mapreduce-sample.js

const fs = require('fs')

class KeyValue {
  constructor(key, value) {
    this.key = key
    this.value = value
  }
}

const mapF = (fileName, contents) =>
  contents.split(/[^A-Za-z]/).filter(a => a.length > 0).map((word) => new KeyValue(word, 1))

const reduceF = (key, values) => {return {[key]: values.reduce((sum, v) => sum + v)}}

const sort = (a, b) => {
  if (a.key < b.key) {
    return -1;
  } else if (a.key > b.key) {
    return 1;
  } else {
    return 0;
  }
}

const start = () => {
  // './contents/pg*.txt'

  ["./contents/pg-being_ernest.txt", "./contents/pg-dorian_gray.txt", "./contents/pg-frankenstein.txt"]

  // map
  let intermediate = []
  fileNames.forEach((name, _) => {
    contents = fs.readFileSync(name, 'utf8');
    const results = mapF(name, contents)
    intermediate.push.apply(intermediate, results)
  });

  // shuffle
  intermediate.sort(sort)

  // reduce
  let i = 0
  let results = []
  while (i < intermediate.length) {
    let j = i
    let values = []
    while (j < intermediate.length && intermediate[i].key == intermediate[j].key) {
      values.push(intermediate[j].value)

      j = j + 1
    }

    const reduceResult = reduceF(intermediate[i].key, values)
    results.push(reduceResult)
    i = j
  }

  console.log(results);
}

start()