Switch to async.mapLimit (#170)

This will use the number of threads -1 which should significantly speed things up with a huge list of files.

Author: XhmikosR

lib/htmllint.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const os = require('os');
 const path = require('path');
 const { execFile } = require('child_process');
 const async = require('async');
@@ -15,6 +16,10 @@ const MAX_CHARS = 5000;
 // https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback
 const MAX_BUFFER = 20_000 * 1024;
 
+// eslint-disable-next-line unicorn/explicit-length-check
+const CPUS = os.cpus() && os.cpus().length;
+const THREADS = CPUS > 2 ? CPUS - 1 : 1;
+
 // Replace left/right quotation marks with normal quotation marks
 function normalizeQuotationMarks(string) {
   if (string) {
@@ -132,7 +137,7 @@ function htmllint(config, done) {
     const files = config.files.map(file => path.normalize(file));
     const chunks = chunkify(files, MAX_CHARS);
 
-    async.mapSeries(chunks, (chunk, cb) => {
+    async.mapLimit(chunks, THREADS, (chunk, cb) => {
       const args = javaArgs(java, chunk, config);
 
       execFile('java', args, { maxBuffer: MAX_BUFFER, shell: true }, (error, stdout, stderr) => {