Support time discretization in scripts

This can be used to ensure parameter refresh times are aligned (as
opposed to using Date.now() - X). It also allows for possible future
optimizations by exploiting the fact that the value of Date.now(X)
remains static for a given amount of time.

The variance value can be used to shift the discretization threshold
points based on the hash of the device ID for load distribution.

The djb2 hashing algorithm has been replaced with MD5 for better
distribution.

lib/scheduling.js

@@ -0,0 +1,78 @@
+/**
+ * Copyright 2013-2017  Zaid Abdulla
+ *
+ * This file is part of GenieACS.
+ *
+ * GenieACS is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * GenieACS is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with GenieACS.  If not, see <http://www.gnu.org/licenses/>.
+ */
+"use strict";
+
+const crypto = require("crypto");
+
+const later = require("later");
+
+
+function md532(str) {
+  const digest = crypto
+    .createHash("md5")
+    .update(str)
+    .digest();
+  return (
+    digest.readUInt32LE(0) ^
+    digest.readUInt32LE(4) ^
+    digest.readUInt32LE(8) ^
+    digest.readUInt32LE(12)
+  );
+}
+
+
+function variance(deviceId, variance) {
+  return (md532(deviceId) >>> 0) % variance;
+}
+
+
+function interval(timestamp, interval, offset = 0) {
+  return Math.trunc((timestamp + offset) / interval) * interval - offset;
+}
+
+
+function parseCron(cronExp) {
+  let parts = cronExp.trim().split(/\s+/);
+  if (parts.length === 5)
+    parts.unshift("*");
+
+  return later.schedule(later.parse.cron(parts.join(" "), true));
+}
+
+
+function cron(timestamp, schedule, offset = 0) {
+  // TODO later.js doesn't throw erorr if expression is invalid!
+  const ret = [0, 0];
+
+  let prev = schedule.prev(1, new Date(timestamp + offset));
+  if (prev)
+    ret[0] = prev.setMilliseconds(0) - offset;
+
+  let next = schedule.next(1, new Date(timestamp + offset + 1000));
+  if (next)
+    ret[1] = next.setMilliseconds(0) - offset;
+
+  return ret;
+}
+
+
+exports.variance = variance;
+exports.interval = interval;
+exports.parseCron = parseCron;
+exports.cron = cron;