Speed up calculating next run time of cron

lib/perfectsched.rb

@@ -52,7 +52,7 @@ module PerfectSched
     require File.expand_path(v, File.dirname(__FILE__))
   }
 
-  require 'cron-spec'
+  require 'chrono'
   require 'tzinfo'
 
   def self.open(config, &block)
@@ -71,28 +71,13 @@ def self.open(config, &block)
   end
 
   def self.cron_time(cron, timestamp, timezone)
-    begin
-      tab = CronSpec::CronSpecification.new(cron)
-    rescue
-      raise ArgumentError, "invalid cron format: #{$!}: #{cron.inspect}"
-    end
-
-    begin
-      tz = TZInfo::Timezone.get(timezone) if timezone
-    rescue
-      raise ArgumentError, "unknown timezone: #{$!}: #{timezone.inspect}"
+    unless /\A\s*(?:[0-9,*,\-\/]+\s+){4}[0-9,*,\-\/]+\z/ =~ cron
+      raise ArgumentError, "invalid cron format: #{cron.inspect}"
     end
 
-    ts = (timestamp + 59) / 60 * 60
-    while true
-      t = Time.at(ts).utc
-      t = tz.utc_to_local(t) if tz
-      if tab.is_specification_in_effect?(t)
-        return ts
-      end
-      ts += 60
-      # FIXME break
-    end
+    ts = timestamp - 1 # compatibility
+    t = Time.find_zone!(timezone || 'UTC'.freeze).at(ts)
+    Chrono::NextTime.new(now: t, source: cron).to_time.to_i
   end
 
   def self.next_time(cron, before_timestamp, timezone)

perfectsched.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.executables = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   gem.require_paths = ['lib']
 
-  gem.add_dependency "cron-spec", [">= 0.1.2", "<= 0.1.2"]
+  gem.add_dependency "chrono", "~> 0.1.0"
   gem.add_dependency "sequel", "~> 3.48.0"
   gem.add_dependency "tzinfo", "~> 1.1"
   gem.add_dependency "perfectqueue", "~> 0.8.41"

spec/perfect_sched_spec.rb

@@ -39,7 +39,7 @@
       ts = PerfectSched.next_time('0 * * * *', 0, nil)
       expect(ts).not_to be_nil
     end
-    xit 'calculates 4 years quickly' do
+    it 'calculates 4 years quickly' do
       t = Time.utc(2012,2,29)
       ts = PerfectSched.next_time('0 0 29 2 *', t.to_i, nil)
       expect(ts).to eq(Time.utc(2016,2,29).to_i)