def delete_nonderivable_nonterminals(grammar): new_grammar = Grammar() new_grammar.axiom = grammar.axiom new_grammar.terminals = grammar.terminals unwatched = list([new_grammar.axiom]) watched = set() while unwatched: nonterminal = unwatched[0] unwatched = unwatched.remove(nonterminal) or [] watched.add(nonterminal) rules = find_rules_for_nonterminal(grammar.rules, nonterminal) for rule in rules: for symbol in rule.right_side: if isinstance(symbol, Nonterminal): if symbol not in watched and symbol not in unwatched: unwatched.append(symbol) new_grammar.nonterminals = watched new_rules = [] for rule in grammar.rules: if rule.left_side[0] in watched: new_rules.append(rule) new_grammar.rules = new_rules return new_grammar
def convert_grammar(grammar, disappearing_nonterminals): print "algorithm 1" new_grammar = Grammar() # build new nonterminals set new_grammar.nonterminals = build_new_nonterminals_set( grammar, disappearing_nonterminals) # find new axiom if grammar.axiom in disappearing_nonterminals: # language has empty chain new_grammar.axiom = Nonterminal(grammar.axiom.name, False) new_grammar.nonterminals.add(new_grammar.axiom) new_grammar.nonterminals.remove(grammar.axiom) else: new_grammar.axiom = grammar.axiom # build new rules new_grammar.rules = build_new_rules( grammar, disappearing_nonterminals, new_grammar.nonterminals) # copy terminals new_grammar.terminals = set(grammar.terminals) return new_grammar
def delete_useless_nonterminals(grammar): new_rules = list(grammar.rules) new_nonterminals = set(grammar.nonterminals) while True: useless_nonterminals = find_useless_nonterminals(new_rules) if not useless_nonterminals: break new_rules = delete_useless_nonterminals_from_rules(new_rules, useless_nonterminals) new_rules = delete_useless_nonterminals_rules(new_rules, useless_nonterminals) new_nonterminals.difference_update(useless_nonterminals) new_grammar = Grammar() new_grammar.axiom = grammar.axiom new_grammar.terminals = set(grammar.terminals) new_grammar.nonterminals = new_nonterminals new_grammar.rules = new_rules return new_grammar
def convert_to_greibach(grammar):
print "algorithm 3"
converted_grammar = delete_empty_rules(grammar)
print "converted grammar:"
print converted_grammar
new_grammar = Grammar()
new_grammar.axiom = converted_grammar.axiom
new_grammar.terminals = set(converted_grammar.terminals)
sorted_nonterminals = sort_nonterminals(
converted_grammar.nonterminals, converted_grammar.rules)
print "sorted nonterminals:", [str(s) for s in sorted_nonterminals]
new_grammar.nonterminals = set(sorted_nonterminals)
# rebuild rules
nonterminal = sorted_nonterminals[-1]
new_rules = find_rules_for_nonterminal(converted_grammar.rules, nonterminal)
for idx in range(len(sorted_nonterminals) - 2, -1, -1):
nonterminal = sorted_nonterminals[idx]
rules = find_rules_for_nonterminal(converted_grammar.rules, nonterminal)
for rule in rules:
if isinstance(rule.right_side[0], Nonterminal):
new_rules.extend(replace_rule(new_rules, rule))
else:
new_rules.append(rule)
# add rules for terminals
for terminal in converted_grammar.terminals:
new_nonterminal = Nonterminal("X{%s}" % str(terminal))
new_grammar.nonterminals.add(new_nonterminal)
new_rules.append(Rule([new_nonterminal], [terminal]))
# replace nonleft terminal to new nonterminals
for rule in new_rules:
rule =replace_nonleft_terminal_to_nonterminal(rule, terminal, new_nonterminal)
new_grammar.rules = new_rules
print "grammar:", new_grammar
return delete_nonderivable_nonterminals(new_grammar)
def build_new_grammar(grammar): new_grammar = Grammar() new_grammar.terminals = set(grammar.terminals) new_grammar.axiom = ComplexNonterminal( [grammar.axiom], grammar.axiom.is_nullable) new_rules = [] unwatched, watched = [new_grammar.axiom], set() while unwatched: complex_nonterminal = unwatched[0] unwatched.remove(complex_nonterminal) watched.add(complex_nonterminal) if complex_nonterminal.starts_with_nonterminal(): new_rules.extend(build_rules_starts_with_nonterminal( grammar.rules, complex_nonterminal, watched, unwatched)) elif complex_nonterminal.starts_with_terminal(): new_rules.extend(build_rules_starts_with_terminal( grammar.rules, complex_nonterminal, watched, unwatched)) new_grammar.rules = new_rules new_grammar.nonterminals = watched return new_grammar
from grammar import (Symbol , EmptySymbol,Terminal,
Nonterminal, Rule, Grammar)
from conversion import (has_empty_chain, convert_grammar, find_disappearing_nonterminals,
delete_empty_rules, delete_useless_nonterminals, convert_to_greibach)
grammar = Grammar()
S = Nonterminal('S')
A = Nonterminal('A')
B = Nonterminal('B')
grammar.nonterminals.update([A, B, S])
grammar.axiom = S
a = Terminal('a')
b = Terminal('b')
grammar.terminals.update([a, b])
e = EmptySymbol()
grammar.rules.append(Rule([S], [A, B]))
grammar.rules.append(Rule([A], [a, A]))
grammar.rules.append(Rule([A], [e]))
grammar.rules.append(Rule([B], [b, A]))
grammar.rules.append(Rule([B], [e]))
print "grammar:\n", grammar
#disappearing_nonterminals = find_disappearing_nonterminals(grammar)
#new_grammar = convert_grammar(grammar, disappearing_nonterminals)