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require 'kpeg/position'
module KPeg
class Parser < StringScanner
def initialize(str, grammar, log=false)
super str
@grammar = grammar
# A 2 level hash.
@memoizations = Hash.new { |h,k| h[k] = {} }
@failing_offset = nil
@failing_op = nil
@log = log
end
attr_reader :grammar, :memoizations, :failing_offset
attr_accessor :failing_op
include Position
def switch_grammar(gram)
begin
old = @grammar
@grammar = gram
yield
ensure
@grammar = old
end
end
def fail(op)
@failing_offset = pos
@failing_op = op
return nil
end
def expected_string
case @failing_op
when Choice
return Range.new(@failing_op.start, @failing_op.fin)
when Dot
return nil
else
@failing_op.string
end
end
class LeftRecursive
def initialize(detected=false)
@detected = detected
end
attr_accessor :detected
end
class MemoEntry
def initialize(ans, pos)
@ans = ans
@pos = pos
@uses = 1
end
attr_reader :ans, :pos, :uses
def inc!
@uses += 1
end
def move!(ans, pos)
@ans = ans
@pos = pos
end
end
# Call a rule without memoization
def invoke(rule)
rule.op.match(self)
end
def apply(rule)
ans = nil
if m = @memoizations[rule][pos]
m.inc!
self.pos = m.pos
if m.ans.kind_of? LeftRecursive
m.ans.detected = true
if @log
puts "LR #{rule.name} @ #{self.inspect}"
end
return nil
end
ans = m.ans
else
lr = LeftRecursive.new(false)
m = MemoEntry.new(lr, pos)
@memoizations[rule][pos] = m
start_pos = pos
if @log
puts "START #{rule.name} @ #{self.inspect}"
end
ans = rule.op.match(self)
m.move! ans, pos
# Don't bother trying to grow the left recursion
# if it's failing straight away (thus there is no seed)
if ans and lr.detected
ans = grow_lr(rule, start_pos, m)
end
end
if @log
if ans
puts " OK #{rule.name} @ #{self.inspect}"
else
puts " FAIL #{rule.name} @ #{self.inspect}"
end
end
return ans
end
def grow_lr(rule, start_pos, m)
while true
self.pos = start_pos
ans = rule.op.match(self)
return nil unless ans
break if pos <= m.pos
m.move! ans, pos
end
self.pos = m.pos
return m.ans
end
def failed?
!!@failing_op
end
def parse(name=nil)
if name
rule = @grammar.find(name)
unless rule
raise "Unknown rule - #{name}"
end
else
rule = @grammar.root
end
match = apply rule
if pos == string.size
@failing_op = nil
end
return match
end
def expectation
error_pos = @failing_offset
line_no = current_line(error_pos)
col_no = current_column(error_pos)
expected = expected_string()
prefix = nil
case expected
when String
prefix = expected.inspect
when Range
prefix = "to be between #{expected.begin} and #{expected.end}"
when Array
prefix = "to be one of #{expected.inspect}"
when nil
prefix = "anything (no more input)"
else
prefix = "unknown"
end
return "Expected #{prefix} at line #{line_no}, column #{col_no} (offset #{error_pos})"
end
end
end
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