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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, data = nil)
@addr = addr
@data = data
end
def car
@addr
end
def car=(addr)
@addr = addr
end
def cdr
@data
end
def cdr=(data)
@data = data
end
def to_s
if car.nil?
"()"
elsif cdr.nil?
"(#{car} . ())"
else
"(#{car} . #{cdr})"
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 members_to_tree(members)
case members.length
when 0
nil
when 1
SExp.new(members[0], SExp.new)
when 2
SExp.new(members[0], members[1])
else
SExp.new(members[0], members_to_tree(members[1..-1]))
end
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)
members_to_tree(members)
end
def parse_sexps(tokens)
sexps = []
until tokens.empty?
sexps << parse_sexp(tokens)
end
sexps
end
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