1 files changed, 102 insertions, 0 deletions
diff --git a/lib/declarative_policy/condition.rb b/lib/declarative_policy/condition.rb
new file mode 100644
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+++ b/lib/declarative_policy/condition.rb
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+module DeclarativePolicy
+  # A Condition is the data structure that is created by the
+  # `condition` declaration on DeclarativePolicy::Base. It is
+  # more or less just a struct of the data passed to that
+  # declaration. It holds on to the block to be instance_eval'd
+  # on a context (instance of Base) later, via #compute.
+  class Condition
+    attr_reader :name, :description, :scope
+    attr_reader :manual_score
+    attr_reader :context_key
+    def initialize(name, opts = {}, &compute)
+      @name = name
+      @compute = compute
+      @scope = opts.fetch(:scope, :normal)
+      @description = opts.delete(:description)
+      @context_key = opts[:context_key]
+      @manual_score = opts.fetch(:score, nil)
+    end
+
+    def compute(context)
+      !!context.instance_eval(&@compute)
+    end
+
+    def key
+      "#{@context_key}/#{@name}"
+    end
+  end
+
+  # In contrast to a Condition, a ManifestCondition contains
+  # a Condition and a context object, and is capable of calculating
+  # a result itself. This is the return value of Base#condition.
+  class ManifestCondition
+    def initialize(condition, context)
+      @condition = condition
+      @context = context
+    end
+
+    # The main entry point - does this condition pass? We reach into
+    # the context's cache here so that we can share in the global
+    # cache (often RequestStore or similar).
+    def pass?
+      @context.cache(cache_key) { @condition.compute(@context) }
+    end
+
+    # Whether we've already computed this condition.
+    def cached?
+      @context.cached?(cache_key)
+    end
+
+    # This is used to score Rule::Condition. See Rule::Condition#score
+    # and Runner#steps_by_score for how scores are used.
+    #
+    # The number here is intended to represent, abstractly, how
+    # expensive it would be to calculate this condition.
+    #
+    # See #cache_key for info about @condition.scope.
+    def score
+      # If we've been cached, no computation is necessary.
+      return 0 if cached?
+
+      # Use the override from condition(score: ...) if present
+      return @condition.manual_score if @condition.manual_score
+
+      # Global scope rules are cheap due to max cache sharing
+      return 2 if  @condition.scope == :global
+
+      # "Normal" rules can't share caches with any other policies
+      return 16 if @condition.scope == :normal
+
+      # otherwise, we're :user or :subject scope, so it's 4 if
+      # the caller has declared a preference
+      return 4 if @condition.scope == DeclarativePolicy.preferred_scope
+
+      # and 8 for all other :user or :subject scope conditions.
+      8
+    end
+
+    private
+
+    # This method controls the caching for the condition. This is where
+    # the condition(scope: ...) option comes into play. Notice that
+    # depending on the scope, we may cache only by the user or only by
+    # the subject, resulting in sharing across different policy objects.
+    def cache_key
+      case @condition.scope
+      when :normal  then "/dp/condition/#{@condition.key}/#{user_key},#{subject_key}"
+      when :user    then "/dp/condition/#{@condition.key}/#{user_key}"
+      when :subject then "/dp/condition/#{@condition.key}/#{subject_key}"
+      when :global  then "/dp/condition/#{@condition.key}"
+      else raise 'invalid scope'
+      end
+    end
+
+    def user_key
+      Cache.user_key(@context.user)
+    end
+
+    def subject_key
+      Cache.subject_key(@context.subject)
+    end
+  end
+end