Data commit

Task/Atomic-updates/Ruby/atomic-updates.rb

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+require 'thread'
+
+# A collection of buckets, filled with random non-negative integers.
+# There are atomic operations to look at the bucket contents, and
+# to move amounts between buckets.
+class BucketStore
+
+  # Creates a BucketStore with +nbuckets+ buckets. Fills each bucket
+  # with a random non-negative integer.
+  def initialize nbuckets
+    # Create an array for the buckets
+    @buckets = (0...nbuckets).map { rand(1024) }
+
+    # Mutex used to make operations atomic
+    @mutex = Mutex.new
+  end
+
+  # Returns an array with the contents of all buckets.
+  def buckets
+    @mutex.synchronize { Array.new(@buckets) }
+  end
+
+  # Transfers _amount_ to bucket at array index _destination_,
+  # from bucket at array index _source_.
+  def transfer destination, source, amount
+    # Do nothing if both buckets are same
+    return nil if destination == source
+
+    @mutex.synchronize do
+      # Clamp amount to prevent negative value in bucket
+      amount = [amount, @buckets[source]].min
+
+      @buckets[source] -= amount
+      @buckets[destination] += amount
+    end
+    nil
+  end
+end
+
+# Create bucket store
+bucket_store = BucketStore.new 8
+
+# Get total amount in the store
+TOTAL = bucket_store.buckets.inject { |a, b| a += b }
+
+# Start a thread to equalize buckets
+Thread.new do
+  loop do
+    # Pick 2 buckets
+    buckets = bucket_store.buckets
+    first = rand buckets.length
+    second = rand buckets.length
+
+    # Swap buckets so that _first_ has not more than _second_
+    first, second = second, first if buckets[first] > buckets[second]
+
+    # Transfer half of the difference, rounded down
+    bucket_store.transfer first, second, (buckets[second] - buckets[first]) / 2
+  end
+end
+
+# Start a thread to distribute values among buckets
+Thread.new do
+  loop do
+    # Pick 2 buckets
+    buckets = bucket_store.buckets
+    first = rand buckets.length
+    second = rand buckets.length
+
+    # Transfer random amount to _first_ from _second_
+    bucket_store.transfer first, second, rand(buckets[second])
+  end
+end
+
+# Loop to display buckets
+loop do
+  sleep 1
+
+  buckets = bucket_store.buckets
+
+  # Compute the total value in all buckets.
+  # We calculate this outside BucketStore so BucketStore can't cheat by
+  # always reporting the same value.
+  n = buckets.inject { |a, b| a += b }
+
+  # Display buckets and total
+  printf "%s, total %d\n", (buckets.map { |v| sprintf "%4d", v }.join " "), n
+
+  if n != TOTAL
+    # This should never happen
+    $stderr.puts "ERROR: Total changed from #{TOTAL} to #{n}"
+    exit 1
+  end
+end