Duplicates

duplicates.js

const not = fn => (...args) => !fn(...args);

const notEqual = (v1, v2) => !Object.is(v1, v2);

const withDuplicates = (filterIndex, arr, isEqual = Object.is) => arr.filter(
  (filteredValue, filteredIndex) => filterIndex(
    filteredIndex,
    arr.findIndex(curValue => isEqual(curValue, filteredValue)) 
  )
);

const getDuplicates = (arr, isEqual) => withDuplicates(notEqual, arr, isEqual);

const getUniques = (arr, isEqual) => withDuplicates(Object.is, arr, isEqual);

const isFirstOrNotEqPrev = (arr, compareFn, value, index) =>
  index === 0 || compareFn(value, arr[index - 1]) !== 0;

const isNotLastAndEqNext = (arr, compareFn, value, index) =>
  index < arr.length - 1 && compareFn(value, arr[index + 1]) === 0;

const getDuplicatesFilter = (arr, compareFn) => (value, index) =>
  isFirstOrNotEqPrev(arr, compareFn, value, index)
  && isNotLastAndEqNext(arr, compareFn, value, index);

const getUniquesFilter = (arr, compareFn) =>
    not(getDuplicatesFilter(arr, compareFn));

const getDuplicatesFaster = (arr, compareFn) => (
  arr.length < 2 ?
    [] :
    arr
      .sort(compareFn)
      .filter(getDuplicatesFilter(arr, compareFn))
);

const getUniquesFaster = (arr, compareFn) => (
  arr.length < 2 ?
    Array.from(arr) :
    arr
      .sort(compareFn)
      .filter(getUniquesFilter(arr, compareFn))
);