

Let’s start with a simple sketch implementation. We’ll use Python since it is expressive and it makes it easier to focus on the algorithm rather than syntax around it.

MIN_MERGE = 4

def sort(arr):
    lo, hi = 0, len(arr)
    stack = []
    nRemaining = hi
    minRun = MIN_MERGE

    while nRemaining > 0:
        runLen = min(nRemaining, minRun)
        insertionSort(arr, lo, lo + runLen)
        stack.append((lo, runLen))

        lo += runLen
        nRemaining -= runLen

    while len(stack) > 1:
        base2, len2 = stack.pop()
        base1, len1 = stack.pop()
        merge(arr, base1, base2, base2 + len2)
        stack.append((base1, len1 + len2))

First, we initialize a few variables:

lo, hi = 0, len(arr)
stack = []
nRemaining = hi
minRun = MIN_MERGE

MIN_MERGE represents the minimum number of elements we want to merge, and is a constant. We’ll talk more about this once we look at some optimizations later on.

lo and hi represent the range in the array we will operate on. Note ranges are always half-open (arr[lo] included, arr[hi] excluded, potentially out of bounds). stack is the run stack, nRemaining is the number of elements we still need to process. minRun is the minimum run length. For this first iteration, we’ll just use MIN_MERGE.

Next, we traverse the array and come up with our runs:

while nRemaining > 0:
        runLen = min(nRemaining, minRun)
        insertionSort(arr, lo, lo + runLen)
        stack.append((lo, runLen))

        lo += runLen
        nRemaining -= runLen

Our run in this case will be the minimum between minRun and the remaining elements of the array (so for the final run, we don’t go out of bounds). We sort the run using insertionSort, then we push the run start index and length onto the stack. We advance lo by the length of the run and we similarly decrement nRemaining, the number of elements still to be processed.

Next, we merge the runs:

while len(stack) > 1:
    base2, len2 = stack.pop()
    base1, len1 = stack.pop()
    merge(arr, base1, base2, base2 + len2)
    stack.append((base1, len1 + len2))

We pop 2 runs from the top of the stack, merge them, and push the new run back onto the stack. With this basic implementation, a stack is technically not really needed, but I’m trying to preserve the general shape of the optimized solution.

We called a couple of helper functions: insertionSort and merge. Here is insertionSort:

def insertionSort(arr, lo, hi):
    i = lo + 1

    while i < hi:
        elem = arr[i]
        j = i - 1

        while elem < arr[j] and j >= lo:
        j -= 1

        arr.pop(i)
        arr.insert(j + 1, elem)

        i += 1

Insertion sort traverses the array from the lower bound + 1 to the higher bound and maintains the invariant that all elements preceding i are sorted. So for any element arr[i], we find a spot j in the range [lo, i) where this element should fit. We then insert it there and shift the remaining elements in [j + 1, i) one spot to the right. Note this algorithm is quite inefficient on large data sets, but performs well on small inputs.

Our merge algorithm is:

def merge(arr, lo, mid, hi):
    t = arr[lo:mid]
    i, j, k = lo, mid, 0

    while k < mid - lo and j < hi:
        if t[k] < arr[j]:
            arr[i] = t[k]
            k += 1
        else:
            arr[i] = arr[j]
            j += 1
        i += 1

    if k < mid - lo:
        arr[i:hi] = t[k:mid - lo]

We are merging the consecutive (sorted) ranges [lo, mid) and [mid, hi). One way to do this (which our implementation uses), is to copy [lo, mid) to a temporary buffer t. We then traverse the [mid, hi) range with j and the buffer with k. We pick the smallest of t[k] and arr[j] to insert at arr[i] (incrementing the corresponding index), then we increment i. At some point, either j or k reaches the end. If j makes it to the end first, it means we still have some elements in t we need to copy over. If k makes it to the end first, we don’t need to do anything: the remaining elements in [j, hi) are where they are supposed to be.

We now have a full implementation of a very simple Timsort. If we run it on the [5, 2, 3, 4, 9, 1, 6, 8, 10, 7] input, the following steps take place:

  • We pick up the first run, [5, 2, 3, 4] and sort it using insertionSort. This becomes [2, 3, 4, 5]. We push its start index and length on the stack ((0, 4)).
  • We next take [9, 1, 6, 8], sort it to [1, 6, 8, 9], and push (4, 4) on the stack.
  • Finally, we only have [10, 7]. We sort this short run to [7, 10] and push (6, 2) on the stack.

Note all our sorting happens in-place, so by now the whole input became [2, 3, 4, 5, 1, 6, 8, 9, 7, 10]. We then proceed to merge runs from the top of the stack:

  • First, we merge [1, 6, 8, 9] with [7, 10, which yields [1, 6, 7, 8, 9, 10]. We pop the two runs from the stack and push (4, 6), the index and length of this new run.
  • Next, we merge [2, 3, 4, 5] with [1, 6, 7, 8, 9, 10], and update the stack accordingly. At this point, we only have 1 run on the stack ([0, 10)). We are done.

Some Optimizations

So far, we haven’t relied that much on the fact that our input might be naturally partially sorted. Instead of simply calling insertionSort on minRun elements, we can actually check whether elements are already ordered. If they are, we don’t need to do anything with them. Even better, if the run of elements is longer than minRun, we keep going.

Elements might also come naturally sorted in descending order, while we are sorting in ascending order. No problem: we can take a range of elements coming in descending order and reverse it to produce a run in ascending order. Let’s call this function countRunAndMakeAscending:

def countRunAndMakeAscending(arr, lo, hi):
    runHi = lo + 1
    if runHi == hi:
        return 1

    if arr[lo] > arr[runHi]: # Descending run
        while runHi < hi and arr[runHi] < arr[runHi - 1]:
            runHi += 1
        reverseRange(arr, lo, runHi)
    else: # Ascending run
        while runHi < hi and arr[runHi] >= arr[runHi - 1]:
            runHi += 1

    return runHi - lo

We return the length of the run starting from lo, going to at most hi - 1. If we have a natural descending run, we reverse the range before returning. Here is reverseRange:

def reverseRange(arr, lo, hi):
    hi -= 1
    while lo < hi:
        arr[lo], arr[hi] = arr[hi], arr[lo]
        lo += 1
        hi -= 1

We can’t get rid of sorting though: we might have worst-case scenario cases with very small runs, in which case we still need a range of at least minRun size. Based on the result of countRunAndMarkAscending, if it is smaller than minRun, we will “force” a few more elements into the run and sort it. Our new implementation looks like this:

def sort(arr):
    lo, hi = 0, len(arr)
    stack = []
    nRemaining = hi
    minRun = MIN_MERGE

    while nRemaining > 0:
        runLen = countRunAndMakeAscending(arr, lo, hi)

        if runLen < minRun:
            force = min(nRemaining, minRun)
            insertionSort(arr, lo, lo + force)
            runLen = force

        stack.append((lo, runLen))

        lo += runLen
        nRemaining -= runLen

    while len(stack) > 1:
        base2, len2 = stack.pop()
        base1, len1 = stack.pop()
        merge(arr, base1, base2, base2 + len2)
        stack.append((base1, len1 + len2))

Highlighting the changed part:

runLen = countRunAndMakeAscending(arr, lo, hi)

if runLen < minRun:
    force = min(nRemaining, minRun)
    insertionSort(arr, lo, lo + force)
    runLen = force

Instead of simply taking the next minRun elements, we try to find a run. If the run we find is smaller than minRun, we force it to be minRun by insertion-sorting into it more elements. If it is larger than or equal to minRun on the other hand, we don’t have to do any sorting.

It gets better: now we know after calling countRunAndMakeAscending that the range [lo, lo + runLen) is already sorted. We can hint this to our sorting function and have it start sorting only from lo + runLen. We can update insertionSort to take a hint of where to start from:

def insertionSort(arr, lo, hi, start):
    if start == lo:
        start += 1

    while start < hi:
        elem = arr[start]
        j = start - 1

        while elem < arr[j] and j >= lo:
        j -= 1

        arr.pop(start)
        arr.insert(j + 1, elem)

        start += 1

This version is very similar to our previous one. Instead of using a local i variable to iterate over the range [lo + 1, hi), we just use start. If start is lo, we increment it before the loop (just like we used to initialize i to lo + 1).

We can now pass this hint in from our main function:

while nRemaining > 0:
    runLen = countRunAndMakeAscending(arr, lo, hi)

    if runLen < minRun:
        force = min(nRemaining, minRun)
        insertionSort(arr, lo, lo + force, lo + runLen)
        runLen = force

At this point, we’re starting to get a lot of value from naturally sorted runs: we either don’t do any sorting, or just sort at most minRun - runLen elements into the range.

A further optimization for sorting: we can replace insertion sort with binary sort. Binary sort works much like insertion sort, but instead of checking where element i fits into [lo, i) by comparing it with i - 1, then i - 2 and so on, it relies on the fact that [lo, i) is already sorted and performs a binary search to find the right spot. Here is an implementation, which also takes a start hint:

def binarySort(arr, lo, hi, start):
    if start == lo:
        start += 1

    while start < hi:
        pivot = arr[start]
        left, right = lo, start

        while left < right:
            mid = (left + right) // 2

            if pivot < arr[mid]:
                right = mid
            else:
                left = mid + 1

        arr.pop(start)
        arr.insert(left, pivot)

        start += 1

Our main function now looks like this:

def sort(arr):
    lo, hi = 0, len(arr)
    stack = []
    nRemaining = hi
    minRun = MIN_MERGE

    while nRemaining > 0:
        runLen = countRunAndMakeAscending(arr, lo, hi)

        if runLen < minRun:
            force = min(nRemaining, minRun)
            binarySort(arr, lo, lo + force, lo + runLen)
            runLen = force

        stack.append((lo, runLen))

        lo += runLen
        nRemaining -= runLen

    while len(stack) > 1:
        base2, len2 = stack.pop()
        base1, len1 = stack.pop()
        merge(arr, base1, base2, base2 + len2)
        stack.append((base1, len1 + len2))

Balanced Merges

Another key optimization of Timsort is trying as much as possible to merge runs of balanced sizes. The closer the size, the better average performance as a combination of additional space required and number of operations.

So far we just pushed everything onto a stack, then merged the top 2 elements of the stack until we ended up with a single run. We actually want to do something a bit different: we want our stack to maintain a couple of invariants:

  1. stack[i - 1][1] > stack[i][1] + stack[i + 1][1] - the length of a run needs to be larger than the sum of the lengths of the following runs.
  2. stack[i][1] > stack[i + 1][1] - the length of a run needs to be larger than the following run.

When pushing a new index and run length tuple onto the stack, we check if the invariant still holds. If it doesn’t, we merge stack[i] with the smallest of stack[i - 1], stack[i + 1] and recheck. We continue merging until the invariants are re-established. Let’s call this function mergeCollapse:

def mergeCollapse(arr, stack):
    while len(stack) > 1:
        n = len(stack) - 2
        if (n > 0 and stack[n - 1][1] <= stack[n][1] + stack[n + 1][1]) or \
        (n > 1 and stack[n - 2][1] <= stack[n][1] + stack[n - 1][1]):
        if stack[n - 1][1] < stack[n + 1][1]:
            n -= 1
        elif n < 0 or stack[n][1] > stack[n + 1][1]:
            break

        mergeAt(arr, stack, n)

We start from the top of the stack - 2. If n > 0 and the invariant doesn’t hold for stack[n - 1], stack[n], and stack[n + 1] or if n > 1 and the invariant doesn’t hold for stack[n - 2], stack[n - 1] and stack[n], we need to merge. We decide whether we want to merge stack[n] with stack[n + 1] or stack[n - 1] with stack[n] depending on which one is smallest (if stack[n - 1] is smaller, then we decrement n to trigger the merge at n - 1.

If the invariant holds, we check for the other invariant: stack[n][1] > stack[n + 1][1]. If this second invariant holds, we’re done and we can break out of the loop (we do the same if we ran out of elements). If not, we trigger a merge by calling mergeAt and repeat until we either merge everything or the invariant is reestablished.

We start by checking only the top few elements of the stack, since we expect the rest of the stack to hold the invariants. We only call this function when we push a new run on the stack, in which case we need to ensure we merge as needed.

Let’s take a look at mergeAt. This function simply merges the runs at positions n and n + 1 on the stack:

def mergeAt(arr, stack, i):
    assert i == len(stack) - 2 or i == len(stack) - 3

    base1, len1 = stack[i]
    base2, len2 = stack[i + 1]

    stack[i] = (base1, len1 + len2)

    if i == len(stack) - 3:
        stack[i + 1] = stack[i + 2]
    stack.pop()

    merge(arr, base1, base2, base2 + len2)

Remember we only ever merge either the second from top and top runs or the third from top and second from top runs. So i should be either len(stack) - 2 or len(stack) - 3. We get the first element and run length for the two runs and update the stack: stack[i] starts at the same position but will now have the length of both unmerged runs. If we are merging stack[-3] with stack[-2], we need to copy stack[-1] (top of the stack) to stack[-2] (second to top). Finally, we pop the top of the stack. At this point, the stack is updated. We call merge on the two runs to update arr too.

We can now maintain a healthy balance for merges. Remember, the whole reason for this is to aim to always merge runs similar in size.

Of course, once we are done pushing everything on the stack, we still need to force merging to finish our sort. We’ll do this with mergeForceCollapse:

def mergeForceCollapse(arr, stack):
    while len(stack) > 1:
        n = len(stack) - 2
        if n > 0 and stack[n - 1][1] < stack[n + 1][1]:
            n -= 1

        mergeAt(arr, stack, n)

This function again merges the second from the top run with the smallest of third from the top or top. It continues until all runs are merged into one. Our updates sort looks like this:

def sort(arr):
    lo, hi = 0, len(arr)
    stack = []
    nRemaining = hi
    minRun = MIN_MERGE

    while nRemaining > 0:
        runLen = countRunAndMakeAscending(arr, lo, hi)

        if runLen < minRun:
            force = min(nRemaining, minRun)
            binarySort(arr, lo, lo + force, lo + runLen)
            runLen = force

        stack.append((lo, runLen))
        mergeCollapse(arr, stack)

        lo += runLen
        nRemaining -= runLen

    mergeForceCollapse(arr, stack)

Instead of pushing everything onto the stack and merging everything at the end, we now call mergeCollapse after each push to keep the runs balanced. At the end, we call mergeForceCollapse to force-merge the stack.

Run Lengths

We used a constant minimum run length so far, but mentioned earlier that it is in fact determined as a function of the size of the input. We will determine this with minRunLength:

def minRunLength(n):
    r = 0
    while n >= MIN_MERGE:
        r |= n & 1
        n >>= 1
    return n + r

This function takes the length of the input and does the following:

  • If n is smaller than MIN_MERGE, returns n - the input size is too small to use complicated optimizations on.
  • If n is a power of 2, the algorithm will return MIN_MERGE / 2. Note: MIN_MERGE is also a power of 2. In our initial sketch we set it to 4, but in practice this is usually 32 or 64.
  • Otherwise return a number k between MIN_MERGE / 2 and MIN_MERGE so that n / k is close to but strictly less than a power of 2.

It does this by shifting n one bit to the right until it is less than MIN_MERGE. In case any shifted bit is 1, it means n is not a power of 2. In that case, we set r to 1 and return n + 1.

The reason we do all of this work is to again strive to keep merges balanced. If we get an input like 2048 and our MIN_MERGE is 64, we get back 32. That means that, if we don’t have any great runs in our input, we end up with 64 runs, each of length 32. We saw in the previous section how we balance the stack. Consider we’re pushing these runs onto the stack:

  • We push the run (0, 32) on the stack (first 32 elements).
  • We push the run (32, 32) on the stack (next 32 elements).
  • This triggers a merge since the run (0, 32) is not greater than the run (32, 32). The stack becomes (0, 64).
  • We push the run (64, 32) on the stack (next 32 elements).
  • We push the run (96, 32) on the stack (next 32 elements).
  • This again triggers a merge, since the length of the run (0, 64) (64) is not greater than the length of the next two runs, both of which are 32. The run (64, 32) gets merged with the smaller run, (96, 32). The stack becomes [(0, 64), (64, 64)].
  • The second invariant no longer holds: the first run is not longer than then next one. Another merged is triggered and the stack becomes [(0, 128)].

This goes on in the same fashion, and all merges end up being perfectly balanced. This works great for powers of 2.

Now let’s consider another case: what if the input is 2112? If we would still use 32 as our minimum run length, we would get 66 runs of length 32. The first 64 will trigger perfectly balanced merges as before, but then we end up with the stack [(0, 2048), (2048, 32), (2080, 32)]. This collapses to [(0, 2048), (2048, 64)], triggering a completely unbalanced merge (2048 on one side and 64 on the other).

To keep things balanced, if our input is not a power of 2, we pick a minimum run length that is close to but strictly less than a power of 2. Let’s update our MIN_MERGE to be 32, and update our sort to call minRunLength instead of automatically setting it to MIN_MERGE. We’ll throw in another quick optimization: if the whole input is smaller than MIN_MERGE, don’t even bother with the whole thing: find a starting run then binary sort the rest, without any merging.

MIN_MERGE = 32

def sort(arr):
    lo, hi = 0, len(arr)
    stack = []
    nRemaining = hi
    if nRemaining < MIN_MERGE:
        initRunLen = countRunAndMakeAscending(arr, lo, hi)
        binarySort(arr, lo, hi, lo + initRunLen)
        return
    minRun = minRunLength(len(arr))
    while nRemaining > 0:
        runLen = countRunAndMakeAscending(arr, lo, hi)
        if runLen < minRun:
            force = min(nRemaining, minRun)
            binarySort(arr, lo, lo + force, lo + runLen)
            runLen = force
        stack.append((lo, runLen))
        mergeCollapse(arr, stack)
        lo += runLen
        nRemaining -= runLen
    mergeForceCollapse(arr, stack)

Optimized Merging

We can optimize merging further. Our initial implementation of merge simply copied the first run into a buffer, then performed the merge. We can do better than that.

What if the second run is smaller? Maybe we’d prefer always merging the smaller run into the larger one. Let’s look at an optimized version of merge. First, we’ll replace merge with two functions, mergeLo and mergeHi. mergeLo will copy elements from the first run into the temporary buffer, while mergeHi will copy elements from the second run. Our original merge becomes mergeLo, and we can add a mergeHi:

def mergeHi(arr, lo, mid, hi):
    t = arr[mid:hi]
    i, j, k = hi - 1, mid - 1, hi - mid - 1
    while k >= 0 and j >= lo:
        if t[k] > arr[j]:
            arr[i] = t[k]
            k -= 1
        else:
            arr[i] = arr[j]
            j -= 1
        i -= 1

    if k >= 0:
        arr[lo:i + 1] = t[0:k + 1]

This is very similar with merge, except it copies the second (mid to hi) run into a temporary buffer and traverses the runs and the buffer from end to start.

When we trigger the merge, another optimization we can do is check elements from the first run and see if they are smaller than the first element in the second run. While they are smaller, we can simply ignore them when merging - they are already in position. We do this by taking the first element of the second run and seeing where it would fit in the first run.

Similarly, elements from the end of the second run which are greater than the last element in the first run are already in place. We don’t need to touch them. We take the last element of the first run and check where it would fit in the first run.

We can use binary search for this. Note that we need two version in order to maintain the stable property of the sort: a searchLeft, which returns the first index where a new element should be inserted, and a searchRight, which returns the last index. For example, if we have a run like [1, 2, 5, 5, 5, 5, 7, 8] and we are looking for where to insert another 5, it really depends where it comes from. If it comes from the run before this one, we need the left-most spot (before the first 5 in the run). On the other hand, if it comes from the run after this one, we need to place it after the last 5. That ensures that the relative order of elements is preserved. Here is an implementation for searchLeft and searchRight:

def searchLeft(key, arr, base, len):
    left, right = base, base + len
    while left < right:
        mid = left + (right - left) // 2
        if key > arr[mid]:
            left = mid + 1
        else:
            right = mid

    return left - base

def searchRight(key, arr, base, len):
    left, right = base, len

    while left < right:
        mid = left + (right - left) // 2
        if key < arr[mid]:
            right = mid
        else:
            left = mid + 1

    return left - base

Both functions return the offset from base where key should be inserted.

We can now update our mergeAt function with the new capabilities:

def mergeAt(arr, stack, i):
    base1, len1 = stack[i]
    base2, len2 = stack[i + 1]

    stack[i] = (base1, len1 + len2)
    if i == len(stack) - 3:
        stack[i + 1] = stack[i + 2]
    stack.pop()

    k = searchRight(arr[base2], arr, base1, len1)
    base1 += k
    len1 -= k
    if len1 == 0:
        return

    len2 = searchLeft(arr[base1 + len1 - 1], arr, base2, len2)
    if len2 == 0:
        return

    if len1 > len2:
        mergeLo(arr, base1, base2, base2 + len2)
    else:
        mergeHi(arr, base1, base2, base2 + len2)

The first part stays the same: we get base1, len1, base2, and len2 and update the stack. Next, instead of merging right away, we first search for where the first element of the second run would go into the first run. We know the elements in [base1, k) won’t move, so we can remove them from the merge by moving base1 to the right k elements (we also need to update len1). Similarly, we search for where the last element of the first run (arr[base1 + len1 - 1]) would fit into the second run. We know all elements beyond that are already in place, so we update len2 to be this offset.

In case either of the searches exhausts a run, we simply return. Otherwise, depending on which run is longer, we call mergeLo or mergeHi.

Galloping

But wait, there’s more! Binary search always performs log(len + 1) comparisons where len is the length of the array we are searching for regardless of where our element belongs. Galloping attempts to find the spot faster.

Galloping starts by comparing the element we are searching for in array A with A[0], A[1], A[3], … A[i^2 - 1]. With these comparisons, we will end up finding a range between some A[(k - 1)^2 - 1] and A[k^2 - 1] that would contain the element we are searching for. We then run a binary search only within that interval.

There are some tradeoffs here: on large datasets or purely random data, binary search performs better. But on inputs which contain natural runs, galloping tends to find things faster. Galloping also performs better when we expect to find the interval early on. Let’s look at an implementation of gallopLeft as an alternative to searchLeft:

def gallopLeft(key, arr, base, len, hint):
    lastOfs, ofs = 0, 1

    if key > arr[base + hint]:
        maxOfs = len - hint
        while ofs < maxOfs and key > arr[base + hint + ofs]:
            lastOfs = ofs
            ofs = (ofs << 1) + 1

        if ofs > maxOfs:
            ofs = maxOfs

        lastOfs += hint
        ofs += hint
    else: # key <= arr[base + hint]
        maxOfs = hint + 1
        while ofs < maxOfs and key <= arr[base + hint - ofs]:
            lastOfs = ofs
            ofs = (ofs << 1) + 1

        if ofs > maxOfs:
            ofs = maxOfs

        lastOfs, ofs = hint - ofs, hint - lastOfs

    # arr[base + lastOfs] < key <= arr[base + ofs]
    lastOfs += 1
    while lastOfs < ofs:
        mid = lastOfs + (ofs - lastOfs) // 2
        if key > arr[base + mid]:
            lastOfs = mid + 1
        else:
            ofs = mid
    return ofs

We start by initializing 2 offsets: lastOfs and ofs to represent the offsets between which we expect to find our key. Note the function also takes a hint, so callers can provide a tentative starting place.

Let’s go over the parts of this function:

if key > arr[base + hint]:
    maxOfs = len - hint
    while ofs < maxOfs and key > arr[base + hint + ofs]:
        lastOfs = ofs
        ofs = (ofs << 1) + 1

    if ofs > maxOfs:
        ofs = maxOfs

    lastOfs += hint
    ofs += hint

We first find the two offsets. If the key we are searching for is greater than (right of) our starting element (arr[base + hint]), then our maximum possible offset is len - hint. While ofs is hasn’t overflowed and the key is still larger than arr[base + hint + ofs], we keep updating ofs to be the next power of 2 minus 1. We keep track of the previous offset in lastOfs. Once we’re done, we add hint to both offsets (we do that because we add hint to all indices in our loop, but not to ofs since we keep it a power of 2 minus 1). If key > arr[base + hint] is not true, in other words, our key is left of our starting element:

else: # key <= arr[base + hint]
    maxOfs = hint + 1
    while ofs < maxOfs and key <= arr[base + hint - ofs]:
        lastOfs = ofs
        ofs = (ofs << 1) + 1

    if ofs > maxOfs:
        ofs = maxOfs

    lastOfs, ofs = hint - ofs, hint - lastOfs

In this case, our maximum possible offset is hint + 1. We gallop again, but now we are looking at elements left of our starting point, arr[base + hint - ofs] where ofs keeps increasing. Once we find the range, we update our offsets: lastOfs becomes hint - ofs and ofs becomes hint - lastOfs. The hint - `` part is again because that is what we actually used as indices. The swap is because we were moving left, and we need ``lastOfs to be the one on the left, ofs the one on the right.

We now identified the range within which we’ll find our key, between arr[base + lastOfs] and arr[base + ofs]. The last part of the function is just a binary search within this interval.

The gallopRight function is very similar to gallopLeft:

def gallopRight(key, arr, base, len, hint):
    ofs, lastOfs = 1, 0

    if key < arr[base + hint]:
        maxOfs = hint + 1
        while ofs < maxOfs and key < arr[base + hint - ofs]:
            lastOfs = ofs
            ofs = (ofs << 1) + 1

        if ofs > maxOfs:
            ofs = maxOfs
        lastOfs, ofs = hint - ofs, hint - lastOfs
    else:
        maxOfs = len - hint
        while ofs < maxOfs and key >= arr[base + hint + ofs]:
            lastOfs = ofs
            ofs = (ofs << 1) + 1

        if ofs > maxOfs:
            ofs = maxOfs

        lastOfs += hint;
        ofs += hint;

    lastOfs += 1
    while lastOfs < ofs:
        mid = lastOfs + ((ofs - lastOfs) // 2)
        if key < arr[base + mid]:
            ofs = mid
        else:
            lastOfs = mid + 1
    return ofs

We won’t cover this in details: the difference is here, like with searchRight, we want to find the rightmost index where key belongs instead of the leftmost one, so the algorithm changes accordingly.

The very neat thing about galloping is that its use isn’t limited to only when we set up the merge. We can also gallop while merging. Let’s go over mergeLo example, since mergeHi is a mirror of this.

In mergeLo, we first copy all elements from the first run to a buffer, then we iterate over the array and at each position we copy either an element from the buffer or one from the second run, depending on which one is smaller. While we do this, we can keep track of how many times the buffer or the second run “won”. If one of these wins consistently, we can assume it will keep winning for a while longer.

For example, if we merge [5, 6, 7, 8, 9] with [0, 1, 2, 3, 4], we initialize the buffer with [5, 6, 7, 8, 9], but for the next 5 comparisons, the second run wins (0 < 5, 1 < 5 …). Now imagine much longer runs. Instead of comparing all elements one by one, we switch to a galloping mode:

We find the last spot where the next element of the second run would fit into the buffer, and immediately copy the preceding elements of the buffer into the array. For example, if our buffer is [12, 13, 14, 15, 17] and the element we are considering from the second run is [16], we know we can copy [12, 13, 14, 15] into the array. Similarly, we find the first spot the next element in the buffer would fit into the remaining second run, and copy elements before that from the second run to their position. The galloping mode aims to reduce the number of comparisons and bulk copy data when possible (using a memcpy equivalent where available). While galloping, we still keep track of how many elements we were able to skip comparing individually. If this falls below the galloping threshold, we switch back to “regular” mode. Here is an updated mergeLo implementation:

MIN_GALLOP = 7
minGallop = MIN_GALLOP

def mergeLo(arr, lo, mid, hi):
    t = arr[lo:mid]
    i, j, k = lo, mid, 0
    global minGallop
    done = False

    while not done:
        count1, count2 = 0, 0
        while (count1 | count2) < minGallop:
            if t[k] < arr[j]:
                arr[i] = t[k]
                count1 += 1
                count2 = 0
                k += 1
            else:
                arr[i] = arr[j]
                count1 = 0
                count2 += 1
                j += 1
            i += 1

            if k == mid - lo or j == hi:
                done = True
                break

        if done:
            break

        while count1 >= MIN_GALLOP or count2 >= MIN_GALLOP:
            count1 = gallopRight(arr[j], t, k, mid - lo - k, 0)
            if count1 != 0:
                arr[i:i + count1] = t[k:k + count1]
                i += count1
                k += count1
                if k == mid - lo:
                    done = True
                    break

            arr[i] = arr[j]
            i += 1
            j += 1
            if j == hi:
                done = True
                break

            count2 = gallopLeft(t[k], arr, j, hi - j, 0)
            if count2 != 0:
                arr[i:i + count2] = arr[j:j + count2]
                i += count2
                j += count2
                if j == hi:
                    done = True
                    break

            arr[i] = t[k]
            i += 1
            k += 1
            if k == mid - lo:
                done = True
                break

            minGallop -= 1

        if minGallop < 0:
            minGallop = 0
        minGallop += 2

    if k < mid - lo:
        arr[i:hi] = t[k:mid - lo]

We introduced a new MIN_GALLOP constant which is the threshold after we want to start galloping. We also maintain a minGallop variable across merges.

We have a couple of nested while loops, but the idea is pretty straightforward. The first nested while does the normal merge but now keeps track of how many times in the row did we end up picking an element from the buffer:

count1, count2 = 0, 0
while (count1 | count2) < minGallop:
    if t[k] < arr[j]:
        arr[i] = t[k]
        count1 += 1
        count2 = 0
        k += 1
    else:
        arr[i] = arr[j]
        count1 = 0
        count2 += 1
        j += 1
    i += 1

    if k == mid - lo or j == hi:
        done = True
        break

if done:
    break

Whenever we increment one counter, we set the other to 0, so at any point, at most one of them is different than 0. We can exit the while loop in two ways: either one of the counters reaches the gallop threshold, or we run out of elements in one of the arrays.

If we ran out of elements we are done, so we break out of the outer loop. Otherwise we are in gallop mode:

while count1 >= MIN_GALLOP or count2 >= MIN_GALLOP:
    count1 = gallopRight(arr[j], t, k, mid - lo - k, 0)
    if count1 != 0:
        arr[i:i + count1] = t[k:k + count1]
        i += count1
        k += count1
        if k == mid - lo:
            done = True
            break

    arr[i] = arr[j]
    i += 1
    j += 1
    if j == hi:
        done = True
        break

    count2 = gallopLeft(t[k], arr, j, hi - j, 0)
    if count2 != 0:
        arr[i:i + count2] = arr[j:j + count2]
        i += count2
        j += count2
        if j == hi:
            done = True
            break

    arr[i] = t[k]
    i += 1
    k += 1
    if k == mid - lo:
        done = True
        break

    minGallop -= 1

if minGallop < 0:
    minGallop = 0
minGallop += 2

We first try to find where the next element in the second run would fit into the buffer. That becomes our count1. If we get an offset greater than 0, we can bulk copy the previous elements from the buffer ([k, k + count1)) to the range [i, i + count1) and increment both k and i by count1. Once we’re done, we know for sure we need to copy the next element from the second run (a[j]), so we do that.

We then do the opposite: gallop left to find where the next element from the buffer would fit into the second run. That becomes our count2 and if it is greater than 0, we bulk copy elements from the second run. Once we’re done, we again now that the next element to copy is at t[k], so we do that.

This loop repeats while either count1 or count2 is greater than MIN_GALLOP. If galloping works, we also update minGallop to favor future galloping. Each time we iterate, we decrement minGallop. Once we’re out of the loop, if it is due to both count1 and count2 being smaller than MIN_GALLOP, we again adjust minGallop - first, if it became negative, we make it 0. We then add 2 to penalize galloping because our last iteration didn’t meet MIN_GALLOP. As a reminder, minGallop is used as the threshold in the first loop. These tweaks to minGallop aim to optimize, depending on the data, when to enter gallop mode and when to keep merging in normal mode.

minGallop state should be maintained across multiple merges, and only reset when we start a new sort - so we would make minGallop = MIN_GALLOP in our main sort function, but otherwise rely on the same value we are updating in minGallop for subsequent calls of mergeLo and mergeHi. We made minGallop a global to keep the code (relatively) simpler. To avoid globals, we should either put all functions in a class and have minGallop be a member, or pass it through as an argument through all functions that need it.

Finally, we copy the remaining elements in the buffer, if any:

if k < mid - lo:
    arr[i:hi] = t[k:mid - lo]

We also have the mirrored mergeHi version:

def mergeHi(arr, lo, mid, hi):
    t = arr[mid:hi]
    i, j, k = hi - 1, mid - 1, hi - mid - 1
    global minGallop
    done = False

    while not done:
        count1, count2 = 0, 0
        while (count1 | count2) < minGallop:
            if t[k] > arr[j]:
                arr[i] = t[k]
                count1 += 1
                count2 = 0
                k -= 1
            else:
                arr[i] = arr[j]
                count1 = 0
                count2 += 1
                j -= 1
            i -= 1

            if k == -1 or j == lo - 1:
                done = True
                break

        if done:
            break

        while count1 >= MIN_GALLOP or count2 >= MIN_GALLOP:
            count1 = j - lo + 1 - gallopRight(t[k], arr, lo, j - lo + 1, j - lo)
            if count1 != 0:
                arr[i - count1 + 1:i + 1] = arr[j - count1 + 1:j + 1]
                i -= count1
                j -= count1
                if j == lo - 1:
                    done = True
                    break

            arr[i] = t[k]
            i -= 1
            k -= 1

            if k == -1:
                done = True
                break

            count2 = k + 1 - gallopLeft(arr[j], t, 0, k + 1, k)
            if count2 != 0:
                arr[i - count2 + 1:i + 1] = t[k - count2 + 1:k + 1]
                i -= count2
                k -= count2
                if k == -1:
                    done = True
                    break

            arr[i] = arr[j]
            i -= 1
            j -= 1
            if j == lo - 1:
                done = True
                break

            minGallop -= 1

        if minGallop < 0:
            minGallop = 0
        minGallop += 2

    if k >= 0:
        arr[lo:i + 1] = t[0:k + 1]

This is very similar to the previous one, so I won’t break it into pieces and explain, just note that since we are starting from the end of the range and we go backwards, we use closed ranges: i, j, and k always point to the last element of the range, not the one past the last.

Summary

This is a very efficient sorting algorithm which relies on observed properties of datasets in the real world. Quick recap:

  • Depending on the size of the input, we determine a good size for runs, so we can get balanced merges.
  • We traverse the array and identify runs. If the run is descending, we reverse it. If we don’t get enough elements in a run to hopefully get balanced merges, we extend the run by adding more elements and sorting them using binary sort.
  • We push runs on a stack which maintains a couple of invariants to, again, keep merges balanced: the second to top run of the stack must be longer than the top run and the third to top run must be longer than the sum of the second and top runs.
  • If an invariant is violated, we start merging until we reestablish it. We merge the second from the top run with the shortest of third from top or top (again aiming for balanced overall merging). Merges always merge consecutive runs.
  • Merge is optimized such that we first identify elements at the beginning of the first run and the end of the second run which are already in place, and we skip them.
  • Next, depending on which of the runs is larger, we merge either from left or from right.
  • Merge happens in two modes: we compare and merge normally, until we see one of the two runs we’re merging consistently gets picked. Once we pass a certain threshold, we switch to galloping mode.
  • Galloping aims to provide better performance than binary sort on smaller datasets, where we expect to find the position we’re searching for earlier rather than later in the search. Galloping tries to find a k such that the position we looking for is within A[(k - 1)^2] and A[k^2], then performs a binary search in the interval.
  • Merging in galloping mode tries to find a range of elements in the run that tends to win. This range can be bulk-copied in the merge portion of the array more efficiently and skipping extra comparisons.
  • If galloping becomes less effective, merge switches back to normal mode.
  • Another heuristic keeps track of how well galloping mode performs and either encourages or discourages entering galloping mode again. This is persisted across multiple merges in a single sort.

Thoughts

Is this sorting algorithm beautiful? Maybe not from a purely syntactical/readability perspective. Compare it with the recursive quicksort implementation in Haskell:

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
    let smallerSorted = quicksort [a | a <- xs, a <= x]
        biggerSorted = quicksort [a | a <- xs, a > x]
    in  smallerSorted ++ [x] ++ biggerSorted

Timsort is not a succinct algorithm. There are special cases, optimizations for left to right and right to left cases, galloping, which tries to beat binary search in some situations, multi-mode merges and so on.

That said, everything in it has one purpose: sort real world data efficiently. I find it beautiful for the amount of research that went into it, the major insight that real world data is usually partially sorted, and for how it adapts to various patterns in the data to improve efficiency.

Most real world software looks more like Timsort than the Haskell quicksort above. And while there is, unfortunately, way too much accidental complexity in the world of software, there is a limit to how much we can simplify before we can no longer model reality, or operate efficiently. And, ultimately, that is what matters.

References

The final version of the code in this blog post is in this GitHub gist (be advised: implementation might be buggy).

Tim Peters has a very detailed explanation of the algorithm and all optimizations in the Python codebase as listsort.txt. I do recommend reading this as it talks about all the research and benchmarks that went into developing Timsort.

The C implementation of Timsort in the Python codebase is listobject.c.

The Python implementation relies on a lot of Python runtime constructs, so it might be harder to read. My implementation is derived from the OpenJDK implementation which I found very readable. That one is here on GitHub.