Add CachingTools::HashCaching to simplify the creation of nested, autofilling hashes.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4059 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

3 files changed, 145 insertions, 0 deletions

diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index dbe6aa91c2..87a8428d0f 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Add CachingTools::HashCaching to simplify the creation of nested, autofilling hashes. [Nicholas Seckar]
+
 * Remove a hack intended to avoid unloading the same class twice, but which would not work anyways. [Nicholas Seckar]
 
 * Update Object.subclasses_of to locate nested classes. This affects Object.remove_subclasses_of in that nested classes will now be unloaded. [Nicholas Seckar]
diff --git a/activesupport/lib/active_support/caching_tools.rb b/activesupport/lib/active_support/caching_tools.rb
new file mode 100644
index 0000000000..c889c148b4
--- /dev/null
+++ b/activesupport/lib/active_support/caching_tools.rb
@@ -0,0 +1,62 @@
+module ActiveSupport
+  module CachingTools #:nodoc:
+    
+    # Provide shortcuts to simply the creation of nested default hashes. This
+    # pattern is useful, common practice, and unsightly when done manually.
+    module HashCaching
+      # Dynamically create a nested hash structure used to cache calls to +method_name+
+      # The cache method is named +#{method_name}_cache+ unless :as => :alternate_name
+      # is given.
+      #
+      # The hash structure is created using nested Hash.new. For example:
+      # 
+      #   def slow_method(a, b) a ** b end
+      # 
+      # can be cached using hash_cache :slow_method, which will define the method
+      # slow_method_cache. We can then find the result of a ** b using:
+      # 
+      #   slow_method_cache[a][b]
+      # 
+      # The hash structure returned by slow_method_cache would look like this:
+      # 
+      #   Hash.new do |as, a|
+      #     as[a] = Hash.new do |bs, b|
+      #       bs[b] = slow_method(a, b)
+      #     end
+      #   end
+      # 
+      # The generated code is actually compressed onto a single line to maintain
+      # sensible backtrace signatures.
+      #
+      def hash_cache(method_name, options = {})
+        selector = options[:as] || "#{method_name}_cache"
+        method = self.instance_method(method_name)
+        
+        args = []
+        code = "def #{selector}(); @#{selector} ||= "
+        
+        (1..method.arity).each do |n|
+          args << "v#{n}"
+          code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
+        end
+        
+        # Add the method call with arguments, followed by closing braces and end.
+        code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
+        
+        # Extract the line number information from the caller. Exceptions arising
+        # in the generated code should point to the +hash_cache :...+ line.
+        if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
+          file, line_number = $1, $2.to_i
+        else # We can't give good trackback info; fallback to this line:
+          file, line_number = __FILE__, __LINE__
+        end
+        
+        # We use eval rather than building proc's because it allows us to avoid
+        # linking the Hash's to this method's binding. Experience has shown that
+        # doing so can cause obtuse memory leaks.
+        class_eval code, file, line_number
+      end
+    end
+    
+  end
+end
diff --git a/activesupport/test/caching_tools_test.rb b/activesupport/test/caching_tools_test.rb
new file mode 100644
index 0000000000..9f3e42e496
--- /dev/null
+++ b/activesupport/test/caching_tools_test.rb
@@ -0,0 +1,81 @@
+require 'test/unit'
+require File.dirname(__FILE__)+'/../lib/active_support/caching_tools'
+
+class HashCachingTests < Test::Unit::TestCase
+  
+  def cached(&proc)
+    return @cached if @cached
+    
+    @cached_class = Class.new(&proc)
+    @cached_class.class_eval do
+      extend ActiveSupport::CachingTools::HashCaching
+      hash_cache :slow_method
+    end
+    @cached = @cached_class.new
+  end
+  
+  def test_cache_access_should_call_method
+    cached do
+      def slow_method(a) raise "I should be here: #{a}"; end
+    end
+    assert_raises(RuntimeError) { cached.slow_method_cache[1] }
+  end
+  
+  def test_cache_access_should_actually_cache
+    cached do
+      def slow_method(a)
+        (@x ||= [])
+        if @x.include?(a) then raise "Called twice for #{a}!"
+        else
+          @x << a
+          a + 1
+        end
+      end
+    end
+    assert_equal 11, cached.slow_method_cache[10]
+    assert_equal 12, cached.slow_method_cache[11]
+    assert_equal 11, cached.slow_method_cache[10]
+    assert_equal 12, cached.slow_method_cache[11]
+  end
+  
+  def test_cache_should_be_clearable
+    cached do
+      def slow_method(a)
+        @x ||= 0
+        @x += 1
+      end
+    end
+    assert_equal 1, cached.slow_method_cache[:a]
+    assert_equal 2, cached.slow_method_cache[:b]
+    assert_equal 3, cached.slow_method_cache[:c]
+    
+    assert_equal 1, cached.slow_method_cache[:a]
+    assert_equal 2, cached.slow_method_cache[:b]
+    assert_equal 3, cached.slow_method_cache[:c]
+    
+    cached.slow_method_cache.clear
+    
+    assert_equal 4, cached.slow_method_cache[:a]
+    assert_equal 5, cached.slow_method_cache[:b]
+    assert_equal 6, cached.slow_method_cache[:c]
+  end
+  
+  def test_deep_caches_should_work_too
+    cached do
+      def slow_method(a, b, c)
+        a + b + c
+      end
+    end
+    assert_equal 3, cached.slow_method_cache[1][1][1]
+    assert_equal 7, cached.slow_method_cache[1][2][4]
+    assert_equal 7, cached.slow_method_cache[1][2][4]
+    assert_equal 7, cached.slow_method_cache[4][2][1]
+    
+    assert_equal({
+      1 => {1 => {1 => 3}, 2 => {4 => 7}},
+      4 => {2 => {1 => 7}}},
+      cached.slow_method_cache
+    )
+  end
+  
+end