What is the fastest search method for a sorted array?

Question

Answering to another question, I wrote the program below to compare different search methods in a sorted array. Basically I compared two implementations of Interpolation search

Answer 1

One way of approaching this is to use a space versus time trade-off. There are any number of ways that could be done. The extreme way would be to simply make an array with the max size being the max value of the sorted array. Initialize each position with the index into sortedArray. Then the search would simply be O(1).

The following version, however, might be a little more realistic and possibly be useful in the real world. It uses a "helper" structure that is initialized on the first call. It maps the search space down to a smaller space by dividing by a number that I pulled out of the air without much testing. It stores the index of the lower bound for a group of values in sortedArray into the helper map. The actual search divides the toFind number by the chosen divisor and extracts the narrowed bounds of sortedArray for a normal binary search.

For example, if the sorted values range from 1 to 1000 and the divisor is 100, then the lookup array might contain 10 "sections". To search for value 250, it would divide it by 100 to yield integer index position 250/100=2. map[2] would contain the sortedArray index for values 200 and larger. map[3] would have the index position of values 300 and larger thus providing a smaller bounding position for a normal binary search. The rest of the function is then an exact copy of your binary search function.

The initialization of the helper map might be more efficient by using a binary search to fill in the positions rather than a simple scan, but it is a one time cost so I didn't bother testing that. This mechanism works well for the given test numbers which are evenly distributed. As written, it would not be as good if the distribution was not even. I think this method could be used with floating point search values too. However, extrapolating it to generic search keys might be harder. For example, I am unsure what the method would be for character data keys. It would need some kind of O(1) lookup/hash that mapped to a specific array position to find the index bounds. It's unclear to me at the moment what that function would be or if it exists.

I kludged the setup of the helper map in the following implementation pretty quickly. It is not pretty and I'm not 100% sure it is correct in all cases but it does show the idea. I ran it with a debug test to compare the results against your existing binarySearch function to be somewhat sure it works correctly.

The following are example numbers:

100000 * 10000 : cycles binary search          = 10197811
100000 * 10000 : cycles interpolation uint64_t = 9007939
100000 * 10000 : cycles interpolation float    = 8386879
100000 * 10000 : cycles binary w/helper        = 6462534

Here is the quick-and-dirty implementation:

#define REDUCTION 100  // pulled out of the air
typedef struct {
    int init;  // have we initialized it?
    int numSections;
    int *map;
    int divisor;
} binhelp;

int binarySearchHelp( binhelp *phelp, int sortedArray[], int toFind, int len)
{
    // Returns index of toFind in sortedArray, or -1 if not found
    int low;
    int high;
    int mid;

    if ( !phelp->init && len > REDUCTION ) {
        int i;
        int numSections = len / REDUCTION;
        int divisor = (( sortedArray[len-1] - 1 ) / numSections ) + 1;
        int threshold;
        int arrayPos;

        phelp->init = 1;
        phelp->divisor = divisor;
        phelp->numSections = numSections;
        phelp->map = (int*)malloc((numSections+2) * sizeof(int));
        phelp->map[0] = 0;
        phelp->map[numSections+1] = len-1;
        arrayPos = 0;
        // Scan through the array and set up the mapping positions.  Simple linear
        // scan but it is a one-time cost.
        for ( i = 1; i <= numSections; i++ ) {
            threshold = i * divisor;
            while ( arrayPos < len && sortedArray[arrayPos] < threshold )
                arrayPos++;
            if ( arrayPos < len )
                phelp->map[i] = arrayPos;
            else
                // kludge to take care of aliasing
                phelp->map[i] = len - 1;
        }
    }

    if ( phelp->init ) {
        int section = toFind / phelp->divisor;
        if ( section > phelp->numSections )
            // it is bigger than all values
            return -1;

        low = phelp->map[section];
        if ( section == phelp->numSections )
            high = len - 1;
        else
            high = phelp->map[section+1];
    } else {
        // use normal start points
        low = 0;
        high = len - 1;
    }

    // the following is a direct copy of the Kriss' binarySearch
    int l = sortedArray[low];
    int h = sortedArray[high];

    while (l <= toFind && h >= toFind) {
        mid = (low + high)/2;

        int m = sortedArray[mid];

        if (m < toFind) {
            l = sortedArray[low = mid + 1];
        } else if (m > toFind) {
            h = sortedArray[high = mid - 1];
        } else {
            return mid;
        }
    }

    if (sortedArray[low] == toFind)
        return low;
    else
        return -1; // Not found
}

The helper structure needs to be initialized (and memory freed):

    help.init = 0;
    unsigned long long totalcycles4 = 0;
    ... make the calls same as for the other ones but pass the structure ...
        binarySearchHelp(&help, arr,searched[j],length);
    if ( help.init )
        free( help.map );
    help.init = 0;