How to sort nearly sorted array in the fastest time possible? (Java)

Question

I have an array of values which is almost, but not quite sorted, with a few values displaced (say, 50 in 100000). How to sort it most efficiently? (performance is absolutely crucial here and should be way faster than O(N)).

I know about smoothsort, but I can't find Java implementation. Does anyone know whether it is already implemented? Or what I can use for this task instead of smoothsort?

Answer 1

Actually, the Wikipedia contains a Java implementation of smoothsort. You can find it here:

http://en.wikipedia.org/wiki/Smoothsort.

Answer 2

Cocktail Sort

If you want a simple algorithm that's easy to implement, you could do a cocktail sort. It would work reasonably well on nearly-sorted input.

Answer 3

As Botz3000 noted, you can't perform such an operation faster than O(N). The most basic element of any algorithm would be to find those entries in the array that are out of order. This requires O(N), even before you figure out what to do with them.

If indeed the number of "out-of-order" elements is orders of magnitude below the total number of elements, you could use the following algorithm (assuming linked list):

1. Find all out-of-order items and extract the from the original list to a separate list, O(N)
2. The result is two lists: a sorted list and a short extracted list
3. For each of the extracted elements, insert them into the sorted list. That would be O(log(N)) for each, total is O(Xlog(N)), where X is the number of the extracted elements. If X is very small relative to N, you end up with a total of O(N).

Answer 4

There are many good algorithms for this.

Smoothsort is my personal favorite... I actually worked all the math out here if you're curious why it works so well.

A fairly good algorithm for already-sorted data is natural mergesort, which is a bottom-up version of mergesort that works by treating the input as a sequence of sorted subranges, then making multiple passes over the range merging adjacent sorted ranges. It runs in O(n) time if the data is already sorted (because it can detect that there's only one sorted range), and O(n lg n) in the worst case. This algorithm works quite well if the data is "block sorted"; that is, it consists of a lot of sorted blocks placed right next to one another.

Straight insertion sort definitely works well for mostly-sorted data, but can degenerate very badly on a lot of inputs. Some really good sorts (like introsort) actually use this property of insertion sort to do a "cleanup step" on the input.

Answer 5

[Sun] JDK7 has (or will have) an implementation of Tim sort (from Python). It's a merge sort that takes advantage of order already existing in the array.

Answer 6

Smoothsort or Timsort are great algorithms, and would be sensible things to use.

I'd add that what you might not realise is that the humble insertion sort is adaptive. Indeed, for really almost sorted lists, as you seem to have, my understanding (which i can't back up with a reference) is that it's faster than the more sophisticated algorithms. The problem is that if the input isn't almost-sorted, it rapidly degrades to O(n^2). Still, it's very simple to implement correctly, so if you know for sure that your input is always almost-sorted, it would be a good choice.

Answer 7

Just to put it on the table, a well implemented bubble-sort would certainly be the simplest algorithm here. With a worst-case of O(n*m), m being the number of displacements. The m part depends heavily on the pattern of displacements, usually total complexity would be O(n).

Answer 8

You are right about the impossibility of reaching O(N), but assuming a multi-core machine (which I have), we can cheat a little by using a parallel sorting algorithm.

Answer 9

Implement what we called in school a Shell's sort. That's bubblesorting sub-arrays. A sub-array with step k is an array of elements with indicies 0, k, 2k, 3k...

If you choose k = 3i+1, and perform multiple bubble sorts, starting from higher i-s downto 0, the times will be smaller on nearly-sorted array.

Answer 10

This is the original Java implementation of Smoothsort that used to be available via the Wikipedia article.

// by keeping these constants, we can avoid the tiresome business
// of keeping track of Dijkstra's b and c. Instead of keeping
// b and c, I will keep an index into this array.

static final int LP[] = { 1, 1, 3, 5, 9, 15, 25, 41, 67, 109,
    177, 287, 465, 753, 1219, 1973, 3193, 5167, 8361, 13529, 21891,
    35421, 57313, 92735, 150049, 242785, 392835, 635621, 1028457,
    1664079, 2692537, 4356617, 7049155, 11405773, 18454929, 29860703,
    48315633, 78176337, 126491971, 204668309, 331160281, 535828591,
    866988873 // the next number is > 31 bits.
};

public static <C extends Comparable<? super C>> void sort(C[] m,
    int lo, int hi) {
  int head = lo; // the offset of the first element of the prefix into m

  // These variables need a little explaining. If our string of heaps
  // is of length 38, then the heaps will be of size 25+9+3+1, which are
  // Leonardo numbers 6, 4, 2, 1. 
  // Turning this into a binary number, we get b01010110 = 0x56. We represent
  // this number as a pair of numbers by right-shifting all the zeros and 
  // storing the mantissa and exponent as "p" and "pshift".
  // This is handy, because the exponent is the index into L[] giving the
  // size of the rightmost heap, and because we can instantly find out if
  // the rightmost two heaps are consecutive Leonardo numbers by checking
  // (p&3)==3

  int p = 1; // the bitmap of the current standard concatenation >> pshift
  int pshift = 1;

  while (head < hi) {
    if ((p & 3) == 3) {
      // Add 1 by merging the first two blocks into a larger one.
      // The next Leonardo number is one bigger.
      sift(m, pshift, head);
      p >>>= 2;
      pshift += 2;
    } else {
      // adding a new block of length 1
      if (LP[pshift - 1] >= hi - head) {
        // this block is its final size.
        trinkle(m, p, pshift, head, false);
      } else {
        // this block will get merged. Just make it trusty.
        sift(m, pshift, head);
      }

      if (pshift == 1) {
        // LP[1] is being used, so we add use LP[0]
        p <<= 1;
        pshift--;
      } else {
        // shift out to position 1, add LP[1]
        p <<= (pshift - 1);
        pshift = 1;
      }
    }
    p |= 1;
    head++;
  }

  trinkle(m, p, pshift, head, false);

  while (pshift != 1 || p != 1) {
    if (pshift <= 1) {
      // block of length 1. No fiddling needed
      int trail = Integer.numberOfTrailingZeros(p & ~1);
      p >>>= trail;
      pshift += trail;
    } else {
      p <<= 2;
      p ^= 7;
      pshift -= 2;

      // This block gets broken into three bits. The rightmost
      // bit is a block of length 1. The left hand part is split into
      // two, a block of length LP[pshift+1] and one of LP[pshift].
      // Both these two are appropriately heapified, but the root
      // nodes are not necessarily in order. We therefore semitrinkle
      // both of them

      trinkle(m, p >>> 1, pshift + 1, head - LP[pshift] - 1, true);
      trinkle(m, p, pshift, head - 1, true);
    }

    head--;
  }
}

private static <C extends Comparable<? super C>> void sift(C[] m, int pshift,
    int head) {
  // we do not use Floyd's improvements to the heapsort sift, because we
  // are not doing what heapsort does - always moving nodes from near
  // the bottom of the tree to the root.

  C val = m[head];

  while (pshift > 1) {
    int rt = head - 1;
    int lf = head - 1 - LP[pshift - 2];

    if (val.compareTo(m[lf]) >= 0 && val.compareTo(m[rt]) >= 0)
      break;
    if (m[lf].compareTo(m[rt]) >= 0) {
      m[head] = m[lf];
      head = lf;
      pshift -= 1;
    } else {
      m[head] = m[rt];
      head = rt;
      pshift -= 2;
    }
  }

  m[head] = val;
}

private static <C extends Comparable<? super C>> void trinkle(C[] m, int p,
    int pshift, int head, boolean isTrusty) {

  C val = m[head];

  while (p != 1) {
    int stepson = head - LP[pshift];

    if (m[stepson].compareTo(val) <= 0)
      break; // current node is greater than head. Sift.

    // no need to check this if we know the current node is trusty,
    // because we just checked the head (which is val, in the first
    // iteration)
    if (!isTrusty && pshift > 1) {
      int rt = head - 1;
      int lf = head - 1 - LP[pshift - 2];
      if (m[rt].compareTo(m[stepson]) >= 0
          || m[lf].compareTo(m[stepson]) >= 0)
        break;
    }

    m[head] = m[stepson];

    head = stepson;
    int trail = Integer.numberOfTrailingZeros(p & ~1);
    p >>>= trail;
    pshift += trail;
    isTrusty = false;
  }

  if (!isTrusty) {
    m[head] = val;
    sift(m, pshift, head);
  }
}