Implementing Block Search in JavaScript

Block Search

Just like Binary Search, Block Search is also a searching algorithm for sorted arrays. The basic idea is to check fewer elements (than linear search) by jumping ahead by fixed steps or skipping some elements in place of searching all elements.

For example

Suppose we have an array arr of length n and block (to be jumped) of size m. Then we search at the indexes arr[0], arr[m], arr[2 * m], ..., arr[k * m] and so on.

Once we find the interval arr[k * m] < x < arr[(k+1) * m], we perform a linear search operation from the index k * m to find the element x.

The time complexity of this algorithm is −

O(√n)

Example

Following is the code −

const arr = [1, 4, 6, 7, 9, 12, 15, 16, 17, 23, 25, 26, 27, 31];
const target = 25;
const blockSearch = (arr = [], target) => {
   let { length: len } = arr;
   let step = Math.floor(Math.sqrt(len));
   let blockStart = 0
   let currentStep = step;
   while (arr[Math.min(currentStep, len) - 1] < target) {
      blockStart = currentStep;
      currentStep += step;
      if (blockStart >= len)
         return -1;
   }
   while (arr[blockStart] < target){
      blockStart++;
      if (blockStart == Math.min(currentStep, len))
         return -1;
   }
   if (arr[blockStart] == target)
      return blockStart
   else
      return -1;
};
console.log(blockSearch(arr, target));

Output

Following is the output on console −

10