57-pick-peaks.js

// In this kata, you will write a function that returns the positions and
// the values of the "peaks" (or local maxima) of a numeric array.

// For example, the array arr = [0, 1, 2, 5, 1, 0] has a peak at position 3
// with a value of 5 (since arr[3] equals 5).

// The output will be returned as an object with two properties: pos and peaks.
// Both of these properties should be arrays. If there is no peak in the given array,
// then the output should be {pos: [], peaks: []}.

// Example: pickPeaks([3, 2, 3, 6, 4, 1, 2, 3, 2, 1, 2, 3]) should return
// {pos: [3, 7], peaks: [6, 3]} (or equivalent in other languages)

// All input arrays will be valid integer arrays (although it could still be empty),
// so you won't need to validate the input.

// The first and last elements of the array will not be considered as peaks
// (in the context of a mathematical function, we don't know what is after
// and before and therefore, we don't know if it is a peak or not).

// Also, beware of plateaus !!! [1, 2, 2, 2, 1] has a peak while [1, 2, 2, 2, 3] does not.
// In case of a plateau-peak, please only return the position and value of the beginning
// of the plateau. For example: pickPeaks([1, 2, 2, 2, 1]) returns {pos: [1], peaks: [2]}
// (or equivalent in other languages)

export default arr => arr.reduce((acc, e, i) => {
  if (i === 0 || i === arr.length - 1) return acc;
  if ((e > arr[i + 1] || e > arr.slice(i).find(e1 => e1 !== e)) && e > arr[i - 1]) {
    return { pos: [...acc.pos, i], peaks: [...acc.peaks, e] };
  }
  return acc;
}, { pos: [], peaks: [] });