const map = fn => arr => (Array.isArray(arr)
  ? arr.map(fn)
  : pipe(
    Object.keys,
    map(key => ({ [key]: fn(arr[key]) })),
    mergeAll,
  )(arr));

const join = str => arr => arr.join(str);
const concat = val => arr => arr.concat(val);
const filter = fn => arr => arr.filter(fn);
const values = obj => map(k => obj[k])(Object.keys(obj));
const reduce = (fn, val) => (arr) => {
  if (!arr.length) return val;

  return (val === null || val === undefined)
    ? arr.reduce(fn)
    : arr.reduce(fn, val);
};

const flatten = reduce(
  (acc, cur) => (Array.isArray(cur)
    ? compose(concat, flatten)(cur)(acc)
    : concat(cur)(acc)),
  [],
);

const merge = (acc, cur) => ({ ...acc, ...cur });
const mergeAll = reduce(merge, {});

/**
 * 从左向右结合函数
 * @param {Function[]} funcs 函数数组
 */
const pipe = (...funcs) => {
  if (funcs.length === 0) return arg => arg;
  if (funcs.length === 1) return funcs[0];

  return funcs.reduce((a, b) => (...args) => b(a(...args)));
};

/**
 * 从右向左结合函数
 * @param {Function[]} funcs 函数数组
 */
const compose = (...funcs) => {
  if (funcs.length === 0) return arg => arg;
  if (funcs.length === 1) return funcs[0];

  return funcs.reduce((a, b) => (...args) => a(b(...args)));
};

export {
  map,
  join,
  pipe,
  merge,
  concat,
  reduce,
  filter,
  values,
  compose,
  flatten,
  mergeAll,
};