Class: Parslet::Atoms::Context

Inherits:
Object
  • Object
show all
Defined in:
lib/parslet/atoms/context.rb

Overview

Helper class that implements a transient cache that maps position and parslet object to results. This is used for memoization in the packrat style.

Also, error reporter is stored here and error reporting happens through this class. This makes the reporting pluggable.

Defined Under Namespace

Classes: LRStack

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(reporter = Parslet::ErrorReporter::Tree.new) ⇒ Context

Returns a new instance of Context.

Parameters:

  • reporter (#err, #err_at) (defaults to: Parslet::ErrorReporter::Tree.new) —

    Error reporter (leave empty for default reporter)



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# File 'lib/parslet/atoms/context.rb', line 29

def initialize(reporter=Parslet::ErrorReporter::Tree.new)
  @cache = Hash.new { |h, k| h[k] = {} }
  @reporter = reporter
  @heads = {}
  @lr_stack = LRStack.new([])
end

Instance Attribute Details

#lr_stack ⇒ Object (readonly)

Returns the value of attribute lr_stack.



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# File 'lib/parslet/atoms/context.rb', line 25

def lr_stack
  @lr_stack
end

Instance Method Details

#err(*args) ⇒ Object

Report an error.

See Also:



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# File 'lib/parslet/atoms/context.rb', line 80

def err(*args)
  return [false, @reporter.err(*args)] if @reporter
  return [false, nil]
end

#err_at(*args) ⇒ Object

Report an error at a given position.

See Also:



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# File 'lib/parslet/atoms/context.rb', line 72

def err_at(*args)
  return [false, @reporter.err_at(*args)] if @reporter
  return [false, nil]
end

#heads ⇒ Object 



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# File 'lib/parslet/atoms/context.rb', line 36

def heads
  @heads
end

#lookup(obj, pos) ⇒ Object

private



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# File 'lib/parslet/atoms/context.rb', line 86

def lookup(obj, pos)
  @cache[pos][obj]
end

#set(obj, pos, val) ⇒ Object 



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# File 'lib/parslet/atoms/context.rb', line 90

def set(obj, pos, val)
  @cache[pos][obj] = val
end

#try_with_cache(obj, source) ⇒ Object

Caches a parse answer for obj at source.pos. Applying the same parslet at one position of input always yields the same result, unless the input has changed.

We need the entire source here so we can ask for how many characters were consumed by a successful parse. Imitation of such a parse must advance the input pos by the same amount of bytes.



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# File 'lib/parslet/atoms/context.rb', line 48

def try_with_cache(obj, source)
  beg = source.pos
    
  # Not in cache yet? Return early.
  unless entry = lookup(obj, beg)
    result = obj.try(source, self)

    set obj, beg, [result, source.pos-beg]
    return result
  end

  # the condition in unless has returned true, so entry is not nil.
  result, advance = entry

  # The data we're skipping here has been read before. (since it is in 
  # the cache) PLUS the actual contents are not interesting anymore since
  # we know obj matches at beg. So skip reading.
  source.pos = beg + advance
  return result
end