Recursive Predictive parser for a simple grammar: s -> A s B | c ; c -> C c | C

simple.rb

# svn+ssh://orion/var/svn/casiano/LPP/rubytesting/simplerecdescent
# s -> A s B
#    | c
# c -> C c
#    | C

class Lexer
  attr_accessor :input, :tokens, :pos

  def initialize(input)
    @input = input 
    @tokens = @input.scan(/\S/)
    @pos = -1
  end

  def next
    @pos += 1
    if @pos < @tokens.length
     @tokens[@pos] 
    else 
      nil
    end
  end
end

def lexer
  @lookahead = @lex.next
end

def match(t)
  if @lookahead == t
    lexer
  else
    raise SyntaxError, "Error. Expecting '#{t}', found #{@lookahead}"
  end
end

def s
  if @lookahead    == 'A' then
    lexer
    s
    match('B')
    puts "s -> A s B"
  elsif @lookahead == 'C' then
    c
    puts "s -> c"
  else
    raise SyntaxError, "Error. Expecting 'A' or 'C'"
  end
end

def c
  if @lookahead == 'C' then
    lexer
  end
  if @lookahead == 'C' then
    c
    puts "c -> C c"
  elsif ['B', nil].include? @lookahead then
    puts "c -> C"
    return
  else
    raise SyntaxError, "Error. Expecting 'C' or 'B' or end of input but found '#@lookahead'"
  end
end

@lex = Lexer.new(ARGV.shift || 'ACCB')
puts @lex.input
lexer
s