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module DeclarativePolicy
# This object represents one step in the runtime decision of whether
# an ability is allowed. It contains a Rule and a context (instance
# of DeclarativePolicy::Base), which contains the user, the subject,
# and the cache. It also contains an "action", which is the symbol
# :prevent or :enable.
class Step
attr_reader :context, :rule, :action
def initialize(context, rule, action)
@context = context
@rule = rule
@action = action
end
# In the flattening process, duplicate steps may be generated in the
# same rule. This allows us to eliminate those (see Runner#steps_by_score
# and note its use of a Set)
def ==(other)
@context == other.context && @rule == other.rule && @action == other.action
end
# In the runner, steps are sorted dynamically by score, so that
# we are sure to compute them in close to the optimal order.
#
# See also Rule#score, ManifestCondition#score, and Runner#steps_by_score.
def score
# we slightly prefer the preventative actions
# since they are more likely to short-circuit
case @action
when :prevent
@rule.score(@context) * (7.0 / 8)
when :enable
@rule.score(@context)
end
end
def with_action(action)
Step.new(@context, @rule, action)
end
def enable?
@action == :enable
end
def prevent?
@action == :prevent
end
# This rather complex method allows us to split rules into parts so that
# they can be sorted independently for better optimization
def flattened(roots)
case @rule
when Rule::Or
# A single `Or` step is the same as each of its elements as separate steps
@rule.rules.flat_map { |r| Step.new(@context, r, @action).flattened(roots) }
when Rule::Ability
# This looks like a weird micro-optimization but it buys us quite a lot
# in some cases. If we depend on an Ability (i.e. a `can?(...)` rule),
# and that ability *only* has :enable actions (modulo some actions that
# we already have taken care of), then its rules can be safely inlined.
steps = @context.runner(@rule.ability).steps.reject { |s| roots.include?(s) }
if steps.all?(&:enable?)
# in the case that we are a :prevent step, each inlined step becomes
# an independent :prevent, even though it was an :enable in its initial
# context.
steps.map! { |s| s.with_action(:prevent) } if prevent?
steps.flat_map { |s| s.flattened(roots) }
else
[self]
end
else
[self]
end
end
def pass?
@rule.pass?(@context)
end
def repr
"#{@action} when #{@rule.repr} (#{@context.repr})"
end
end
end
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