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class Atom
def initialize(content = nil)
@content = content
end
attr_reader :content
def to_s
content
end
def self.from_token(token)
new(token.content)
end
end
class SExp
def initialize(addr = nil, decr = nil)
@addr = addr
@decr = decr
end
def car
@addr
end
def car=(addr)
@addr = addr
end
def cdr
@decr
end
def cdr=(decr)
@decr = decr
end
def nil?
car.nil? && cdr.nil?
end
def to_s
if car.nil?
"()"
elsif cdr.nil?
"(#{car} . ())"
else
"(#{car} . #{cdr})"
end
end
def to_a
res = []
at = self
until at.nil?
res << at.car
at = at.cdr
end
res
end
def self.from_a(list)
if list.empty?
SExp.new
else
SExp.new(list[0], SExp.from_a(list[1..-1]))
end
end
end
def peek_token(tokens)
tokens.first
end
def get_token(tokens)
tokens.shift
end
def num_members(tokens)
members = 0
level = 1
at = 1
until level == 0
case tokens.at(at)
when T_LParen
members += 1 if level == 1
level += 1
when T_RParen
level -= 1
when T_Atom
members += 1 if level == 1
end
at += 1
end
members
end
def parse_sexp(tokens)
need = num_members(tokens)
get_token(tokens)
members = []
need.times do
case peek_token(tokens)
when T_Atom
members << Atom.from_token(get_token(tokens))
when T_LParen
members << parse_sexp(tokens)
end
end
get_token(tokens)
SExp.from_a(members)
end
def parse_sexps(tokens)
sexps = []
until tokens.empty?
sexps << parse_sexp(tokens)
end
sexps
end
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