Class: BinData::LazyEvalEnv

Inherits:
Object
  • Object
show all
Defined in:
lib/bindata/lazy.rb

Overview

The enviroment in which a lazily evaluated lamba is called. These lambdas are those that are passed to data objects as parameters. Each lambda has access to the following:

parent

the environment of the parent data object

params

any extra parameters that have been passed to the data object. The value of a parameter is either a lambda, a symbol or a literal value (such as a Fixnum).

Unknown methods are resolved in the context of the parent environment, first as keys in the extra parameters, and secondly as methods in the parent data object. This makes the lambda easier to read as we just write field instead of obj.field.

Constant Summary collapse

@@empty_hash =

An empty hash shared by all instances

Hash.new.freeze
@@variables_cache = 
{}

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(parent = nil) ⇒ LazyEvalEnv

Creates a new environment. parent is the environment of the parent data object.



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# File 'lib/bindata/lazy.rb', line 23

def initialize(parent = nil)
  @parent = parent
  @variables = @@empty_hash
  @overrides = @@empty_hash
  @params    = @@empty_hash
end

Dynamic Method Handling

This class handles dynamic methods through the method_missing method

#method_missing(symbol, *args) ⇒ Object 



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# File 'lib/bindata/lazy.rb', line 92

def method_missing(symbol, *args)
  if @overrides.include?(symbol)
    @overrides[symbol]
  elsif @variables.include?(symbol)
    @variables[symbol]
  elsif @parent
    obj = symbol
    if @parent.params and @parent.params.has_key?(symbol)
      obj = @parent.params[symbol]
    elsif @parent.data_object and @parent.data_object.respond_to?(symbol)
      obj = @parent.data_object.__send__(symbol, *args)
    end
    @parent.lazy_eval(obj)
  else
    super
  end
end

Instance Attribute Details

#data_object=(value) ⇒ Object

Sets the attribute data_object

Parameters:

  • value —

    the value to set the attribute data_object to.



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# File 'lib/bindata/lazy.rb', line 30

def data_object=(value)
  @data_object = value
end

#params ⇒ Object

Returns the value of attribute params.



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# File 'lib/bindata/lazy.rb', line 29

def params
  @params
end

#parent ⇒ Object (readonly)

Returns the value of attribute parent.



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# File 'lib/bindata/lazy.rb', line 29

def parent
  @parent
end

Instance Method Details

#add_variable(sym, value) ⇒ Object

Add a variable with a pre-assigned value to this environment. sym will be accessible as a variable for any lambda evaluated with #lazy_eval.



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# File 'lib/bindata/lazy.rb', line 43

def add_variable(sym, value)
  sym = sym.to_sym
  if @variables.equal?(@@empty_hash)
    # optimise the case where only 1 variable is added as this
    # is the most common occurance (BinData::Arrays adding index)
    key = [sym, value]
    @variables = @@variables_cache[key]
    if @variables.nil?
      # cache this variable and value so it can be shared with
      # other LazyEvalEnvs to keep memory usage down
      @variables = {sym => value}.freeze
      @@variables_cache[key] = @variables
    end
  else
    if @variables.length == 1
      key = @variables.keys[0]
      @variables = {key => @variables[key]}
    end
    @variables[sym] = value
  end
end

#lazy_eval(obj, overrides = nil) ⇒ Object

Evaluates obj in the context of this environment. Evaluation recurses until it yields a value that is not a symbol or lambda. overrides is an optional params like hash



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# File 'lib/bindata/lazy.rb', line 80

def lazy_eval(obj, overrides = nil)
  @overrides = overrides if overrides
  if obj.is_a? Symbol
    # treat :foo as lambda { foo }
    obj = __send__(obj)
  elsif obj.respond_to? :arity
    obj = instance_eval(&obj)
  end
  @overrides = @@empty_hash
  obj
end

#offset_of(sym) ⇒ Object

TODO: offset_of needs to be better thought out



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# File 'lib/bindata/lazy.rb', line 66

def offset_of(sym)
  @parent.data_object.offset_of(sym)
rescue
  nil
end

#parent_data_object ⇒ Object

Returns the data_object for the parent environment.



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# File 'lib/bindata/lazy.rb', line 73

def parent_data_object
  @parent.nil? ? nil : @parent.data_object
end