import { memoize } from 'lodash'

const letter = /[a-z]/i
const isLetter = x => letter.test(x)

function getLetter(name: string) {
  let letter = '0-9'
  if (name) {
    letter = name[0].toLowerCase()
    if (!isLetter(name[0])) {
      letter = '0-9'
    }
  }
  return letter.toUpperCase()
}

export const groupByFirstLetter = memoize((key: string = 'name') => {
  return function groupByLetter<A>(item: A, index: number, items: A[]): { separator?: string } {
    if (items[index - 1]) {
      const lastLetter = getLetter(items[index - 1][key])
      const thisLetter = getLetter(item[key])
      if (thisLetter !== lastLetter) {
        return {
          separator: thisLetter,
        }
      }
    } else {
      // first index
      return {
        separator: getLetter(item[key]),
      }
    }
    return item
  }
})