LocalMaximumPoint.js

/**
 * [LocalMaxima](https://www.geeksforgeeks.org/find-indices-of-all-local-maxima-and-local-minima-in-an-array/) is an algorithm to find relative bigger numbers compared to their neighbors
 *
 * Notes:
 * - works by using divide and conquer
 * - the function gets the array A with n Real numbers and returns the index of local max point (if more than one exists return the first one)
 *
 * @complexity: O(log(n)) (on average )
 * @complexity: O(log(n)) (worst case)
 * @flow
 */
const findMaxPointIndex = (
 array,
 rangeStartIndex,
 rangeEndIndex,
 originalLength
) => {
 // find index range middle point
 const middleIndex =
 rangeStartIndex + parseInt((rangeEndIndex - rangeStartIndex) / 2)

 // handle array bounds
 if (
 (middleIndex === 0 || array[middleIndex - 1] <= array[middleIndex]) &&
 (middleIndex === originalLength - 1 ||
 array[middleIndex + 1] <= array[middleIndex])
 ) {
 return middleIndex
 } else if (middleIndex > 0 && array[middleIndex - 1] > array[middleIndex]) {
 return findMaxPointIndex(
 array,
 rangeStartIndex,
 middleIndex - 1,
 originalLength
 )
 } else {
 // regular local max
 return findMaxPointIndex(
 array,
 middleIndex + 1,
 rangeEndIndex,
 originalLength
 )
 }
}

const LocalMaximumPoint = (A) => findMaxPointIndex(A, 0, A.length - 1, A.length)

export { LocalMaximumPoint }