From: Ruby Quiz Date: 2006-02-17T01:05:54+09:00 Subject: [SUMMARY] FasterGenerator (#66) There were two kinds of solutions to this problem. The first were Array-based solutions. The logic here is that we can do the iteration, shove all the results in an Array, and then hand them out one-at-a-time. Here is Christoffer Lerno's solution, doing exactly that: class MyGenerator attr_reader :index def initialize(enum = nil) if enum then @array = enum.to_a else @array = Array.new yield self end @index = 0 end def current raise EOFError unless next? @array[@index] end def next value = current @index += 1 return value end def next? return @index < @array.length end def rewind @index = 0 self end def each(&block) @array.each(&block) end def yield(value) @array << value end def pos return @index end def end? return !next? end end Even though Generator supposedly works off of the each() method, it claims to accept an Enumerable argument. If we go on that assumption, there should be a to_a() method which will return all the objects each() iteration would. (If you are uncomfortable with this logic, you could iterate and add them to an Array by hand.) You can see Christoffer putting this shortcut to work in initialize(), filling @array with a simple call to to_a(). The rest of the methods are trivial. An @index is saved pointing into the @array, which can be incremented or reset to walk the objects. The good news: this is wicked fast in comparison to the old or even the new Generator library. The bad news: it doesn't work for all cases. First, an iterator does not have to be finite, and trying to shove something like a lazily evaluated infinite stream in an Array is going to go poorly. Furthermore, iteration may be sensitive to external factors like when it is executed. In those cases, running the full iteration up front may produce different results. For these reasons the standard Generator library requires a more general solution. However, I would still tuck the Array-and-index trick away in the back of your mind, as it may just come in handy some day. If you can be sure you aren't dealing with any of the edge cases mentioned above and sure you can spare the memory, using an Array and an index is simple and fast. For all other times, you have the standard Generator library. We know that the library use to work with continuations, but now it has a new implementation. The new technique was the other one used by the solutions. It goes like this: launch a Thread to iterate over the elements, stop the Thread just before each iteration, and wake it up each time you need a new item. Let's see how Jacob Fugal implemented that (with a small patch from Ross Bamford). Here's the setup: require 'thread' class FasterGenerator def initialize( enum=nil, &block ) @position = 0 @values = [] @done = false @mutex = Mutex.new @block = block if enum @block = proc{ |g| enum.each{ |x| g.yield x } } end end # ... This almost looks like the Array-and-index trick again, when you see @position and @values, but this version isn't going to slurp the values all at once. @done is just a flag to tell us when we have exhausted the values and @mutex is a locking tool from the thread library. Now @block is a little more interesting. Generator can be called in two ways, either with an Enumerable object, or with a block that yield()s values to the Generator. The code above merges the two cases, by providing a block that yield()s through each() when the Enumerable object is given. That means the next piece of the puzzle is the yield() method: # ... def yield( x ) @mutex.synchronize{ @values << x } Thread.stop end # ... We're starting to see threading work here. This method grabs the lock, adds the object to @values, drops the lock, and halts the Thread that fetched the object. The lock is to ensure that multiple Threads don't stomp on the internal @values queue at the same time and the halting is the pause before each iteration I mentioned earlier. So, let's see see what current() and next() look like: # ... def current collect_value raise EOFError if eof? @values[@position] end # ... def next x = current @position += 1 x end # ... Seems like all the magic here is hidden in the collect_value() method and everything else is just indexing. Here's the magic method: # ... private def collect_value @thread ||= Thread.new { Thread.stop @block[self] @mutex.synchronize{ @done = true } } Thread.pass until @thread.stop? @thread.run if @thread.status and need_value? Thread.pass while need_value? end def need_value? @mutex.synchronize{ not @position < @values.size and not @done } end # ... collect_value() makes sure we have a Thread running, or creates it if needed. The Thread is trivial: halt (to pause before the first iteration), run the block, grab the lock, toggle the @done flag, and release the lock. Once we are sure there is a Thread, we can pass() the calling Thread until the generator Thread gets a chance to run() and reaches its stop()ing point. need_value?() is just a check to make sure we don't have one queued, and we're not finished with iteration. The only challenge left is knowing when we are at the end?() of iteration: # ... def next? return (not end?) end def end? collect_value return eof? end # ... private # ... def eof? @mutex.synchronize{ not @position < @values.size and @done } end end The secret to end?() is that we must make sure we have a value queued, if possible. That's because we pause just after each item is added. We might be in the very last pause before iteration would end. Calling collect_value() will find the next item or cause the @done flag to be set. Either way, we then have enough information to properly answer the question. Here are the rest of the methods for Jacob's solution: # ... def pos @position end # ... def rewind @position = 0 self end def each self.rewind while self.next? yield self.next end end def index pos end # ... There shouldn't be any surprises left in there. Some other solutions that used this same technique were faster, or even slower. This seems primarily related to which tools were used to control Thread synchronization. Mutex, for example, is fairly slow while `Thread.critical = true` is quick and probably all you need in this instance. Let me send out a big thank you to all who tried this quiz and an even bigger thank you to Ross Bamford for helping me out with the timings. Tomorrow we have a deep meditation on metaprogramming...