Add Iterators examples

007_iter_1.rb

@@ -0,0 +1,44 @@
+class Thing; end
+
+list = Array.new(1000) { Thing.new }
+puts ObjectSpace.each_object(Thing).count
+
+list.each.with_index do |item, i|
+  GC.start
+  puts ObjectSpace.each_object(Thing).count if i == 500
+end
+
+list = nil
+GC.start
+puts ObjectSpace.each_object(Thing).count
+
+puts 'With shift deallocation'
+
+list = Array.new(1000) { Thing.new }
+puts ObjectSpace.each_object(Thing).count
+
+while list.count > 0
+  GC.start
+  puts ObjectSpace.each_object(Thing).count if list.count == 500
+  item = list.shift
+end
+
+GC.start
+puts ObjectSpace.each_object(Thing).count
+
+# each! Pattern
+
+puts 'With each! pattern'
+
+class Array
+  def each!
+    while count > 0
+      yield shift # Shift 1 item and execute block on it without &block capturing
+    end
+  end
+end
+
+Array.new(1000).each { |e| e }
+GC.start
+puts ObjectSpace.each_object(Thing).count
+

007_iter_2.rb

@@ -0,0 +1,29 @@
+GC.disable
+before = ObjectSpace.count_objects
+
+Array.new(10000).each do |i|
+  [0,1].each do |j|
+    # j
+  end
+end
+
+after = ObjectSpace.count_objects
+puts '# of arrays: %d' % (after[:T_ARRAY] - before[:T_ARRAY])
+puts '# of nodes: %d' % (after[:T_NODE] - before[:T_NODE])
+
+GC.enable
+GC.start
+GC.disable
+
+before2 = ObjectSpace.count_objects
+
+Array.new(10000).each do |i|
+  [0, 1].each_with_index do |j, index|
+    # [j, index]
+  end
+end
+
+after2 = ObjectSpace.count_objects
+puts '# of arrays: %d' % (after2[:T_ARRAY] - before2[:T_ARRAY])
+puts '# of nodes: %d' % (after2[:T_NODE] - before2[:T_NODE])
+

008_date_parsing.rb

@@ -0,0 +1,30 @@
+require 'date'
+require 'benchmark'
+
+date = '2014-05-23'
+time = Benchmark.realtime do
+  100_000.times do
+    Date.parse(date)
+  end
+end
+
+puts 'Date#parse: %.3f' % time
+
+GC.start
+time = Benchmark.realtime do
+  100_000.times do
+    Date.strptime(date, '%Y-%m-%d')
+  end
+end
+
+puts 'Date.strptime: %.3f' % time
+
+GC.start
+time = Benchmark.realtime do
+  100_000.times do
+    Date.civil(date[0, 4].to_i, date[5, 2].to_i, date[8, 2].to_i)
+  end
+end
+
+puts 'Manual parsing: %.3f' % time
+

README.md

@@ -14,13 +14,13 @@ See [001_gc.rb](001_gc.rb)
 1. 80% of performance optimization comes from memory optimization
   See [002_memory.rb](002_memory.rb)
 
-2. GC::Profiler has memory and CPU overhead (See [wrapper.rb] for custom wrapper example).
+2. GC::Profiler has memory and CPU overhead (See [wrapper.rb](wrapper.rb) for custom wrapper example).
 
 3. Save memory by avoiding copiyng objects, modify them in place if it is possible (use ! methods).
 
   See [004_string_bang.rb](004_string_bang.rb)
 
-4. If your String is less than 40 bytes - user '<<', not '+=' method to concatenate it and Ruby will not allocate additional object.
+4. If your String is less than 40 bytes - user `<<`, not `+=` method to concatenate it and Ruby will not allocate additional object.
 
   See [004_array_bang.rb](004_array_bang.rb) (w/ GC)
 
@@ -34,3 +34,52 @@ See [001_gc.rb](001_gc.rb)
   See [006_callbacks_3.rb](006_callbacks_3.rb)
   Try to avoid `&block` and use `yield` instead.
 
+7. Iterators use block arguments, so use them carefuly
+
+  Issues:
+  1. GC will not collect iterable before iterator is finished
+  2. Iterators create temp objects
+
+  Solutions:
+
+  1. Free objects from collection during iteration & use `each!` pattern
+    See [007_iter_1.rb](007_iter_1.rb)
+  2. Look at C code to find object allocations
+    See [007_iter_2.rb](007_iter_2.rb) (for ruby < 2.3.0)
+
+    Table of `T_NODE` allocations per iterator item for ruby 2.1:
+
+    | Iterator         | Enum | Array | Range |  
+    | ---------------: | ---- | ----- | ----- |  
+    | all?             | 3    | 3     | 3     |  
+    | any?             | 2    | 2     | 2     |  
+    | collect          | 0    | 1     | 1     |  
+    | cycle            | 0    | 1     | 1     |  
+    | delete_if        | 0    | —     | 0     |  
+    | detect           | 2    | 2     | 2     |  
+    | each             | 0    | 0     | 0     |  
+    | each_index       | 0    | —     | —     |  
+    | each_key         | —    | —     | 0     |  
+    | each_pair        | —    | —     | 0     |  
+    | each_value       | —    | —     | 0     |  
+    | each_with_index  | 2    | 2     | 2     |  
+    | each_with_object | 1    | 1     | 1     |  
+    | fill             | 0    | —     | —     |  
+    | find             | 2    | 2     | 2     |  
+    | find_all         | 1    | 1     | 1     |  
+    | grep             | 2    | 2     | 2     |  
+    | inject           | 2    | 2     | 2     |  
+    | map              | 0    | 1     | 1     |  
+    | none?            | 2    | 2     | 2     |  
+    | one?             | 2    | 2     | 2     |  
+    | reduce           | 2    | 2     | 2     |  
+    | reject           | 0    | 1     | 0     |  
+    | reverse          | 0    | —     | —     |  
+    | reverse_each     | 0    | 1     | 1     |  
+    | select           | 0    | 1     | 0     |  
+
+8. Date parsing is slow
+  See [008_date_parsing.rb](008_date_parsing.rb)
+
+9. `Object#class`, `Object#is_a?`, `Object#kind_of?` are slow when used inside iterators
+