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'use strict';

function falsyValue(item) {
  if (Array.isArray(item)) {
    if (item.length) {
      return false;
    }
  } else if (item !== undefined && item !== null
    && typeof item === 'object') {
    if (Object.keys(item).length) {
      return false;
    }
  } else if (item) {
    return false;
  }

  return true;
}

function filterObject(func, items) {
  const result = {};
  Object.keys(items).forEach((key) => {
    if (func) {
      if (func(items[key], key)) {
        result[key] = items[key];
      }
    } else if (!falsyValue(items[key])) {
      result[key] = items[key];
    }
  });

  return result;
}

function filterArray(func, items) {
  if (func) {
    return items.filter(func);
  }
  const result = [];
  for (let i = 0; i < items.length; i += 1) {
    const item = items[i];
    if (!falsyValue(item)) {
      result.push(item);
    }
  }

  return result;
}

module.exports = function filter(fn) {
  const func = fn || false;
  let filteredItems = null;
  if (Array.isArray(this.items)) {
    filteredItems = filterArray(func, this.items);
  } else {
    filteredItems = filterObject(func, this.items);
  }

  return new this.constructor(filteredItems);
};